Large-Area Avalanche Photodiode Detector Array Upgrade for a Ruby-Laser Thomson Scattering System T.M. Biewer, D.J. Den Hartog, D.J. Holly Department of Physics, University of Wisconsin-Madison M.R. Stoneking Physics Department, Lawrence University A low-cost upgrade has been implemented on the Madison Symmetric Torus (MST) ruby-laser Thomson scattering (TS) system to increase spectral coverage and substantially improve the signal-to-noise ratio M adison S ymmetric T orus (SNR). The spectral resolution has been increased from 5-channels on the long-wavelength side to an 11- channel system which covers both sides of the laser line. Coverage on both sides of the spectrum allows for more accurate detection of subtle changes in the distribution function, particularly relativistic spectral shifts of high-temperature plasmas during auxiliary current drive. The previous micro-channel plate (MCP) detector was replaced with an array of modular large-area avalanche Department photodiode of Physics (APD) detectors, which have University of Wisconsin approximately sixteen times the quantum efficiency of the MCP detector. Scattered light collection has also been upgraded, allowing the radial viewing location of the TS system to be easily changed between plasma discharges. Improved SNR and upgraded light collection hardware in the Thomson scattering system have facilitated first-time measurements of the evolution of the electron temperature profile in the MST under a variety of discharge conditions, leading to increased understanding of the underlying dynamics of reversedfield pinch plasmas. This work was supported by the U.S. D.O.E. High Temperature Plasma Diagnostics Conference, Madison, WI
Motivation and Outline Motivation: Improving the signal-to-noise ratio in the MST ruby-laser Thomson scattering diagnostic was accomplished through a series of hardware upgrades. The result is a diagnostic which has led to an increased understanding of MST and RFP physics. Outline: Light Collection Upgrade Fiber Optic Bundle Edge Chord Addition Light Detection Upgrade Spectrometer Modifications APD Installation Timing Hardware Experimental Results
The Madison Symmetric Torus Reversed Field Pinch
Ruby-Laser Thomson Scattering Diagnostic JK-Lumonics PDS1 ruby laser. 5 J, 50 ns single pulse system This laser-head has been operational for over 15 years. Reliable but limited Jarrell-Ash MonoSpec-27 spectrometer. 600, 1200, 1800 g/mm gratings Modified for new exit-plane Advanced Photonix 5 mm Large Area-APD s. 11 modules High quantum efficiency Internally cooled
Light Collection Upgrade Fiber Optic Bundle Replaced difficult to adjust, fixed mirror collection of TS light. Dove-tail mounts allow: Easy, between-discharge re-positioning of collection optics. Simple alignment with orthogonal translation stages. Initial radial coverage on inboard and outboard of axis to r/a~0.63. Edge Chord Addition Laser beam can be translated 30 cm towards the collection optics. Utilizes the same collection optics. Correction lenses added. Expands radial coverage to r/a~0.90. Single mirror adjustment changes beam steering between edge and core configuration.
Thomson Scattering Diagnostic Overview
Edge laser chord extends the radial coverage. Locations of Thomson scattering volumes with the % sensitivity to electrons from perpendicular and parallel dist. functions for Standard 300 ka plasmas. MST poloidal cross-section of scattering centers. View 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 r/a 0.627 0.455 0.283 0.115 0.080 0.244 0.415 0.587 0.882 0.769 0.681 0.630 0.625 0.666 0.746 0.854-16.93-13.53 123.91 147.06 151.39 153.19 22.28 31.41 43.26 57.76 73.61 88.59 101.10 110.82 99.5 96.8 76.2 64.3 50.0 99.9 96.1 84.5 64.1 38.2 13.9 3.1 6.6 4.5 3.2 23.8 35.7 50.0 0.1 3.9 15.5 35.9 61.8 86.1 96.9 93.4 f -18.46 0 % perp 63.0 77.0 88.5 % par 37.0 23.0 11.5 MST radial location of scattering centers. Horizontal bars show radial range of scattering volume for each view.
