russian research center “kurchatov institute” nuclear fusion institute development of the hfs...

RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE”

NUCLEAR FUSION INSTITUTE

Development of the HFS ITER reflectometryDevelopment of the HFS ITER reflectometry (REFLECTOMETRY FOR THE MAIN PLASMA (HFS))

Task- F.09, Packet № 6Includs: Upper port plug, in-vessel components, equipment after

bioshield, acquisition and processing systemLocation: Upper port № 8

Presented by V.A. Vershkov

NFI RRC “Kurchatov Institute”,Moscow, Russian Federation

9-th IRW Meeting, Lisbon, Portugal, 4 – 6 May 2009

RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE”

NUCLEAR FUSION INSTITUTE

OUTLINEOUTLINE

1.1.Principles of HFS reflectometryPrinciples of HFS reflectometry

2. Advantages and problems2. Advantages and problems

3. Analysis of the physical issues3. Analysis of the physical issues

4. Components of the HFS reflectometry and their 4. Components of the HFS reflectometry and their characteristicscharacteristics

5.Conclusions5.Conclusions

4 5 6 7 80

50

100

150

200

250

300

54

3

2

1

Critic

al f

requenci

es

[GH

z]

Major radius [m]

1 – Low frequency extraodinary mode

2 – ordinary mode

3 – Electron cyclotron frequency

4 – Upper frequency extraordinary mode

5 – Second harmonic of the cyclotron frequency

Principles of the HFS reflectometry. Advantages and problemsPrinciples of the HFS reflectometry. Advantages and problems

Advantages: 1.Using low frequency extraodinary mode it is possible to observe the plasma core even with flat density profile2.Very week relativistic corrections to the permittivity.3.Low frequency range (10-80 GHz) with widely available high power generators.Problems:1.Technical: integration of highly oversized waveguide (20×12 mm) in prescribed geometry2.Physical: Estimated high level of the phase fluctuations, exceeded in a order of magnitude the typical reflectometry limit of 1.5 radians.

RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE”3 of 33

Physical problemsPhysical problemsThere are several physical problems, arising due to the expected flat density profile with

high pedestal value and expected high phase fluctuation level.I. The first problem arises even in the calm plasma and it is connected with the parasitic

reflection in O-mode (due to the field line inclination of about 130) and reflection from the jump of the permittivity at the pedestal. Both parasitic reflection occur in the pedestal zone and make difficult to extract the real reflection in that area.

II.The second problem arise from the estimated high level of the phase fluctuations of the reflected signal. This high level of the phase fluctuations influenced all functions of the HFS reflectometry, namely:

1. The accuracy of the density profile measurements2. Possibility of characterizing the amplitude and Fourier spectrum of the local

density fluctuations from the measured one3. Abilities of the reflectometry to measure MHD and Alfvenic modes.

The last two items connected to the fact, that high level of the phase fluctuations result in a 2π jumps and spreading of the spectrum of the reflected wave, giving in the limit to flat spectrum of -function, which smashes pecularities of the density spectrum.

As the most serious problems arise due to the assumptions about the level and wavelength of the density fluctuation at the HFS, it is of primary importance to predict reasonable values and structures of turbulence and properties of the TAE at the HFS!!!


Enhancement of the linear reflectometry limit 1.5 rad Enhancement of the linear reflectometry limit 1.5 rad


All reflectometer systems for core plasma measurements will operate with strong perturbation of reflected phase due to the plasma turbulence.

XL-mode has an advantage in both non-relativistic and relativistic case, but as the enhancement factor over limit is expected 5-8, even XL-mode should have problems.

4 5 6 7 80

10

20

30

0

10

20

relativistic

Radius [m]

non-relativistic

O-mode XL-mode XU-mode

Turbulence: r=2 cm,

n/n=1%

Ratio

of est

imate

d p

hase

fluct

uatio

ns

to the

refle

ctom

etr

y lim

it le

vel (

pi/2

radia

n)

Parameters for XL mode simulation with turbulence Parameters for XL mode simulation with turbulence

Simulation input values:

σn/n was taken according to the mixing criteria from the pressure profile

Density profile is flat according to the scenario 2

Results:Edge fluctuations may prevent the XL mode penetration to the reflection layer

