Book of Abstracts for the6th International Reflectometry Workshop

San Diego, May 5-7, 2003

The abstracts are listed in alphabetical order, by surname of the first author.

Reflectometry on Alcator C-Mod: Status and future upgrades

N. P. Basse, Y. Lin, J. H. Irby

Plasma Science and Fusion Center

Massachusetts Institute of Technology

MA-02139 Cambridge

USA

G. J. Kramer, R. Nazikian

Princeton Plasma Physics Laboratory

NJ-08543 Princeton

USA

The reflectometer system currently installed on the Alcator C-Mod tokamak is anamplitude modulated (AM) reflectometer working in O-mode [1]. It consists of fivechannels with center frequencies 50, 60, 75, 88 and 110 GHz. These frequenciescorrespond to densities in the range between 0.31 × 1020 m−3 and 1.5 × 1020 m−3.

Four of the channels use the traditional phase detection scheme for AM reflectometry,i.e. measuring the phase difference between the upper (USB) and lower (LSB)sidebands of the AM waves. This is not ideal for fluctuation measurements, since itreduces the sensitivity by subtracting correlated fluctuations in the USB and LSBsignals. The 88 GHz channel has therefore been upgraded to separate and measure theUSB and LSB signals independently [2].

The focus of the reflectometry system has shifted from profile measurements towardsfluctuation studies. The proposed upgrades to the reflectometer are the following:

1. Add two high frequency channels to the system: 130 and 140 GHz, correspondingto densities of 2.1 × 1020 m−3 and 2.4 × 1020 m−3. This will enable theinvestigation of internal transport barrier turbulence dynamics.

2. Upgrade the 50, 60, 75 and 110 GHz channels to measure the USB and LSBsignals separately.

3. Move data acquisition from CAMAC to compact PCI modules.

The first experimental Alcator C-Mod campaign of 2003 has now begun (April 2003)and will continue until July 2003. The plan is for point 1 to be completed by June 2003,allowing us to make first high frequency measurements before the summer shutdown.

References

[1] P. Stek, Ph.D. thesis, Massachusetts Institute of Technology (1997)Y. Lin, Ph.D. thesis, Massachusetts Institute of Technology (2001)

[2] Y. Lin, J. Irby, P. Stek et al., Rev. Sci. Instrum. 70, 1078 (1999)

Profile reflectometry on TORE SUPRA

F. Clairet, C. Bottereau, J.M. Chareau, R. Sabot, F. Gabillet and L. Vermare

Association Euratom-CEA, CEA/DSM/DRFC, Centre de Cadarache, 13108 St-Paul-lez-Durance, France

S. Heuraux

LPMI, Université Henri Poincaré Nancy I, BP 239 54506 Vandoeuvre-lès-Nancy, France

G. Leclert

Laboratoire P.I.I.M., Faculté de saint Jérôme, Case 321, 13397 Marseille, France

Within the framework of the recently upgraded configuration of the TS tokamak, the

profile reflectometry uses a X-mode configuration and covers now a full frequency band from

50 to 110 GHz. After one year of operation we resume the main results and the difficulties we

have encountered. It has been confirmed that deleterious effects of the plasma turbulence

upon the reflected signal are resolved using ultra-fast sweep capabilities in association with a

high sensitive heterodyne detection. Optimisation of the output power combined with suitable

antenna set-up has allowed overcoming of the ECE radiation noise generated by suprathermal

electrons during lower hybrid (LH) additional power. A damaging effect of the edge magnetic

ripple onto the reflected amplitude has been clearly identified and surmounted by adjusting

the reflectometer position. So far, automatic profile recoveries from raw data have reached

nearly 100% without averaging needs. A new acquisition system in VME format is now

operating. It allows for 25 Msample memory capabilities per channel at sampling frequency

up to 200 MHz and fulfils the data storage requirements particularly during burst mode

operations. Experimental results obtained with the reflectometer installed for measurements in

front of the LH antenna will be presented to point out the parasitic reflection problems

induced in such configuration. Finally, extended perspectives of measurements for MHD

activity and small-scale turbulence using frequency swept systems will be presented.

6th Intl. Reflectometer Workshop, GA, San Diego May 5-7th 2003

Latest results and future plans for Doppler reflectometry on ASDEX

Upgrade

G.D.Conway, J.Schirmer, W.Suttrop, S.Klenge∗, E.Holzhauer∗, D.Wagner, H.Zohm

and the ASDEX Upgrade Team

Max-Planck-Institut fur Plasmaphysik, Euratom-Association IPP, Garching, Germany

∗ Institut fur Plasmaforschung, Universitat Stuttgart, Germany

A second V-band (50−75 GHz) channel has been added to the ASDEX Upgrade Doppler

reflectometer system. Both channels have steppable launch frequencies, with selectable

O-mode or X-mode polarization, giving a radial coverage from the tokamak edge to mid-

radius, and, at low densities to the plasma core. Separate low-field-side O and X-mode

bistatic hog-horn antenna pairs with elliptic focusing mirrors (corrugated to optimize side

lobe suppression) are used. The antennas are tilted primarily in the poloidal direction,

but also aligned with the edge magnetic field line inclination to measure the true per-

pendicular velocity v⊥ = 2πfDk⊥ = fDλo/2 sin θ. The actual incident angle θ between

beam and cutoff layer varies with the plasma shape, cutoff layer position and refraction.

