Abstract Measurement capabilities for the Pegasus ST are increasing to support the scientific studies of plasma behavior at very-low A. Global parameters are obtained from equilibrium reconstructions constrained by a magnetics set consisting of internal and external fluxloops, a poloidal array of B coils, and a diamagnetic loop. Pulse height analysis using a CCD detector will give a timeresolved electron temperature profile with a shotwise spatial scan. A tangentially viewing bolometer array measures profiles of radiated power. A 28 GHz single-chord microwave interferometer is used to measure line-averaged density, while a CCD-based visible bremsstrahlung array gives an approximate density profile. SPRED provides an impurity spectrum between 1-11 nm at 5 khz. MHD activity is characterized by Mirnov coil arrays and a 19 channel poloidal soft x-ray array. The proposed next generation of diagnostics includes time-evolving density, temperature and current profiles using active neutral beam spectroscopy.
Overview Pegasus has an extensive set of magnetics diagnostics for equilibrium and stability analysis The first generation of plasma diagnostics measure <T e >, N e l, P rad (r,t) and plasma shape The proposed next generation of plasma diagnostics will give profile measurements
Magnetics used for Equilibrium Reconstruction & Mode Analysis Flux loops Mirnovs on limiter Toroidal Mirnov Array
Two coils installed for redundancy Internal Rogowski Coil for I p The core armor and cones induce current in the plasma Rogowski - A vacuum shot accounts for these induced currents and is subtracted to get an accurate plasma current 4 Current (ka) 2-2 Installed Plasma Rogowski coils -1 1 2 3 4 time (ms) Current measured with plasma Rogowski for vacuum shot Plasma Current (ka) 16 12 8 4 1 12 14 16 18 time (ms) 2 22 24 Stainless Steel foil protection Installed plasma Rogowski coil and protective steel foil OH coil current used for subtraction currents measured with vac shot used for subtraction
mwb Diamagnetic Loop Used to Constrain Pressure For Pegasus, B tor due to plasma is relatively large. - Alignment to ±1mm is adequate. Toroidal Flux (Wb).16.14.12.1.8.6.4.2. Vacuum Shot Subtracted Plasma Signal Plasma Shot -.2 -.15 -.1 -.5..5.1 4 3 2 1 12 14 16 18 2 22 24 Time (ms) Time (s) A compensation loop is used to remove signal noise due to TF switching transients
Schematic Layout of Mirnov Coils Side View Top View 1 2 3 4 5 6 7 8 9 1 2 3 13 12 11 5-9 4,1 13 12 11 1 High-Res Core Mirnov Coils (21) Poloidal Mirnov Coils (22) LFS Toroidal Mirnov Coils (6) HFS Toroidal Mirnov Coils (7)
Mirnov Coils Allow Full Mode Identification The 6-coil LFS toroidal array is used to obtain values of n - Uneven coil spacing allows for resolution up to n=12 d B (AU) 1..8.6.4.2. 27 18 9 3 45 f = Toroidal Mirnov Signals Mode Poloidal Angle (x2p ) 2. 1.5 1..5...15 2/1 Poloidal Phase at the Wall 2. Outboard Coils.2.16.17 Time (ms) Centerstack Coils.4. 6.18 Physical Poloidal Angle (x2p ) Outboard Coils. 8.19 Spectral techniques are used to extract resonant frequencies and phase delays - Cross-power gives spectrum - Cross-phase gives phase shift 1. Phase Lag (x2p ) 1.5 1..5.. Reconstructed Toroidal Mode Numbers n=2 (7.4 khz).2.4.6 n=1 (3.5 khz).8 Physical Toroidal Angle (x2p ) 1.