Strong scattering occurs400 450 500 550 600

0,0

0,5

1,0

0,0

0,5

1,00

5

10

0

5

10

15

Fprobe

= 36 GHz

Fprobe

= 40 GHz

n

e [1

019 m

-3]

z·

i ~ 0.3

z ~ 3 - 5 cm

r =

z/3 ~ 1 - 2.5 cm n

/n [%

]


Geometry and turbulence simulations in 2D full wave calculationsGeometry and turbulence simulations in 2D full wave calculations

Z

2D electric field

Permittivity (XL-mode)

Blue corresponds to ε=1, white – to ε<0. σn/n=0.47%


Pulse reflection from unperturbed plasma – parasitic effectsPulse reflection from unperturbed plasma – parasitic effects

0 100.00

0.05

0.10 X-mode

O-mode

End of horn

Bis

tatic

ampl

itude

[a.u

.]

Time [ns]

0.00

0.02

0.04

0.06

0.08

0.10X-mode

O-mode

End of horn

Mon

osta

ticam

plitu

de [a

.u.]

0.0

0.5

1.0

O-mode X-mode Entire reflected signal

Inpu

t am

plitu

de [a

.u.]

F=34 GHz


Beside the main reflection from cut-off layer, several parasitic reflections could be observed – O-mode reflection due to magnetic field line inclination, reflection from steep gradient (spreading of the pulse due to high dispersion) and secondary reflections.

0 10 200

2

4

6

8

10

12X-mode

O-mode

End of horn

Bis

tatic

am

plit

ud

e [a

.u.]

Time [ns]

0

2

4

6X-mode

O-mode

End of horn

Mo

no

sta

tica

mp

litu

de

[a.u

.]

0.0

0.5

1.0 F=36 GHz

O-mode X-mode Entire reflected signal

Inp

ut

am

plit

ud

e [a

.u.]

Pulse propagation in turbulent plasmas. 2D full wave simulation.Pulse propagation in turbulent plasmas. 2D full wave simulation.

σ(ne)/ne=0.47%, k×ρi=0.3. Density turbulence level was taken 1/4 of the mixing length

criteria of the pressure profile. The characteristics of the LFS were taken, while HFS turbulence has the different nature!!! (Question to theory!!)

Several turbulence realization were simulated and avereged.The results showed that the averaged pulse in bistatic and monostatic approached to the limit delay, which about 2 ns less then 1D estimation without turbulence. This time delay should be taken into account in data processing

0 10 20 30 40 500.0

0.5

1.0

1.5

2.00.0

0.2

0.4

0.6

0.8

1.0

Initial pulse 1D Exact value Unperturbed plasma Turbulent plasma

(mean across 23 profiles)

Bis

tatic

Am

plit

ud

e [a

.u.]

Time [nsec]

Mo

no

sta

ticA

mp

litu

de

[a

.u.]

0 10 20 30 40 500.0

0.5

1.0

1.5

2.00.0

0.5

1.0

1.5

2.0

1D Exact value Single reflected signal Mean signal mean across 23 profiles

Bis

tatic

Am

plitu

de [a

.u.]

Time [nsec]

Mon

osta

ticA

mpl

itude

[a.u

.]

0

10

20

30

40

50

60

Tim

e [n

s]

Monostatic

Sin

gle

puls

e

Bistatic

2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

Number of time slice

Tim

e [n

s]

2 4 6 8 10 12 14 16 18 20 22

Mea

n pu

lse

Number of time slice


Delay shift due to the turbulenceDelay shift due to the turbulence

0.0 0.1 0.2 0.3

-1.5

-1.0

-0.5

0.0

0.5

15.5

16.0

16.5

17.0

17.5

Del

ay d

iffer

ence

[ns

]

n [%]

unperturbed plasma positive perturbation mean value negative perturbation

Del

ay [

ns]

4.70 4.72 4.74 4.76 4.78 4.80

12

14

16

-0.02

-0.01

0.00

0.01

0.02

Del

ay [

ns]

n [%]

unperturbed plasma positive perturbation negative perturbation

Pla

sma

perm

ittiv

ity

1D geometric optics approach could reveal the nature of delay shift towards the launched/receiving antenna. The delay proportional to ε-1/2, so positive and negative fluctuations near the reflection point give non-symmetric response of plasma permittivity profile near the reflection point.