However, typical angles range from 5−18o giving a wavenumber sensitivity range of k⊥ of

3− 8 cm−1, with resulting Doppler shifts fD of upto 5 MHz. Heterodyne and full IQ de-

tection is employed with 80 MHz IF and 20 MHz data sampling via S-Link and MDSplus

server/client interface. Launch power is several mW via oversized circular waveguide

transmission lines with a resulting dynamic range of > 40 dB. New high spatial resolution

radial v⊥ profiles will be presented for a variety of discharge conditions, including L-mode,

H-mode, ITB etc. with forward and reversed BT and co and counter injection. The in-

trinsic phase velocity is predicted to be small, hence the measured v⊥ = vE×B + vphase

should be dominated by the E × B velocity, from which the radial electric field Er pro-

file can be extracted. Typical, H-mode v⊥ profiles show the velocity rising from zero at

the edge, with a sign reversal around the region of steep pressure gradient (due to the

dominance of the diamagnetic drift velocity over the bulk fluid velocity) then falling back

towards zero on axis. The roles of the constituent velocity components are confirmed by

profile reversals when BT and Ip are reversed. Some indications of future directions will

be given, including Doppler correlation measurements, the addition of two more channels

in W-band (75− 110 GHz) to increase core coverage, plus a new wide-band quasi-optical

transmission line and antenna system with poloidally steerable mirrors.

6th Intl. Reflectometer Workshop, GA, San Diego May 5-7th 2003

Some observations on plasma turbulence measurements in ASDEX Upgrade and JET

G.D.Conway, ASDEX Upgrade Team and CFN Reflectometer Group

Max-Planck-Institut fur Plasmaphysik, Euratom-Association IPP, Garching, Germany

Reflectometry has over the years become an important diagnostic for density fluctuation and turbulence studies, contributing directly to

the understanding to tokamak confinement behaviour. A selection of reflectometer fluctuation measurements from the ASDEX Upgrade

and JET tokamaks are presented with physics interpretations. These include attempts to discriminate between the effects of velocity

shearing and magnetic shear on turbulence levels and radial correlation lengths within transport barrier regions, plus the localisation of

coherent edge modes associated with type-II ELM discharges etc.

6th Intl. Reflectometer Workshop, GA, San Diego May 5-7th 2003

Reflectometer simulation studies

G.D.Conway, B.Kurzan, B.Scott, E.Holzhauer∗ and M.Kaufmann

Max-Planck-Institut fur Plasmaphysik, Euratom-Association IPP, Garching, Germany

∗ Institut fur Plasmaforschung, Universitat Stuttgart, Germany

In this talk the latest results on the coupling of numerical turbulence and reflectometer simulation codes are presented. The aim of

the work is two-fold. First, to provide a direct route for the validation and testing of numerical turbulence simulation predictions.

This involves simulating the edge turbulence of a real ASDEX Upgrade L-mode tokamak discharge using the DALFTI Landau fluid

code, applying a model for the reflectometer response function to the numerical data, and then finally comparing with real experimental

homodyne reflectometer data using the same data analysis techniques. Secondly, with the same (and hopefully realistic) numerical density

perturbation data a benchmarking of various reflectometer simulation codes can be performed. In this case, a 2D finite-difference time

domain full-wave code, a 2D time-invariant network code and the 2D physical optics model. The benchmarking shows excellent agreement

between the full-wave and network codes, but the need for a 0.3 amplitude correction factor for the physical optics code case. First step

comparisons between experimental data and numerical turbulence plus full-wave reflectometer kernel show ‘reasonable’ but not perfect

agreement. Further simulations using the GEM code (electromagnetic gyrofluid model) and heterodyne reflectometer data are in progress.

2D Re�ectometry simulation as a tool for evaluating data processing

capabilities

F. da Silva, †S. Heuraux, P. Varela and M. Manso

Associação EURATOM/IST�Centro de Fusão Nuclear,Instituto Superior Técnico, 1046-001 Lisboa, Portugal

†Laboratoire de Physique des Milieux Ionisés et Applications, Unité du CNRS 7040,Université Henri Poincaré, Nancy 1, BP 239, 54506 Vand÷uvre Cedex, France

Abstract

In FMCW broadband re�ectometry data analysis tools are essential to extract the density pro�le,

automatically, in the presence of plasma turbulence. Here the capabilities of the burst-mode method

are assessed using a 2D FDTD full wave code. We brie�y review the numerical methods employed

in the 2D re�ectometry simulation and the basic principles of the burst-mode method. Homogeneous

plasma turbulence is assumed and probing frequencies in the range 30�40 GHz (Ka band) are used

throughout the work. The turbulence level is de�ned as δneT RB/ne(xc35 GHz) × 100 %, where δneT RB

is

the RMS value of the �uctuations and ne(xc35 GHz) the density at the cut-o� position for f = 35 GHz.

Simulations show a clear change in the plasma response when the amplitude of the density �uctuations

increases above δneT RB/ne(xc35 GHz) ≈ 3.5 %, which can be easily explained by the transition from a

quasi-monotonic pro�le into a more complex one, including sequences of hills and holes due to plasma

turbulence. This results in a measured density pro�le with a gradient steeper than the real one, explained

by the asymmetric response of the phase derivative ∂ϕ/∂f (abrupt increase of ∂ϕ/∂f from a hole and

slow decrease at a hill). It was found empirically from a large number of re�ectometry measurements

at ASDEX Upgrade that, for a sweep rate of 25 µs and a repetition rate of 10 µs, 8 sweeps are adequate

for the burst-mode analysis, in most plasma regimes. To support this �nding, numerical simulations

assuming parabolic density pro�les and a realistic spatial distribution of the plasma density �uctuations

were made. Simulations con�rm that number and also show that a higher number of sweeps (samples)

does not improve the accuracy of the measured pro�les and a smaller number of samples does not contain

enough information to reconstruct the unperturbed density pro�le. The limitations and advantages of

the burst-mode method are compared with single sweep and averaging analysis. Simulations were also

done to determine the conditions where density plateaus due to magnetic islands can be recovered

using burst-mode analysis. This is particularly important, to investigate the possibility of localizing

automatically rational surfaces in the presence of plasma turbulence. A �rst study was performed

assuming a unperturbed density pro�le to determine the data acquisition time requirements for the

typical plateau characteristics (size, location, rotation frequency). For a turbulent plasma a comparison

is presented between the single sweep and burst-mode analysis. It is shown that simulation data allow

to evaluate the experimental requirements of burst-mode for the detection of magnetic islands in the

vicinity of rational surfaces with strong density �uctuations. A short discussion is presented on the

usefulness of simulation studies to prepare and interpret new applications for re�ectometry.

