HIGH BANDWIDTH FIReTIP OPERATION ON NSTXW-C. Tsai, C.W. Domier, K.C. Lee, N.C. Luhmann, Jr., University of California at Davis, USAH.K. Park, Pohang University of Science and Technology, Korea

Supported by UC DAVISPLASMA DIAGNOSTICS GROUP

Lasers

Optical system

Retro-Reflectors

Probe BeamsColor: Ch#

NSTX PlasmaThe FIReTIP System

Far InfraRed Tangential Interferometer/ Polarimeter system installed on NSTX

FIR wavelength of 119 m (2.5 THz)

Seven chords as a fan beam Michelson systemfor 2-D profile determination

Retro-reflectors (5 external, 2 internal) used to make double-pass measurements

Schottky diode mixers for signal down-conversion

NSTX FIReTIP System

NSTX Plasma/4

Plate

Reference Mixer

Signal Mixer

Polarizer

A B A* B*

f1= f0 – 1.0 MHz

f2= f0 + 1.0 MHz

fLO= f0 – 5.0 MHz

f0

f0

f0

Polarimetry: phase difference of (A* - B*)Interferometry: phase difference of (B - B*)

f1 – f2 = 2.0 MHzfLO – f1 = 4.0 MHzfLO – f2 = 6.0 MHz

Beat freq. from these 3 lasers:

FIR Laser

FIR Laser

Stark Laser

f1

f2

fLO

fR fL

Electronics

Electron Density Fluctuations During ELMs

(e)Spectrum of FIReTIP ∂ne (kHz)

Example of electron density fluctuations during ELMs as measured by the FIReTIP edge channel.

(d) is a magnified detail of (b).

(e) is a detailed spectrum of (c), where agreed peaks were indicated for each individual ELM by both D-alpha emission and FIReTIP measurements.

During ELMs, the spectrum becomes more like L-mode plasmas than typical quiet H-mode plasmas.

TAE Modes and “Fishbones” ObservedTAE modes (150~200 kHz)

Magnetic coil data showed similarity and difference in different channels

Localized fishbones

Similarity observed in Channels 1, 2 and 3 is due to plasma chords spanning the entire core region

Ch1 Ch2

Ch3 Ch7

Electron density fluctuation spectrum of discharge #113655

Motivation for Increased BandwidthThe video bandwidth of the

FIReTIP system is currently limited to ~250 kHz

A wide range of kineticinstabilities exist on NSTXwith frequencies far inexcess of 250 kHz, such asToroidal Alfvén eigenmodes (TAEs)

Resonant TAEs (rTAEs)

Compressional Alfvén eigenmodes (CAEs)

Global Alfvén eigenmodes (GAEs)

A significant increase in video bandwidth will allow these instabilities to be studied using FIReTIP

From M. Peng, “NSTX Research Plan – FY05-07,”OFES Budget Planning Meeting FY-07, available online at http://www.nstx.pppl.gov/bpm/03-2005/Mpeng_Science_FY05-07.pdf (2005).

FIReTIP Bandwidth Upgrade Approach Employ high modulation bandwidth voltage-controlled oscillators

(VCOs) which can be applied to plasma signals as well as reference signals VCOs now available with 3 dB modulation bandwidth > 6 MHz

Turn off one FIR laser for maximum bandwidth in interferometry-only measurements (eliminates need for bandpass filters) Maintaining a ~4.5 MHz separation between FIR lasers enables a ~4 MHz video

bandwidth to be achieved

Employ wider bandwidth, sharper edge bandpass filters to separate out R- and L-wave signal components as needed Simultaneous interferometry/polarimetry measurements with ~1 MHz bandwidth

Perform phase comparison measurements at high frequencies(current version employs 24.0 MHz) to permit higher cutoff frequency lowpass filters to be employed Lowpass filters with auto-selected 1 MHz/4 MHz bandwidths

(choice of filter depending on mode of operation employed)