Light Detection Upgrade Spectrometer: Jarrell-Ash Monospec 27 Wider entrance slit accepts greater number of photons. Wider entrance slit accepts greater number of photons. Added shutter to protect LA-APD APD s from plasma start-up light. Removed exit-mirror to accomadate new exit-plane fixture. Widened focusing mirror to accept broader spectrum. Widened focusing mirror to accept broader spectrum. Exit-Plane Fixture Exit-Plane Fixture 11 wavelength channels. 11 wavelength channels. Samples light on both sides of laser line. Samples light on both sides of laser line. Kinematic mount and translation stage ease alignment. Fiber-optic coupling to LA- Fiber-optic coupling to LA-APD APD s. Large-Area Avalanche Photodiode Detectors Large-Area Avalanche Photodiode Detectors Replaced MCP and improved SNR Replaced MCP and improved SNR Higher quantum efficiency (85% v. 6%) Higher quantum efficiency (85% v. 6%) Lower gain (300 v. 1x10 Lower gain (300 v. 1x10 6 )
LA-APD Array Wider Mirror Spectrometer and APD Overview Power Distribution Box Exit-plane Fixture Shutter July 8th, 2002
Spectral Coverage of the Diagnostic Expanded spectral coverage from previous MCP configuration. H_alpha Line Ruby Laser Line MCP Coverage 1200 g/mm 1800 g/mm Light from both sides of the laser line is sampled High temperatue relativistic corrections to the distribution function Plasma temperatures range from 50 ev (edge) to over 1 kev (core during current drive) Expected APD Coverage Changing the spectrometer grating, allows the diagnostic to remain sensitive to this range of temperatures. 600 g/mm: 1-3 kev 1200 g/mm: 500-1500 ev 1200 g/mm: 500-1500 ev 1800 g/mm: 100-700 ev 1800 g/mm: 100-700 ev
LA-APD Module Layout APD Module Large Area (5 mm) Large Area (5 mm) 5V APD cooler Modular 12V 12V meter load meter load Bias meter APD Bias APD Bias Temp. meter? APD Bias Load Temp. Load delay line Commercially available from Advanced Photonix,, Inc. www.advancedphotonix.com Power input: Power input: Dual ±12 V for APD bias. 5 V for thermoelectric cooler. 5 V for thermoelectric cooler. 80% quantum efficiency when cooled digitizer 3way splitter Gain of 300 Gain of 300 digitizer digitizer
MST event timing controls the diagnostic. 300 ka Typical Plasma Current 5V TTL 0 t (ms) 10 20 30 40 50 60 adjustable fire laser trigger 0 t (ms) spectrometer shutter opening gate 1.25 ms delay 0 t (ns) laser fires 0.5 1 1.5 2.0 laser fast photodiode output plasma background photons 100 200 digitizer gate: bkgd (pre-laser) digitizer gate: laser digitizer gate: bkgd (post-laser) TS photons 300 The shutter is gated open following plasma startup: 6 ms opening time. Start-up light would damage APD s. The ruby laser can be set to pulse either (1.25 ms delay to pump flash lamps) : At a fixed time. Triggered by sawtooth event. The laser fast-photodiode controls subsequent timing APD output is sent through lengths of delay-cable. 3 staggered digitizers gate on for 100 ns to integrate the TS and background light.
Timing Diagram of TS Hardware MST Thomson Scattering APD Timing Layout x11 channels Spectrometer light signal Power Dist. box Laser Energy Monitor shutter control Ruby Laser Plasma APD Power laser fire Delay line Fast Photodiode MST Master Control CAMAC 2249 dig. shutter gate APD Power Supply and Spec. Shutter TS-timing box light signal x11 channels bkgnd TS lgt. bkgnd 3-way power splitter digitizer gates Delay line x11 channels
Experimental Results Light collection upgrades added the capability to measure T e profiles (12 radial points) at a fixed time in a single day. Previously 2 points measured in 2 days. Previously 2 points measured in 2 days. Light detection upgrades allowed the T e profile to be measured with accuracy in the edge. Time evolving T e profiles (12 time points, 16 radial points) can be measured in ~1 week. T e evolution over sawtooth crash. evolution over sawtooth crash. Accurate relativistic corrections to >1 kev plasmas can be made because of the improved spectral coverage.