Density Profile Characterization in the Stellarator TJ-II

T. Estrada, E. Blanco, J. Sánchez and the TJ-II Team

Laboratorio Nacional de Fusión. Asociación Euratom-CIEMAT, 28040 Madrid, Spain

L. Cupido, A. Silva, M.E. Manso

Associação Euratom-IST, CFN, Instituto Superior Técnico, 1096 Lisboa, Portugal

An Amplitude Modulation reflectometer has been in operation in the stellarator TJ-II during

the last three years [1]. The system works in the frequency range from 25 to 45 GHz with X-

mode polarization, covering densities from 0.03 to 1.0 1019 m-3 (from the plasma edge to ρ ≈

0.5-0.7 in ECH plasmas). The low value of the first reflecting density together with a more

accurate knowledge of the magnetic field and LCMS in stellarators than in tokamaks,

alleviate the initialisation problem.

The AM reflectometer works routinely and it is able to measure even in conditions of high

turbulence, i.e., during the magnetic well scans performed in TJ-II. The agreement found in

the overlapping region between the reflectometry profiles and those measured by the

Thomson Scattering system is in general very good.

TJ-II is a flexible heliac type stellarator that can explore a wide range of rotational transform

and magnetic well values by changing the currents that circulate through the coils. Besides, it

has a low and negative magnetic shear that can be modified by the induction of OH current

(up to ± 10 kA). Experiments have been performed changing these magnitudes to study the

confinement properties of TJ-II plasmas. In general, a correlation between the density profile

behaviour and the global confinement has been found. Reflectometry profiles obtained in

these experiments will be presented showing that:

• The density profile narrows and its gradient decreases as the magnetic well is reduced in

the plasma edge of TJ-II.

• Changes in the density profile are observed that depend on both strength and sign of the

magnetic shear.

In the near future we plan to install a new reflectometer devoted to measure fluctuations and

radial correlation properties in TJ-II plasmas.

[1] T. Estrada, J. Sánchez, B. van Milligen, L. Cupido, A. Silva, M.E. Manso and V. Zhuravlev.

Plasma Phys. Control. Fusion 43 (2001) 1535-1545

Reconstruction of density profiles from combined X and O mode reflectometry

L. Fattorini and M.E. Manso

Centro de Fusão Nuclear, Associação EURATOM / IST, Instituto Superior Técnico

Av. Rovisco Pais, P-1049-001 Lisboa (Portugal)

Abstract

Extraordinary (X) mode reflectometry is being increasingly used to measure the

density profile because it can, in principle, probe a wider plasma region than O mode

for each frequency band and provides the outermost part (SOL) of the density plasma

profile (ne<0.4x1019 m-3). However, as profile inversion depends both on the

detection of the first reflected signal (first fringe), and magnetic field profile it is

important to investigate the errors associated with that input data.

We studied numerically the accuracy of profile reconstruction assuming typical

ASDEX Upgrade scenarios and analyzed the sensitivity of the inverted density

profiles to the (1) first fringe, (2) magnetic field profile and density profile gradient.

Results show that the group delay of the probing microwaves, τg, is particularly

sensitive to changes in the density gradient and magnetic field, especially in the very

edge region (r/a>0.9). Magnetic field errors produce a radial translation as well as

distortions in the reconstructed profile. However, the parameter which has the largest

impact on profile accuracy is the frequency of the first fringe. As this is often difficult

to measure accurately we develop a new technique for profile reconstruction at the

very edge based on combined O and X-mode measurements (in the density region

where they overlap) that allows correcting the first fringe data. Improvements in the

reconstructed X-mode profile are presented.

Taking into account that the FM-CW broadband reflectometry system on ASDEX

Upgrade is equipped with O and X channels, we discuss also the possibility of using

the combined method to extract the information about the magnetic field profile in the

bulk plasma aiming to provide complementary data for magnetic equilibrium

reconstruction.

Correlation Reflectometry Measurements of Magnetic Field Strength and Turbulence in NSTX

M. Gilmore†, S. Kubota, W.A. Peebles, and X.V. Nguyen

Electrical Engineering Department, University of California, Los Angeles, CA 90095, USA A. Ejiri

Graduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0033, Japan

† Currently at the University of New Mexico, Albuquerque, NM 87131, USA Correlation reflectometry has been implemented on the National Spherical Torus Experiment (NSTX) in the 20–30 GHz band in both O-mode and dual mode (O-X) homodyne configurations. In both cases, the system consists of two channels: one operated at fixed frequency, while the second channel is slowly swept in 50 – 100 ms over the full or partial band. Polarization is changed from O-mode to O-X by interchanging a pair of waveguide directional couplers and a pair of ortho mode transducers (OMT’s). Magnetic field strengths, B, in the region 0.8 < r/a < 1.0 determined from O-X cross-correlation measurements interpreted by a 1D numerical model were found to be in good agreement with values from EFIT reconstructions under a variety of conditions (ohmic, L-mode, H-mode, RF-heated). In addition, measurements of turbulent correlation lengths in O-mode in the edge of the confinement region (0.9 < r/a < 0.98) in L-mode plasmas during toroidal magnetic field and plasma current scans will be presented. It was found that radial correlation lengths scale with ρ* in scans where nearly constant edge q was maintained, but more complicated dependencies on Btor and Ip were observed when edge q varied.