Soft X-Ray Array - MHD 19 channel poloidal array of photodiodes (courtesy of PPPL) - external kinks observed and correlate with the visible camera images - 2/1 covers large portion of the plasma - 5 mil Be filters are used to get rid of most of the edge radiation - Ni filters will be used next spring to kill off the edge radiation completely SXR Signal (AU).3.2.1 8 4 d B (Gauss). -4 2 21 22 23 Time (s) Correlation between Mirnovs and SXR 2. Edge 8 SXR View of a plasma SXR fluctuation (A.U.) 1.5 1..5 Midplane 12 14 16 18 2 22 24 Time (ms) Large spatial extent of oscillations 6 4 2 Plasma Current (ka)
Global Instabilities Visible on Camera DALSA 256 Camera - 1 frames per second - 1 microsecond exposure/frame - 256x256 pixels - Local PC control Shot 1471 16 ms 17 ms 18 ms 19 ms Center post Visible distortions on plasma boundry
27 GHz Single Chord Interferometer for Line Integrated Density UCLA Collaboration Possible viewing chords 6 Pegasus Density Measurement n e l (x 1 19 m -3 ) 5 4 3 2 1 3.8 3.9 3.1 3.11 Time (ms) 3.12 3.13 3.14
Spectroscopic Measurement of T e up to 4 ev Ross filter system uses photodiodes to detect soft x-rays which depend heavily on T e MIST results indicate applicable range of T e () for each species: C: 7 to 15 ev O: 15 to 4 ev 1 Line Intensity Ratios (CV/CVI) D=1. to 5.m^2/s 1 Line Intensity Ratios (OVII/OVIII) 1 1 ratio ratio 1 1 1.1 5 7 9 11 13 15 17 19 21 ParaT D=5. SharpT D=5. FlatNe D=5. ParaT D=1. SharpT D=1. FlatNe D=1. Center Temp (ev).1 1 2 3 4 5 6 7 8 Para T Sharp T Center Temp (ev)
Ross Filters 2. Ross Filter Data 14 1.5 C V Plasma Current 12 1 1. 8 6 I p (ka).5 4 2. 12 14 16 18 2 time (ms) Set of six diodes installed on PEGASUS for T e estimates - AXUV-1 diodes - 1 x 1 mm active area Filters isolate: Lyman-a line of each ion CV 2 Å Al, 25 Å Ag, 1 µm CaF 2 CVI 4 Å Al, 6 Å V OVII 5 Å Al, 5 Å Mn OVIII 4 Å Al, 45 Å Fe Diodes
Time Resolved SXR Pulse Height Analysis: T e (t) Pulse Height Analysis systems measure SXR spectra - Individual photon energies are measured to reconstruct the plasma emissivity spectrum - This spectrum is used to estimate T e A CCD is being developed as a time - resolved PHA detector - Thousands of pixels => Thousands of detectors - Increased count rates (better time resolution possible) - High energy resolution Monte Carlo Modeling indicates good statistics without pulse pile-up Monopixels with valid energy 5 4 3 2 1 Split #1={.778,.195,.14,.13} Split #2={.9,.1,,} Split #3={.5,.3,.1,.1} 1 2 3 4 5 Number of Photons Collected Modeling for Split #1 Theory for Split #1 Modeling for Split #2 Theory for Split #2 Modeling for Split #3 Theory for Split #3 6 7 Events Laboratory tests show acceptable resolution 8 6 4 2 5 1 FWHM - 22 ev Energy(AU) 15 Iron Spectrum 2 5.9 kev 25
PHA/CCD System on Pegasus Parallel Shift exposed.81mm 5.99mm CCD SXR/PHA assembly ready for installation.81mm 8.61mm 13.39mm 14mm masked Camera Bellows allows scan for multiple spatial points Mask used for time resolution - Exposed rows are shifted behind the mask at ~8 m sec per row - Rows remain behind mask until plasma extinguished, and are then read out The CCD/PHA system requires direct x-ray illumination of the CCD, therefore the chip must be place in machine vacuum A gold mask has been designed to add the capability of time resolution The split fraction of the camera is higher than expected - If this cannot be resolved, Si-drifted detectors may be used to get measurements of T e in the kev range
16 channel photodiode array installed Bolometers Radius = 12.9 cm Center Raw Emissivity (AU) Radius = 23.8 cm Radius = 34.1 cm Pinhole imaging aperture Diodes and pre-amp assembly Radius = 43.8 cm Edge 1 12 14 16 18 2 time (ms) View of Plasma