Simulation was made for Gaussian perturbation with 2 cm width, located at cut-off radius.

Full wave estimations at high turbulence level are required for qualitative ……..comparison with simulation results.


Broadening of the measured reflectometry spectrum Broadening of the measured reflectometry spectrum due to the high level of the phase fluctuationsdue to the high level of the phase fluctuations

-0,4 -0,2 0,0 0,2 0,40,0

0,5

1,0

Density fluctuations Reflectometry (2D)

Fourier

am

plit

ude [a.u

.]

Frequency [MHz]

(ne)/n

e=0.47%


It could be difficult to measure the turbulence spectra in ITER plasma core due to strong phase perturbations in reflected signal even at rather low level of density perturbation. These perturbations appear due to both strong variations of dielectric permittivity near the cut-off layer at flat density profile and small-angle scattering.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0 Monostatic Bistatic F1

Am

plitu

de o

f per

turb

atio

n [a

.u.]

(/(n/n

Reflectometer sensitivity to TAE modes with m ~ 17. (/(n/n scan.

2D Simulation of reflectometry sensitivity to AEM modes in unperturbed plasmas. Scen 22D Simulation of reflectometry sensitivity to AEM modes in unperturbed plasmas. Scen 2

-10 0 10 20 300.0

0.2

0.4

0.6

0.8

1.0

To plasma center

Bistatic channel

Sen

citiv

ity,

Erm

s

k/E

rms

0

Radial position of TAE mode w/r cut-off layer [cm]

Monostatic channelReflection layer position

-10 0 10 20 30

Reflection layer position

To plasma center

Radial position of TAE mode w/r cut-off layer [cm] 0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0Bistatic channel

Senci

tivity

, Erm

s

k/E

rms

0

Poloidal m number

Monostatic channel

0 20 40 60 80 100 120 140 160 180

Poloidal m number


Simulation results shows that in unperturbed plasma reflectometry is sensitive to AEM even at significant distances from mode position to cut-off layer.

Reflectometry provides measurements for fluctuations with poloidal m number up to 150

Reflectometry response at XL-mode is sensitive to ratio of density and magnetic filed perturbations.

Conclusions from the 2D full-wave simulations for Conclusions from the 2D full-wave simulations for the capabilities of the HFS reflectometrythe capabilities of the HFS reflectometry

1. HFS reflectometry is capable of the density profile measurements, even at the highest levels of turbulence with the corrections for the decrease of time delay due to turbulence.

2. Capabilities of the HFS reflectometry for measuring the turbulence characteristics strongly depend on the assumptions about the turbulence properties at the HFS. Thus it is needed theoretical models for estimation HFS turbulence. It should be noted the importance of the decrese of the phase fluctuations with density peaking, which may be the case!!

3. Observation of the MHD and Alfven modes also needed theoretical prediction. Especially:

- for the amplitude of such modes (at the HFS, as strong modes asymmetry may exist!)

- As the lower extraodinary mode is sensitive to the magnetic field perturbation, the ratio and relative phase of TAE should be known


Block-schema of the HFS reflectometryBlock-schema of the HFS reflectometry

RF module - frequency bend I

Antenna 90o

Bend

In-v

esse

l sta

inle

ss s

teel

wav

egui

des

20 x

12

mm

Port entrance

(40o bend)

Port exit(40o bend)

Primary vacuum window

Bioshield with thermo-compensated loop

Secondary vacuum window

Waveguide-quasioptics transitions

Combine/divide system

Full frequency bend 10 - 80 GHz, E01 wave 15-130 GHz, E10 wave

Vacuum part of system

Workstation for RF modules management, signal formation, acquisition and preliminary

data processing

Reflected

Launched

PLASMA

RF module - frequency bend II

RF module - frequency bend III

Source

Heterodine

RF generator

Signal formation scheme

Response detection scheme

Waveguide scheme


The components of HFS reflectometryThe components of HFS reflectometry

1. Vacuum chamber elements:- antenna systems- waveguide bends 90 and 40 degrees- waveguide tract at vacuum chamber wall that consists of stainless steel waveguide parts connected with flanges- primary vacuum windows

2. Atmosphere elements before the bioshield- secondary vacuum windows- waveguide tract with N-shape curving to compensate the thermal shifts