Doppler Reflectometry at W7-AS

M. Hirsch, E. Holzhauer*

Max-Planck-Institut für Plasmaphysik, EURATOM Association, Greifswald, Germany

*Institut für Plasmaforschung, Universität Stuttgart, Stuttgart, Germany

During the last experimental campaign of the stellarator W7-AS a multichannelDoppler reflectometer was set-up and operated routinely. A total of seven independenthomodyne channels probe densities between 0.4 and 5.8 1019 m-3 in x-mode polarization. Thereflectometers share a common monostatic Gaussian antenna with fixed tilt angle of

014++++====tiltθ with respect to the normal onto the reflecting layer. In order to separate theDoppler shifted st1−−−− diffraction order and the th0 order reflection the antenna characteristicis optimized for a narrow instrument function ⊥⊥⊥⊥K∆ in the ⊥⊥⊥⊥K -space of turbulence. For fastspectral analysis an analogue 15 channel spectrum analyzer with bandpass filters at centerfrequencies MHz8MHz3.0 ≤≤≤≤≤≤≤≤ cf has been developed. The optimum bandwidth of thechannels is the maximum compatible with the spectral width of the Doppler shifted signal

(((( )))) ⊥⊥⊥⊥⊥⊥⊥⊥ ⋅⋅⋅⋅⋅⋅⋅⋅==== Kf ∆∆ v21 π . Therefore the maximum achievable temporal resolution ( µs5≈≈≈≈t∆ )resulting from the necessary filter bandwidths is determined by the antenna pattern.

As an example we show turbulence behaviour and slowing down of plasma rotation ata back transition from H- to L-mode with spectra measured every µs4 . The temporalevolution of both Doppler frequency shift and intensity of the diffraction order display strongvariations in the 10 kHz range.

A second symmetric antenna with 014−−−−====tiltθ allows for a differential measurementof the Doppler shift which is necessary as the effective tiltθ may depend on plasma operationand magnetic configuration. The two antennas have crossed sightlines probing spots whichare poloidally separated by cm7 . This allows for an independent determination of ⊥⊥⊥⊥v by atime-of-flight (TOF) measurement using two reflectometers with identical frequency.Doppler- and TOF reflectometry are complementary as the TOF measurement is particularilysuited for low propagation velocities, where vice versa the accuracy of Doppler reflectometryis intrinsically low. As a tracer for the TOF measurement the total power of the Dopplershifted signal is selected with a broadband filter and the TOF is calculated by crosscorrelation.The advantage of this "Doppler TOF reflectometry" is that small scale turbulence( cm7.0≈≈≈≈Λ ) is used as a tracer while the large scale activity seen by conventionalreflectometry is suppressed. First investigations show good agreement of the propagationvelocity of turbulence obtained simultaneously by Doppler and TOF reflectometry.

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Measurement of a Density Profile Using Microwave

A. Itakura, T. Hirai, H. Hojo, J. Kohagura, Y. Shima, S. Tsunoda, K. Yatsuand M. Yoshikawa

Plasma Research Center, University of Tsukuba, 305-8577, Japan

An ultrashort-pulse reflectometry is utilized to observe an electron density profilein the central cell of the GAMMA 10 device. The pulse having 65 ps FWHM islaunched into the plasma in the ordinary wave mode. The frequency range of receivingsystem is 6 to 11 GHz. The electron density profile is reconstructed using the time offlight of the received signal. However, it is difficult to distinguish a density variationand a shift of plasma axis. A microwave interferometer with a movable horn system isalso installed in the central cell. Its frequency is 70 GHz. So, one can define a centre ofplasma axis within a few shots of different radial cords. Using this combined system ofthe ultrashort-pulse reflectometry and the interferometer system, complete densityprofile is acquired in a few shots.

Correlation Reflectometry for Magnetic Field Measurements in Fusion Plasmas

G J Kramer, R Nazikian, and E Valeo

Princeton Plasma Physics Laboratory, Princeton University,Princeton, New Jersey 08543, U.S.A.

Magnetic field measurements are essential for fusion devices. In the next generation of Tokamaks it will become

very difficult to use Motional Stark Effect (MSE) measurements for the determination of the magnetic safety factor

or q-profile because of the expected high plasma densities in which diagnostic neutral beams cannot penetrate to the

plasma center.

An alternative approach to magnetic field measurements in hot plasmas has been proposed based on the reflection

of microwaves of different polarizations from magnetized plasmas. In this method the omnipresent turbulence in the

plasma is used by measuring the peak correlation in the reflected signal between O-mode and X-mode polarized waves.

The absolute value of the magnetic field at the reflection point can then be deduced from the measured O-mode and

right hand side X-mode frequencies.

On a laboratory scale this method has been tested [1, 2]. In this presentation we will use these experiments to

benchmark our 2-D reflectometer code and study the posability to measure the magnetic field in a next generation

Tokamak.

[1] M. Gilmore, W.A. Peebles, and X.V. Nguyen, Plasma Phys. Control. Fusion 42 (2000) L1.[2] M. Gilmore, W.A. Peebles, and X.V. Nguyen, Rev. Sci. Instrum. 72 (2001) 293.

Abstract for the 6th International Reflectometry WorkshopMay 5-7, 2003, San Diego, CA

Magnetic Field Pitch Angle Measurements UsingCorrelation Reflectometry on NSTX

S. Kubota, W.A. Peebles, X.V. NguyenInstitute of Plasma & Fusion Research, University of California, Los Angeles, CA 90095, USA

A. EjiriGraduate School of Frontier Sciences, University of Tokyo, Tokyo 113-0033, Japan

M. GilmoreElectrical Engineering & Computer Science Department, University of New Mexico,

Albuquerque, NM 87131, USA

On NSTX correlation reflectometry is being investigated as a possible diagnostic formagnetic field pitch angle measurements in toroidal fusion devices. This diagnosticutilizes the concept that for naturally occurring turbulence in such devices, the correlationlength along the field lines is large compared to those in the poloidal or radial directions.Some measurements have been made using a pair of vertical horn arrays displacedtoroidally and poloidally. This system uses a pair of fixed-frequency homodynereflectometers with monostatic horns, operating at 50 GHz (O-mode cutoff of 3.1x1013

cm-3). Although the experimental data set is still extremely limited, preliminary analysishas been performed and compared with results from Thomson scattering and EFIT.These results will be presented.