FIReTIP Plasma Coverage Retro-reflectors (5 external, 2 internal)

used to make double-pass measurements

System upgraded from 4 to 5channel operation in 2006;6 channel operation in 2008with addition of channel #5

Channels #1 and #2 usedas overall density monitors

Channels #3, #4 and #5monitor core fluctuations

Channels #5 and #7 monitor edge fluctuations

Edge Channel Measurement at L/H Transition At the L/H transition, the inner FIReTIP channels indicate a density rise

Edge FIReTIP channel (RT=150 cm, outside of separatrix) indicates a sudden density decrease at the L/H transition

►measured at same time as in and out D-alpha asymmetry emission

► Sudden increase of electron density gradient

increase of neutral density gradient

Er increase by gyro-center shift

ExB flow increase

time (sec)

Faraday Rotation Measurements

► Faraday rotation and electron density data on MHD oscillation showed phase difference

► Phase difference information of magnetic

field oscillation

► Different channels showed different oscillation patterns mode structure of MHD

Current FIReTIP Electronics System

X2

X2

X2

50

50

Comparator

Comparator

÷4

÷4

6.00 MHz REF

Phase Comparator

Phase Comparator

To Digitizer (R-Wave Phase)

To Digitizer (FaradayRotation)

A(3-5 MHz)

TTL Clock24.00 MHz

B(5-7 MHz)

A* and B* (3-7 MHz)

÷4

PLL24 MHz 25 MHz

25 MHz

PLL24 MHz

Fringe counter circuits employed with ~300 kHz lowpass filters

14 bit, 1.25 MHz digitizers employed

Bandpass filters employed to separate the R- and L-wave signal components

Effective bandwidth is ~250 kHz due to filter bandwidth

The video bandwidth is limited to ~250 kHz (due primarily to BP filters)

Reduce noise of reference signals; Track FIR laser drifts up to 100 kHz

Not applied to plasma signals as the low modulation bandwidth (~60 kHz) VCOs used in the PLLs are not sufficiently fast for the plasma signals

Phase locked loops

To increase signal amplitude for possible signal processing and define the receiver bandwidth

Plasma signals consist of R- and L-waves, mixed with reference signals, up-converted to 24 MHz and bandpass filtered, and then sent to phase comparators and digitizers for phase calculation

& digitizer

PLL

Phase Comparator

Step 1: New Phase Locked Loop (PLL) CircuitsPLL

BufferVCO

70 MHz

VCO70 MHz

Phase Comparator

PLL

Buffer

fromRef. Signal

fromPlasma Signal

44 MHz LPF

OPAmp

VCO

Mixer

Buffer

6 MHz-165-145-124-10076 – 89ROS-80-711917 MHz-162-143-122-9355 – 80ROS-70-119100 kHz-150-130-110-8825 – 50POS-50 (current)

1 MHz100 kHz10 kHz1 kHz3 dB Mod.

BWPhase Noise (dBc/Hz) SSB @Offset Freq.Frequency

(MHz)VCO Model

A PLL consists of a phase detector, low pass filter, and VCO.

The feedback mechanism forces the VCO’s frequency to settle at the input frequency with a constant phase angle between the oscillator and the input signal.

A VCO with 3 dB modulation bandwidth greater than 6 MHz (as shown bellow) can take advantage of the 6 MHz modulation frequency from the FIR lasers.

PLL Phase-Tracking Performance

FIReTIP currently employs PLLs only to track FIR laser frequency drifts.

New PLLs have been developed with significantly increased lock-in and tracking ranges.Lock-in range measures the frequency range over which the input signal can

be pulled-in to lock.

Tracking range measures the frequency range over which the input signal remains locked.