Standard Plasma Discharge 13-nov-2000 Shot 69 Motivation: To study the dynamics of a fully diagnosed MST plasma over a sawtooth cycle. Typical plasma parameters: Typical plasma parameters: Standard Standard w/ full PFN I p ~ 385 ka ~ 385 ka F ~ -0.24 F ~ -0.24 n e ~ 1.1x10 13 cm -3 Deuterium Sawtooth period ~ 5.5 ms Sawtooth period ~ 5.5 ms Time of interest: ~ 15 ms, i.e. early in the discharge but during plasma flattop flattop
Comparing T e (MSTFIT) from T e (TS) t = -1.75 ms t = -0.25 ms After binning the raw Thomson scattering data and including the edge Langmuir probe T e, MSTFIT splines are fit to the data in each time slice. Examples are shown here. t = +0.75 ms
Temperature Profile Evolution r (m) r (m) time from sawtooth (ms) MSTFIT splines to 16 chord Thomson Scattering First time that the Te profile evolution through a sawtooth cycle has been measured. T e ~400 shots at nominally the same conditions were necessary to map out the profile. ~1 experimental week of data taking is a significant improvement for the MST. 0.5 ms bins of 3 to 5 shots used at each time,space point. Within a time slice, MSTFit used to interpolate smooth curve. time from sawtooth (ms)
Improved T e fitting at high temperatures. Relativistic Gaussian Simple Gaussian Simulated TS data at 1300 ev with 10% noise is fit with a simple Gaussian and with a relativistic correction (Hutchinson). The deviation from a Gaussian distribution is most pronounced in the channels on the short wavelength side of the laser line.
Conclusions The light collection system on the MST ruby-laser TS diagnostic was upgraded: Between-shot change of radial viewing location. Between-shot change of radial viewing location. Radial coverage out to r/a ~0.90. Radial coverage out to r/a ~0.90. The light detection system improved the SNR: The light detection system improved the SNR: Larger slit increases photon counts. Larger slit increases photon counts. Greater spectral coverage: Greater spectral coverage: Both sides of Both sides of laserline. 11 wavelength channels up from 5. 11 wavelength channels up from 5. Cooled LA-APD APD s have higher quantum efficiency. First measurement of time resolved T e profiles in the MST. Accurate relativistic corrections to high temperature plasmas. Accurate relativistic corrections to high temperature plasmas.
Reprints Full color version of this poster is available online at: http://sprott.physics.wisc.edu/biewer/htpd02poster.pdf
11 10 9 8 7 6 5 4 3 2 1
1 2 3 4 5 6 7 8 9 10 11 Laser Line Dump Region 10.0 mm 18.4 mm -40-30 -20-10 +10 +20 +30 +40 +50 +60 wavelength shift (nm) Hydrogen Balmer Alpha Line Ruby Laser Line 694.3 nm
Shots are Ensembled wrt.. The Sawtooth Crash nov-2000 ensemble of ~400 shots Plasma shots are ensembled for 2 reasons: Want an average average measure of quantities, i.e. smooth out the fluctuations Need ~400 shots to get T e (r,t) from Thomson sct. -2.75-2.25-1.75-1.25-0.75-0.25 +0.25 +0.75 +1.25 +1.75 +2.25 +2.75 This 6 ms time window about the crash is sub- divided into 12 time slices, every 0.5 ms. High time resolution signals (e.g. FIR) are computed as 0.1 ms ensembles. Raw TS data is ensembled in 0.5 ms bins.