Abstract for the 6th International Reflectometry WorkshopMay 5-7, 2003, San Diego, CA

Millimeter-Wave FM-CW Reflectometry on NSTX

S. Kubota, W.A. Peebles, X.V. NguyenInstitute of Plasma & Fusion Research, University of California, Los Angeles, CA 90095, USA

A.L. RoquemorePlasma Physics Lab, Princeton University, Princeton, NJ 08543, USA

UCLA operates a set of millimeter-wave/microwave reflectometers on NSTX for routinemeasurements of the electron density. The system has a combined frequency coverage of12 to 50 GHz (in the bands 12-18, 20-32 and 33-50 GHz) for an ordinary-mode cutoffrange of 1.2x1012 to 3.1x1013 cm-3 for coverage between the edge and core regions of theplasma. Profile measurements are made using FM-CW reflectometry with sweep timesbelow 50 µs over the full band. The profile reconstruction algorithm uses CDM(complex demodulation) for signal processing and Thomson scattering measurements tomodel the edge below the lowest cutoff density; signal amplitude is also used as ameasure of turbulence effects. Fast changes in the profile during L-H transitions, ELMsand IREs (internal reconnection events) have been documented. Fluctuationmeasurements are also possible by fixing or stepping the launch frequency. Phenomenasuch as turbulence suppression during L-H transitions and CAEs (compressional Alfveneigenmodes) during neutral beam injection have also been observed.

Abstract submitted for

The 6th International Reflectometry Workshop, San Diego

Application of Ultrashort-Pulse Reflectometry to Large Helical Device

A. Mase, Y. Kogi, L. G. Bruskin, M. Ignatenko

Advanced Science and Technology Center for Cooperative Research, Kyushu University

H. Hojo

Plasma Research Cener, University of Tsukuba

T. Tokuzawa, S. Inagaki, Y. Nagayama, K. Kawahata

National Institute for Fusion science

An ultrashort-pulse reflectometry (USRM) using an impulse generator as a source hasbeen applied to an inductively-coupled steady-state plasma. The reflectometer signal isdirectly recorded to a digitizing scope with 50 GHz bandwidth, and analyzed by a signalrecord analysis (SRA) method of profile reconstruction which relies on a raw signalwaveform rather than on the group delay of each frequency component. We have applied thismethod to Large Helical Device (LHD) of National Institute for Fusion Science. Themaximum Fourier component of the impulse generator (a pulse width of _p=22 ps with avoltage amplitude of 3 V) is limited below ~20 GHz. In order to increase the frequencyregime, a frequency-upconverter or a frequency-doubler can be utilized. Preliminal resultsobtained from LHD plasma will be presented. .

Density fluctuation measurement at internal transport barrier using O-mode reflectometer in JT-60U

N. Oyama, H. Takenaga, L.G. Bruskin, K. Shinohara and the JT-60 TeamJAERI Naka.

Reversed shear (RS) plasma with internal transport barriers (ITBs) is onecandidate of the attractive operation modes in a steady-state tokamak reactor due to its highconfinement and high bootstrap current fraction. In the RS plasma, ITB transport isstrongly linked with density and temperature profiles, current profile, rotation/electric fieldprofile, etc. These linkages introduce various responses to external perturbations in the RSplasma. In order to understand these linkages, the responses to external perturbationsinduced by pellet and ECRF injections are investigated in JT-60U.

In order to investigate the ITB response to the pellet injection, pellet is injectedfrom the high-field-side at the top into the high elongated RS plasma (Ip = 2.2 MA and BT =4 T) with a strong ITB. When the first pellet is injected during the Ip flattop in the relativelyconstant density and stored energy phase, high frequency component of the O-modereflectometer signal is drastically reduced by one order of magnitude, as well as the lowfrequency component, 6 ms after the pellet injection. The cut-off layer is located in the ITBregion in this case. The central density and the stored energy start to increase from thetiming of the reduction of the high frequency component. The reduction of the high and lowfrequency components of the O-mode reflectometer signal indicates the change of densityfluctuation level and/or wave number of the fluctuation. So far, the clear reduction of thedensity fluctuation was not observed in the ITB formation phase of JT-60U RS plasma,although the reduction of the correlation length was observed. In order to understandwhether the confinement improvement is caused by the reduction of the density fluctuationor not, the density fluctuation level is estimated from the analytical solution of time-dependent 2D full-wave equation for various poloidal and radial wave numbers (kθ and kr).In this analysis, the F-index defined as F = σ(P)/<P> is used for the fluctuation index andthe density fluctuation is given by assuming Gaussian k spectrum. The density fluctuationis estimated to be ~1.5% before the pellet injection and ~0.5% after the pellet injection withk θ = kr =3 cm-1. Here, the wave numbers are determined based on the linear stabilityanalysis using the FULL code in other RS plasma. Since the detailed density profile insidethe ITB is not measured in this discharge, the uncertainty of the density profile inducesrelatively large error range of 1.0-1.7% before and 0.4-0.7% after the pellet injection withsame k values. These analyses indicate that the density fluctuation is reduced by the pelletinjection even though the wave number changes in some range. The reduction of the highfrequency component of the O-mode reflectometer signal by the ECRF injection is alsodiscussed in the presentation.

Comparisons of turbulent radial correlation length measurements toturbulence simulations

T.L. Rhodes,1 J.-N. Leboeuf,2 D.W. Ross,3 J. Candy,4 G.R. McKee,5 R.V.Bravenec,3 E.J. Doyle,1 R.J. Groebner,4 W.A. Peebles,1 R.D. Sydora,6 L.Zeng,1 and G. Wang1

1Electrical Engineering Department, University of California, Los Angeles, California 900952Physics Department, University of California, Los Angeles, California, USA3Fusion Research Center, University of Texas, Austin, Texas 87812 USA4General Atomics, P.O. Box 85608, San Diego, California, 92186-5608 USA5University of Wisconsin, Madison, Wisconsin, 53706-1687 USA6Physics Dept., University of Alberta, Canada

Experimental turbulence and transport characteristics in DIII–D plasmas have beencompared with similar quantities calculated from gyro-kinetic and gyro-fluid turbulencesimulations. Turbulent radial correlation lengths ∆r from DIII–D L–mode plasmas scaleas 5 to 10 ρs (ρs = ion gyro-radius evaluated using the electron temperature) and areindependent of the poloidal magnetic field. Comparisons to a global gyrokinetic code(UCAN) show similar behavior (i.e. magnitude, radial scaling, lack of Bz dependence)when zonal flows are included. Experimental ∆r from quiescent double barrier (QDB)plasmas show both a reduction below this L–mode scaling, consistent with reduced coretransport, as well as similarities with UCAN simulations of ∆r. Gyro-fluid flux tubesimulations (GRYFFIN) of L–mode discharges have likewise been performed andcomparisons show similarities between measured and simulated poloidal wave numberspectrum while the simulated ion thermal transport and density fluctuation levels arelarger than experiment by factors of ~1.5 and ~4 respectively.