Optimum PLL circuit performance requires thatInput signals must be first upconverted above the VCO modulation bandwidth

using single-sideband mixers to avoid spurious signal generation, andsignal amplitudes be kept within a relatively narrow range.

ffVCO

Lock-in range

Tracking range

Step 2: AGC and Upconversion Circuits

Input signal levels range from -30 to -10 dBm Voltage controlled amplifier (VCA)

Variable gain (>20 dB dynamic gain range) set by the AGC feedback path

Good linearity (harmonic distortion < 42 dBc)

Gain “locked” during plasma discharge to avoid gain-induced phase shifts

Low frequency (2.5–9.5 MHz) signals up-converted by 21.75 MHz

Applied to both plasma and reference signals

A parallel circuit supplies 30.0 MHz HHFW output signal for detection

VCA

Sample & Hold

DetectorMixer

Level Shifter

Input Signal(2.5–9.5 MHz)

21.75 MHz

21.4 MHz

Trigger Signal

RFDetector

Sample& Hold

VCA 18 dB

LevelShifterTimer

Circuit

50

HHFWOutput Signal

17 MHzAGC

output20 dB

24.25–31.25 MHz

Ch #1

Ch #2

Ch #3

-40 -35 -30 -25 -20 -15 -10 -5 0 5

P in (d B m )

-20

-15

-10

-5

0

Po

ut (

dB

m)

AGC Test Circuit Performance

2.4 dB variation observed at the output over the 20 dB FIReTIP input signal range

AGC circuit tested using combined 4.0/6.0 MHz signal corresponding to FIReTIP plasma signals

Up-converted 25.75 MHz and 27.75 MHz signals as the input to the following PLL circuits(harmonic distortion levels < -42 dBc)

Input signals

Output signals

Estimated FIReTIP

signal range

4.0 MHz6.0 MHz

25.75 MHz 27.75 MHz

fLO = 21.75 MHz

< -42 dBc

PLL Phase-Tracking Performance

-35 -30 -25 -20 -15 -10 -5 0

Input Level (dBm)

0123456789

10111213

Tra

ckin

gR

ange

(MH

z)

-35 -30 -25 -20 -15 -10 -5 0

Input Level (dBm)

0123456789

101112

Lock

-inR

ange

( MH

z )

6 MHz-165-145-124-10076 – 89ROS-80-7119

17 MHz-162-143-122-9355 – 80ROS-70-119

100 kHz-150-130-110-8825 – 50POS-50 (current)

1 MHz100 kHz10 kHz1 kHz

3 dB Mod. BW

Phase Noise (dBc/Hz) SSB @Offset Freq.Frequency (MHz)VCO Model

Old PLLROS-70 PLLROS-80 PLL

Old PLLROS-70 PLLROS-80 PLL

Step 3: Enhanced R- and L-Wave Separation

For interferometry-only measurements, in which only one of the R- and L-wave components is present, the upconverted plasma signals can be directly fed to the phase comparison circuits

Simultaneous interferometry/polarimetry operation requires that the R-and L-wave components be first separated out and then compared

Current system mixes each componentto 24.0 MHz and then bandpass filter;measurement bandwidth is essentiallythat of the filter

A wide bandwidth filter is required toincrease measurement bandwidth,but with sharp band edges for signalselectivity – only available at lowfrequency (see right)

Solution is to downconvert the ~70 MHzsignals to 11 MHz, bandpass filter, andthen upconvert back to ~70 MHz forphase comparison measurements -50

-40

-30

-20

-10

0

-1.2 -0.8 -0.4 0 0.4 0.8 1.2

Old FilterNew Filter

Atte

nuat

ion

(dB

)

Frequency Offset (MHz)

New IF Electronics Layout

Automatic gain control (AGC) andupconversion circuits are utilized tomaximize phase-locking performance

Both reference (2) and plasma (7) signals now go through the PLLs at ~70 MHz (reference+plasma), and

~81 MHz (reference only, used to separate out R- and L-wave signal components)