High Time Resolution VUV Survey Spectrometer SPRED VUV Survey Spectrometer - 1-11 nm spectral range measured simultaneously - single radial view of plasma New high -speed 1-D imaging detector - 2 pixel 1-D camera (DALSA CL-C6) - VUV - visible image intensifier/convertor lens coupled to 1-D camera - Up to 48 frame/sec sample rate D t ~.2 msec/frame Intensity (A.U.) 35 3 25 Intensity (A.U.) 2 15 1 O VI (15. nm) VUV Spectroscopy (SPRED) O IV (55.4 nm) O V (62.9 nm) O V (76.1 nm) D I (12.5 nm) 5 5 1 15 2 Pixel Pixels
Visible Bremsstrahlung: Z eff and N e profile Visible Bremsstrahlung emissivity sensitive to electron temperature Z eff and n e 2 CCD chip before readout CCD chip exposed when line passes slit in mask CCD chip after readout area to be imaged empty area to be skipped image of plasma Line Shift Technique Princeton Instruments camera 523.5 nm filters (BP 1 A) Zoom lens Slit for line shift technique Center Post Slit frame transfer provides for time resolution Signal dominated by visible edge radiation - next generation will have improved filtering Time PEGASUS Toroidal 5 Experiment
Tangential SXR Plasma Imaging: j(r,t), P(R,t) Internal plasma measurements constrain equilibrium reconstructions and help determine j(r) profile Next generation of the SXR profile imaging system is in development - Direct soft x-ray illumination of the CCD chip - Ta mask used for time resolution See poster KP1.95 for more details
DNB Spectroscopy Provides a Variety of Internal Measurements In core plasma region, beam attenuation dominates the spatial variation of the H, D line intensity, giving a simple local estimate of Ne - Use D b line to minimize dependence of excitation rate on local plasma parameters - Use iterative solution to estimate the fex term and derive local Ne(R,t) in the plasma periphery Can recover the plasma density profile with good accuracy, even with no knowledge of the plasma temperature profile Logarithmic Derivatives (m -1 ) 5 4 3 2 1-1 n = 3 n = 4 1 df ex f ex dx 1 I l di l dx n = 3 n = 4 fi 6x1 18 N e (m -3 ) 5 4 3 2 1 Ne Ne_from3 (Da ) Ne_from4 (Db ) -2.. 2. 4 R majo r (m ). 6. 8 1...2.4 R majo r.6 (m).8 1.
T e (r,t) Estimates Available from He Beam Line Ratio Measurements Model Ratio (I 3^3S / I 3^1P ) 1 1 1 4 2 4 2 4 2.1 Outer edge of plasma Metastable = 5% Metastable = 2% 1 2 T e (ev) 3 4 5 Measurements past the magnetic axis are the key - At the outer edge, beam dynamics are in non-linear evolution - State populations are very sensitive to initial metastable fraction - Large differences seen in the ratios for different metastable fractions - Deeper inside the plasma, beam population has equilibrated Model Ratio (I 3^3S / I 3^1P ) 1 2 6 5 4 3 2 Past magnetic axis %Metastable = 5% %Metastable = 2% - Line ratio is mainly sensitive to local temperature - Virtually no difference seen in the ratios for different metastable fractions 1 2 T e (ev) 3 4 5
Future Work Thomson Scattering: Single point; spectrometer available; need laser and optics Optics for the TS system Detectors Li pellet injection - Measures the local magnetic field tilt - infers the j(r) and q(r) profiles - Compliment the CCD soft x-ray imaging system
Summary Global parameters are constrained during equilibrium fitting by magnetics - poloidal fluxloops for geometry - diamagnetic loop for plasma pressure - plasma Rogowski for total plasma current The first generation of plasma diagnostics measure <T e >, N e l, P rad (r,t) and plasma shape Next generation of diagnostics will provide time evolving density, temperature and current profiles - DNB spectroscopy - Thompson Scattering