Poloidal propagation and wavenumber measurements of densityfluctuations using two point reflectometry

T.L. Rhodes,1 G. Kramer,2 W.A Peebles,1 and R. Nazikian2

1Electrical Engineering Department, University of California, Los Angeles, California 900952Princeton Plasma Physics Laboratory, Princeton, New Jersey

Simultaneous measurements from two poloidal positions can give information on thepoloidal properties of density fluctuations including phase velocity, mean wavenumber,wavenumber dispersion, and wavenumber width. A reflectometer technique firstdemonstrated by Verskov, et al. is employed on DIII-D to perform these two pointmeasurements. The data have been encouraging showing increased velocities with neutralbeam injection, reversal of direction across the plasma center, and good comparison toother diagnostics. Quantitative interpretation of the wavenumber and velocity datarequires accurate knowledge of the poloidal separation of the two detection positions.Geometry and raytracing give some estimates of this separation however more accurateestimates are required. To this end a collaboration with PPPL has begun that utilizes theirfull-wave hybrid code to model the reflectometer-plasma system. This diagnostictechnique has substantial promise for flow and turbulence measurements on both presentand future fusion research devices.

New 105-155 GHz reflectometer

for density fluctuation measurements on Tore-Supra

R. Sabot, J.-M. Chareau

Association EURATOM/CEA, CEA/DSM/DRFC,

CEA-Cadarache, 13108 ST PAUL-LEZ-DURANCE (France)

A new broadband X mode reflectometer has been installed on Tore-Supra last year to measure

density fluctuations in plasma centre. Operating in X mode polarisation, in the frequency

range 105-155 GHz, it covers the outer gradient zone and plasma core at nominal magnetic

field, 3.8 T. The high field gradient region is also reachable at medium density (ne(0) ≤ 5 1019

m-3). Different radial positions can be probed in sequence. The overall acquisition time for 10

radii is 200 milliseconds.

This reflectometer is based on the scheme previously developed for the 50-75 and 75-110GHz

density profile reflectometers [1]: one low frequency source, single side band modulation for

heterodyne detection, active multipliers. Main differences lie in the source and antennas. For

fluctuations measurements, source stability is more important than agility. Good localisation

is obtained with high gain gaussian antenna.

We will present results showing that the diagnostic is highly sensitive to density

perturbations. Very low phase noise source and no antenna coupling enable a direct

evaluation of the phase. Regular and large phase variations of few kilohertz are observed in

plasmas with strong MHD activity, but also similar, but smaller, oscillations in plasmas where

MHD activity monitored by Minorv coils is much smaller. The induced cutoff layer

displacement is centimetric, leading to density perturbations of few percent.

We will also present results on the broadband density fluctuations.

[1] F. Clairet, C Bottereau, J.-M. Chareau, R. Sabot, Advances of the density profile

reflectometry on Tore-Supra, to be published in Rev. Sci. Instrum.

RECENT RESULTS

FROM FM-CW REFLECTOMETRY ON ASDEX UPGRADE

A. Silva1, I. Nunes1, L. Cupido1, M. Manso1, F. Serra1, P. Varela1,

G. Conway2, and the ASDEX Upgrade Team2

1Associação EURATOM/IST, Centro de Fusão Nuclear, Instituto Superior Técnico, 1049-001 Lisboa, Portugal 2Max-Planck-Institüt für Plasmaphysik, EURATOM Association, D-85748 Garching, Germany

Abstract

Fully automatic profile evaluation and extended measuring capability (up to the W-band) of the

FM-CW reflectometry diagnostic on ASDEX Upgrade enabled recent progresses in the

understanding of several plasma scenarios, namely ELMy H-mode regimes.

The W-band was upgraded with new IF amplifiers and the frequency range was reduced in order

to optimize the PPL lock bandwidth. Successful measurements were made but some limitations

were found in the maximum density that can be measured. New developments are being made to

increase the input power of this channel and a novel approach is being investigated to further

improve signal to noise ratio. This development will be crucial in to meet the challenge of high

density measurements on ITER.

A comparative analysis of the density profile evolution for both type I and type III ELMs is

presented to illustrate the present capabilities of the diagnostic. For type I ELMs, a delay (70-100

µs) is observed between the onset of the density profile perturbation at the LFS and HFS. This

delay is consistent with radial losses occurring only at LFS and the density perturbation

propagating to the HFS by parallel transport; this phenomenon is indicative of a ballooning

character for type I ELMs. For type III ELMS, time delays are also observed (140-200µs), in

experiments where q95 was varied from 4.5 to 7; the difference on the measured time delays are

commensurate with the variation in connection length and corresponding ion parallel transport

time.

Recent Results of Pulsed Microwave and Ultra-short Pulse Radar Reflectometer

on the LHD.