Plasma Signal2.5~ 9.5 MHz

LO1 LO2

Ref. Signal6 MHz

24.25 ~31.25 MHz

78 MHz21.75MHz

AGC1 PLL VCO-80

Ref. Signal4 MHz

11 MHz

PLL VCO-80

PLL VCO-70

PLL VCO-70

PLL VCO-70

Phase Comparator

Phase Comparator

78 MHz

11 MHz

96 MHz

107 MHz

LO2 LO2

LO1

LO3

LO2

SIG1

SIG2

AGC2

AGC978 MHz

96 MHz

Interferometry only path

21.75 MHz

To Digitizer (R-Wave Phase)

To Digitizer (FaradayRotation)

Step 4: Phase Comparator CircuitsOverview of current approach

Analog signals upconverted to 24 MHz, bandpass filtered, and converted into TTL signals using zero-crossing comparators

Phase delay measured employing S-R flip flop circuits

Interferometer signals employ 16 and 20 circuits digitized at 1.25 MHz16 circuits have input frequency of ~1.5 MHz, output filtered to 330 kHz20 circuits have input frequency of ~1.2 MHz, output filtered to 280 kHz

Polarimeter signals employ 4 circuits digitized at 1.25 MHz, output filtered to 12 kHz due to excessive phase noise (from use of zero-crossing comparators with noisy input signals)

Overview of new approachAnalog signals upconverted to 70 MHz, bandpass filtered (as needed for

polarimetry), phase-locked to reduce noise, and converted into TTL signals using zero-crossing comparators

Phase delay measured employing S-R flip flop circuits

Interferometer signals employ pair of 4 circuits digitized at 12.5 MHz,set 90º apart and output filtered to 5.0 MHz

Polarimeter signals employ pair of 2 circuits digitized at 1.25 MHz,set 90º apart and output filtered to 500 kHz

Comparison Phase Comparator Circuit

70 MHz PLL signal

Compact 90º phase splitter circuit design

Bandwidth for 4 circuit: 5.0 MHz

Bandwidth for 2 circuit: 5.0 MHz

16

20

4

Phase DelayMeasurement

Video Amplifier

Phase DelayMeasurement

Video Amplifier

Phase DelayMeasurement

Video Amplifier

V

Existing arrangement Improved arrangement

24 MHz mixer output signal

Bandwidth for 16 circuit: 330 kHz

Bandwidth for 20 circuit: 280 kHz

for Interferometry

for Polarimetry

2 90º phase splitter (2)

V 16 20

for Interferometry

for Polarimetry

Phase DelayMeasurement

Video Amplifier

90º phase splitter (2)

Summary and Future Work

PLL circuits have been designed and fabricated, with considerable improvement in modulation bandwidth and lock-in/tracking ranges

AGC (and upconversion) circuits have been designed and fabricated to support the new PLL circuits, with only 2.4 dB change in output power observed over a 20 dB range of input signal level

New circuits have been designed to separate the R- and L-wave signal components (required for polarimetry) with both increasedselectivity and increased bandwidth

A new compact 2 phase splitter has developed, and will be employed to achieve high bandwidth FIReTIP phase measurements

The new wide bandwidth FIReTIP electronics will be installed on NSTX in May 2009 after experimental testing and characterization has been completed

AbstractThe Multichannel Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) system has great potential in monitoring high frequency coherent/incoherent density fluctuations and magnetic field profiles on the National Spherical Torus Experiment (NSTX). The measurements are essential in understanding transport physics issues in NSTX as well as for the coming ITER device, in which fundamental understanding of microturbulence MHD issues is essential. Due to the relatively narrow intermediate frequency (IF) video bandwidth of the current FIReTIP(250 kHz) system, the capability to monitor the above variations has been limited in both speed and accuracy.

Current activities have two upgrade goals: (1) to extend the IF video bandwidths from 250 kHz to 4 MHz for the FIReTIP system; and (2) to develop a parallel set of electronics for both systems to support high harmonic fast wave (HHFW) experiments that could induce 30 MHz density fluctuations superimposed on the system. Experimental details regarding the upgrade activities are presented.

*Work supported by the U. S. DoE grants DE-FG02-99ER54518 and DE-AC02-76CH03073