3. Frequency combine/divide system in ceiling region between bioshield and gallery.

4. Launching and receiving RF equipment for 7 frequency bends 5. System for diagnostic control, data acquisition, primary data

processing and connection with CODAC system


The schematics of one waveguide routing in the upper port #8 from The schematics of one waveguide routing in the upper port #8 from antenna to the area after bioshieldantenna to the area after bioshield

Critical components1.Antenna2.900 bend after antenna3.Two “400 bends” at the port entrance and output4.Primary and secondary vacuum windows

Bioshield Ceiling

Diagnostic equipmentPort

Vacuum chamber

Primary vacuum window

Secondary vacuum window


Antenna schematics and prototypeAntenna schematics and prototype

Design of ITER HFS reflectometry antenna system required developing unique combine horn-mirror system due to strong restrictions of system size and small level of receiving signal. Antenna system prototype was made and successfully tested at HFS reflectometry mock-up in RRC “Kurchatov Institute” (RF).


Antenna heating simulation with bleckness=1 Antenna heating simulation with bleckness=1

Thermal simulations were made to estimate the heating of a critical points in antenna system due to neutron flux. Final thermal calculation will be made after finishing antenna design as well as mechanical stress estimations.

stainless steel vanadium molybdenum Tmax=618.5°C Tmax=569°C Tmax=495.5°C


Antenna response in mono and bistatic modeAntenna response in mono and bistatic mode


0

100

200

300

0 5 100

50

100

0 5 10

mirror

horn edge

Lmirror

=500 mmLmirror

=150 mmM

on

ost

atic

am

plit

ud

e [a

.u.]

horn edge

mirror

2nd reflection

mirror

Bis

tatic

am

plit

ud

e[a

.u.]

Time [ns]

mirror

Time [ns]

Temporal Laboratory Test Facility of HFS Temporal Laboratory Test Facility of HFS reflectometryreflectometry


Antenna response in mono and bistatic modeAntenna response in mono and bistatic mode


0

100

200

300

0 5 100

50

100

0 5 10

mirror

horn edge

Lmirror

=500 mmLmirror

=150 mmM

on

ost

atic

am

plit

ud

e [a

.u.]

horn edge

mirror

2nd reflection

mirror

Bis

tatic

am

plit

ud

e[a

.u.]

Time [ns]

mirror

Time [ns]

Antenna response versus the reflection mirror distance

0 100 200 300 400 5000

2

4

0.0

0.1

0.2

0.3

monostatic bistatic free space wave propagation

Distance to mirror [mm]

Del

ay [n

s]

monostatic bistatic

F=26.5 GHz

Mai

n re

flect

ion

ampl

itude


0 500 1000 1500

0

5

10

15

0

20

40

60

80

monostatic bistatic theoretical estimation

De

lay

[ns]

Distance [mm]

F=33.93 GHz monostatic bistatic (*30)

Am

plit

ud

e [m

V]

noise level in monostatic channel

3D simulation Mock-up prototype measurements

Both 3D simulation and prototype mock-up test demonstrate the same properties:

• Strong decrease of monostatic signal at distances above 0.5 m

• Rise of bistatic signal with distance increase in antenna close region and slow decrease at large distances up to 1.8 m

• Pulse propagation times were found to be close to geometric optics predictions

Optimization of 90° bendOptimization of 90° bend

20 40 60 80-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

40 60 80 100 120

X-mode

Non-optimized 400 mm bend Optimized 360 mm bend Optimized 320 mm bend

Atte

nu

atin

[dB

]

Frequency [GHz]

O-mode

Non-optimized 400 mm bend Optimized 360 mm bend Optimized 320 mm bend

Frequency [GHz]


Initial non-optimized hyperbolic cosine bend was made and demonstrated good performance.

Special optimization of 90° bend was developed to decrease the size of the bend and keep the performance.

Experimental transmission of optimized 90° bend in Xl modeExperimental transmission of optimized 90° bend in Xl mode

Exceptable transmission in XL mode up to 110 GHz, except some spikes at 27 and 52 GHz


20 40 60 80 100

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

Atten

uatio

n [d

B]

Frequency [GHz]

Common problem of EU Plasma shape system and Common problem of EU Plasma shape system and HFS reflectometryHFS reflectometry

Inner size 20×12 mm, wall thickness 1 mm. The same as in EU projectCooperation is urgent. It is preferable to start prototype in 2009 because

technology of welding and bending should be developed.