T. Tokuzawa, K. Kawahata, T. Kaneba1, K. Tanaka, and A. Ejiri2

National Institute for Fusion Science, Toki 509-5292, Japan,

1Department of Fusion Science, School of Mathematical and Physical Science,

Graduate University for Advanced Studies, Hayama 240-0193, Japan

2Graduate School of Frontier Sciences, Univ. Tokyo, 113-0033, Japan

We have been developing the pulsed microwave radar reflectometer and ultra-short

pulse reflectometer for the density profile and fluctuation measurements on the Large

Helical Device (LHD). LHD has a complicated structure to make a helical magnetic

field and it causes that the polarization direction of the electromagnetic wave is not

definite for the direction of the magnetic axis. Pulsed radar type reflectometry is a

suitable reflectometric technique, in order to study the effect of the strong magnetic

shear for the polarization of the microwave. Because pulsed radar type reflectometry

measures the delay time of the reflected wave from the cutoff layer in the plasma, it can

distinguish between X-mode and O-mode polarized wave. We have constructed four

channel pulsed microwave radar reflectometer system for the present. The pulse width

and the repetition rate are usually about 2ns and 200KHz, respectively. Adjusting the

direction of the electric field of the microwave to the direction of the magnetic field at

the location of the plasma edge, it is obtained that the desired polarization of the

reflected wave is selectable. By using O-mode operation, the temporal movement of

the cutoff layer and the reconstructed density profile are almost in agreement with the

FIR interferometer signal. By using X-mode operation, the density fluctuation which

the rotating island is conjectured is found to localize in the very low-density ergodic

layer. Also we have developed new reflectometer system using an ultra-short pulse

which the temporal pulse width is 23ps. On the initial experiment, it can be measured

that the cutoff layer is moved by the fuel pellet injection into the plasma.

BURST-MODE ANALYSIS OF BROADBAND REFLECTOMETRY DATA

ON ASDEX UPGRADE P. Varela1, J. Santos1, M. Manso1, G. Conway2, and the ASDEX Upgrade Team2

1Associação EURATOM/IST, Centro de Fusão Nuclear, Instituto Superior Técnico

1049-001 Lisboa, Portugal 2Max-Planck-Institüt für Plasmaphysik, EURATOM Association

D-85748 Garching, Germany

Abstract

A multiple-sweep data processing method, the so-called burst-mode analysis, was developed in

order to improve the accuracy of density profiles in the presence of high levels of plasma

turbulence. Due to its capability to estimate the average density profile while strongly reducing

the effect of spurious group delay components, the burst-mode analysis has proved to be a

valuable tool in obtaining automatic density profiles from broadband reflectometry. In this work

we illustrate the burst-mode analysis in two important applications: the study of the edge

pedestal parameters (position, density and width) in H-mode discharges with type I ELMs, using

for the first time the W band channel, and the measurement of plasma position in several plasma

scenarios. In the first case, the collapse and recovery of the edge pedestal structure is resolved

and the evolution of the pedestal density is given, in good agreement with previous results

obtained in similar ASDEX Upgrade discharges. In the second application, the position of

selected layers with fixed density is tracked along the discharge to estimate the position of the

plasma column. Results are found to be in good agreement with data from magnetic diagnostics.

Status and prospects for Reflectometry in ITER (abstract)

G. Vayakis, A. Malaquias*+ and C. Walker+

ITER ITA, Naka Joint Work Site, Naka, Ibaraki 311-0193, Japan* IST, Portugal

+ ITER ITA, Garching JWS

Reflectometry will be used in ITER to measure the density profile in the main plasma and divertorregions, and to measure the plasma position and shape in order to provide a reference for the magneticdiagnostics in long pulses. An outline description of a system to achieve these goals was compiled in1997[1] and updated since [2-7]. It included low and high field side O-mode systems for themeasurement of the density profile, a low field side X-mode system for the edge profile, a high field sideX mode system operating in the left hand cutoff to measure the core profile, a dedicated O-mode systemfor plasma shape measurement and a multi-band, multiple line of sight O-mode system to measuredivertor density profiles. Since then, the design of the ITER machine has changed and reflectometrymeasurements have improved. These factors have influenced the design in various ways. For example:-

• Whilst measurement of plasma position by reflectometry is now supported by experimental results,to keep implementation manageable the scope of the system has been reduced to the key gaps due tospace restrictions. In fact, all in-vessel systems now have to cope with reduced space for waveguideaccess, now rather well-defined following significant progress the allocation of space for all in-vesselsystems. At the same time, access to the vessel has been improved, with significant simplification ofthe waveguide path through the upper ports.

• In the divertor too, the situation is mixed. On the one hand, it seems that confidence in ECA as a toolto measure divertor pressure has eroded further from the already low levels it enjoyed 5 years ago,and, in general, there seems to be little new to report in terms of experimental evidence forreflectometry in this region. On the other hand, access has been provided for a total of 5 sightlinesspread between 4 toroidal locations, to cover the low (15-60) medium (60-~300) and high (~300 - ~1000 GHz) frequency range. The last range was intended for ECA and comb reflectometrymeasurements; the space allocated to these systems seems increasingly likely to be used by a moreconventional two-colour interferometer (in the short mm-wave or even IR region) along the same 5lines of sight.

• All low dield side systems now benefit from the reduced movements of the vessel during a pulse andfrom improved access to the second vacuum boundary and associated waveguides.

• Measurements by Doppler reflectometry have shown significant promise for provision of a routinefluctuation / rotation diagnostic and in order to accommodate this several of the low field side lineshave been re-allocated to this task.

This talk will present a summary of the present systems and their intended use and space allocation, togetherwith the main design and R&D elements that need to be decided before the functional specification of thesystems can be completed.

[1] G. Vayakis et al, RSI 68 (1) 1997[2] G. Vayakis et al, in Diagnostics for Experimental Thermonuclear Fusion Reactors (PlenumPress, New York, P. Stott et al, eds.) p.97 (1998) [3] V. Vershkov et al, ibid. p. 107, [4] E.J. Doyle et al, ibid. p. 119, [5] N.L.Bretz et al, ibid. p. 129 [6] M. Manso et al, ibid. p. 139[7] G. Vayakis et al, in 23th International Conference on Infrared and Millimeter Waves Conference Digest, Parker and Smith(eds.), University of Essex, Colchester, U.K. (1998) pp. 287-289

N 55 RI 28 03-05-02 W 0.1

Direct comparison of turbulence measurements with Langmuir probes and

reflectometry at the same radial locations in T-10 and reflectometry

simulations with 2D full wave code.

V.A. Vershkov, A.O. Urazbaev, D.A. Shelukhin, S.A. Grashin, V.F. Denisov, V.F.

Chistiakov, E.P. Gorbunov, Yu.V. Skosirev, S.V. Soldatov, V.A. Zhuravlev, T.B. Mialton

Nuclear Fusion Institute, RRC �Kurchatov Institute�, Moscow, Russia.

The work deals with the investigation of periphery turbulence characteristics in

tokamak by multipin Langmuir probes and correlation reflectometry as well as modeling

reflectometry data by 2D full wave electromagnetic code. Density fluctuations were measured

by correlation reflectometry and Langmuir probes at the same plasma radii. It allowed us to

compare qualitatively the results of both diagnostics. Data were in good agreement.

Reflectometry data modeling was carried out with 2D full wave electromagnetic code

to test the reflectometry locality. The real fluctuations parameters were obtained by

comparison of experimental data from several Langmuir probes with 1d stochastic turbulence

model. It allowed us to obtain radial distributions of the main turbulence characteristics.

Computer program was developed to simulate 2D matrix of density fluctuations with

experimentally obtained parameters of turbulence. This 2D matrix of density fluctuations was

used for modeling of reflection of electromagnetic waves from turbulent plasma with full

wave 2D computer code. A good agreement between computing and experimental data was

obtained. The analysis of the results of simulations showed a good radial and poloidal

resolution of reflectometry.

The results of 2D model are compared with 1D calculations.

High Resolution Density Profile Measurement Using a SimultaneousDual-polarization Reflectometer System on DIII-D*

G. Wang, E.J. Doyle, L. Zeng, T.L. Rhodes, and W.A. Peebles

Department of Electrical Engineering and PSTI, University of California, Los Angeles, California 90095

A Q-band (32-50 GHz), simultaneous dual-polarization (O- and X-mode) frequency-modulated

profile reflectometer system has been installed and is now operational on DIII-D. The improved

system measures densities in the range of 0-3.1x1019 m-3, with time resolution as high as 25 µs. It is

a modification of the previous high performance solid-state Q-band profile reflectometer system so

as to improve density coverage from the previous range of 0-0.5x1019 m-3 (X-mode at full magnetic

field BT=2.1 T). The density range is expanded by making simultaneous O- and X-mode

measurements while using a single source. The launch antenna is now oriented at 45° relative to the

edge magnetic field direction, so as to simultaneously launch both O- and X-mode waves. One

receive antenna is used to detect the X-mode reflection, keeping the same measurement capability

as before. A separate additional receive antenna detects the O-mode reflection, covering a density

range of 1.26-3.1x1019 m-3. With this system there is an unmeasured density range which is

dependent on the magnetic field, e.g. 0.5-1.26x1019 m-3 at full magnetic field BT=2.1 T, or 1-

1.26x1019 m-3 at BT=1.65 T. A model is used to calculate the density profile in this range and shows

good agreement with other diagnostics. Measurements show that the contamination of one

polarization signal by the other polarization can be negligible if the receive antennas match the

edge magnetic field pitch angle. Successful implementation of the dual-polarization reflectometer

system on DIII-D demonstrates the feasibility of such applications to current devices and future

machines such as ITER, where improved measurement capability with a reduced number of

microwave sources, antennas, waveguide runs, vacuum windows and cost is highly desirable.

* Work supported by US DOE Grant No. DE-FG03-01ER54615 and Contract No. DE-AC03-89ER511144

Detailed Improvements to the DIII-D FM-CW Profile ReflectometerSystem *

G. Wang, L. Zeng, E.J. Doyle, T.L. Rhodes, and W.A. Peebles

Department of Electrical Engineering and PSTI, University of California, Los Angeles, California 90095

Details are presented of recent improvements to hardware and analysis codes components

of the DIII-D profile reflectometer system which makes it possible to more precisely

determine the zero density plasma start position, an essential issue for utilizing

reflectometry as an independent density profile diagnostics, and to calculate profiles

automatically between shots. The DIII-D system consists of Q- and V-band frequency-

modulated reflectometer systems, covering 33-50 and 50-70 GHz, respectively.

Depending on choice of operating polarization the system can be used for either edge or

core electron density profile measurements. Experimental results over several years have

demonstrated successful independent measurements of edge density profiles with high

spatial and temporal resolution, providing an additional tool for studying edge transport

phenomena like L-H transition and ELMs, while detailed core density profile data

supplement routine Thomson Scattering measurements.

*Work supported by US DOE Grant No. DE-FG03-01ER54615 and Contract No. DE-AC03-99ER54463

Investigation of Plasma Instabilities by Fast ProfileReflectometer Measurements in DIII-D*

L. Zeng, G. Wang, E.J. Doyle, T.L. Rhodes, W.A. Peebles, UCLA, G.R. McKee, R. Fonck,Univ. of Wisconsin-Madison, K.H. Burrell, General Atomics, R. Moyer, UCSD

Profile modifications associated with plasma instabilities have been studied using a variety

of diagnostics and in particular a fast profile reflectometer system. During ELMing H-mode

plasmas, the edge density profile evolution is robustly tracked by fast profile reflectometer

during ELMs, showing that the density profile expands to the vessel wall at the onset of

ELMs within several hundreds µs. The profile expanding speed is consistent with Vr (=

EθθθθxB/B2) as measured by Langmuir probes. The profile modifications during different type

of ELMs and in different density plasma are presented. The results demonstrate enhanced

particle and energy radial transports to the vessel wall at onset of ELM. In contrast, for

Quiescent Double Barrier (QDB) plasmas, high spatial and temporal resolution

measurements show the SOL density profile is modulated at the fundamental frequency of

the Edge Harmonic Oscillation (EHO). The density fluctuation profile is peaked regions of

large density gradient. A comparison between reflectometer and other diagnostic

measurements (BES and Langmuir probe) regarding the EHO density fluctuation profile is

presented.

*Work supported by US DOE Grant No. DE-FG03-01ER54615 and Contract No. DE-AC03-89ER51114