CuNi

stainless steel


Pulse spreading in the waveguide Pulse spreading in the waveguide

10 12 14 16 18 20 22 24 260

2

4

6

8

10

12Initial pulse width 0.5 ns 1.0 ns 2.0 ns

Simulated semi-width at e-1

level for pulse TE10 after transmission through 25 m rectangular waveguide. (20 mm height)

Pu

lse

wid

th [n

s]

Frequency [GHz]

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260

5

10

15

20Initial pulse width 0.5 ns 1.0 ns 2.0 ns

Simulated wideninig of pulse TE10 after transmission through 25 m rectangular waveguide. (20 mm height)

Ra

tio o

f pu

lse

wid

th a

fter

to b

efo

re th

e w

ave

gu

ide

Frequency [GHz]


Slowing of electromagnetic wave in waveguide will be the key issue for working at low frequencies. This effect will be important for both frequency scan and radar technique measurements.

Numerical calculations show that for TE10 wave (XL-mode in plasma) 1 ns pulse broadening due to waveguide dispersion is important for frequencies below 13 GHz.

The choice of pulse width for measurements at low frequencies should be the compromise between pulse broadening and initial pulse width.

Optimization of “40° bend” and the way to primary vacuum Optimization of “40° bend” and the way to primary vacuum windowwindow


Optimization of “40° bend” (entrance of waveguide into port) should be made. This work is required blanket module cutting.

Waveguide exit from port is required additional simulation to optimize the RF properties of the bends.

Primary window Primary window

Primary vacuum window is made by welding 2 mm quartz plane inside the waveguide. Quartz wedges at both size of plane are using for smooth changes of dielectric permittivity in window. Several prototypes of window make up and test now in N. Novgorod. Choice of design should be made at late 2009-early 2010.


Primary window characteristicsPrimary window characteristics

Calculated window attenuation


10 20 30 40 50 60 70 80

-1,5

-1,0

-0,5

0,0

Attenuatio

n [dB

]

Frequency [GHz]

XL-mode

20 40 60 80 100

-1,5

-1,0

-0,5

0,0

Attenuatio

n [dB

]

Frequency [GHz]

XL-mode

Measured attenuation of The 4-th prototype window

Shown the 4-th window example characteristicsProblems , which were steadily worked out:

1.Influence of the resonance properties of the measurments waveguides2.Inaccuracy of the wages fabrication

Nevertheless still not match the simulations!!!

Secondary windowSecondary window

stainless steel waveguide

ROHACELL®


20 40 60 80 100 120

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Atte

nu

atio

n [d

B]

Frequency [GHz]

Rochasell d=20mm

Secondary vacuum window is to be made of 2 cm ROHACELL® foam d=90mg/cm3 with ε < 1.1 and low RF absorption. ROHACELL® will be glued inside waveguide with RF dielectric epoxide or conducting compound. Test of secondary vacuum window is to be made till the end of 2009.

Quasi-optical system of frequency bands separation and summation in the region between bioshield and gallery


Exact principles of the frequency separation system will be chosen later

ConclusionsConclusions

1. The presented preliminary design showed that all components of the HFS reflectomentry are capable to work effectively in the frequency band of Xl mode (10 - 80 GHz) and in O-mode (15 -120 GHz).

2. The last two techical problems should be solved:-The final primary vacuum window design- the production of the SS waveguide 20 x 12 mm, covered inside with 10 of Cupper (which is the common RF and EU problem)

3. The quality of HFS reflectometry measurements :1. The accuracy of the density profile measurements2. Possibility of characterizing the amplitude and Fourier spectrum of

the local density fluctuations from the measured one3. Abilities of the reflectometry to measure MHD and Alfvenic modes.

Depends fully on the assumptions about the peakedness of the density profile and the level and wavelength of the density fluctuation at the HFS. Thus it is of primary importance to predict reasonable density profile peakedness, values and structures of turbulence and properties of the TAE at the HFS!!! RUSSIAN RESEARCH CENTER “KURCHATOV INSTITUTE”

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