Hard Xray Diagnostic for Lower Hybrid Current Drive on Alcator C- Mod J. Liptac, J. Decker, R. Parker, V. Tang, P. Bonoli MIT PSFC Y. Peysson CEA Cadarache APS 3 Albuquerque, NM
Abstract A Lower Hybrid Current Drive (LHCD) system is being installed on C-Mod allowing the exploration of advanced tokamak (AT) regimes through current profile control The LH current profile may be inferred through non-thermal Bremsstrahlung emission measured by a pinhole camera A Fokker-Planck (FP) model coupled to camera data gives information about the LH modified electron distribution function Camera design and FP model will be discussed
LH Waves LHCD is calculated based on Ray tracing Imaginary part of the hot plasma dispersion relation or an FP solver k =(krbr+m/rbθ+n/rbφ)/ B Experimental verification of LHCD location is needed LHCD works through electron Landau damping around v~3vth Creation of non-thermal Bremsstrahlung radiation from high energy tail Bonoli ACCOME Simulation
Non-Thermal Bremsstrahlung Bremsstrahlung radiation is continuous: an electron of energy E can radiate any hν < E No direct measure of distribution function, need model e-i Bremsstrahlung is typically an order of magnitude larger than e-e for E < 5 kev Radiation is anisotropic, peaked in the forward direction Perpendicular Direction 8 6 6 8 E electron = kev Total Bremsstrahlung Cross section (cm x 1 8 ) 1keV kev 5keV 1keV 5 1 15 Parallel Direction
Experimental Layout C-Mod top view D E F G Klystrons Wave Guides LH Antenna C H B J Located on mezzanine 1 klystrons at 5kW each 3MW f =.6GHz Located on C-port Grill composed of 96 waveguides, rows of Dynamic phase change possible, ~ms, allowing for pre-program spectra or feedback control HXR Diagnostic A Located on B-port 3 channel pinhole camera -5keV energy range ~1.5cm spatial resolution ~1ms temporal resolution K
HXR Camera Design 3 channel pinhole camera 1 using CZT detectors and fast digitization techniques C-Mod cross section with HXR camera
HXR Camera Design (cont.) CdZnTe detectors: High Z 9.1, high density 5.8g/cm3 No cooling or magnetic shielding Made in the USA- ev Products Fast digitization allows: Flexibility through software data analysis Energy resolution limited by detector Improved noise rejection/pile up detection Lower system cost Possibility of real time feedback control of n Data management: Up to GB/shot raw data ~15 shot scratch disk Local data processing Processed data archived
HXR Camera Design (cont.) B-port Aluminum RF shield and inner structural support Lead plate shielding variable thickness Aluminum vacuum window Square pinhole Insulated support structure Cable feed through
Pulse Processing Photons Detector Preamplifier Amplifier and Shaper Fast Digitizer Detector Current vs Time 5 Preamp Voltage vs Time 1 Gaussian Shaper Output I e 35 I h.5.9.8 3 3.5.7 Current (na) 5 15 Voltage (mv) 3.5 Voltage (V).6.5. 1.5.3 1 1. 5.5.1...6.8 1 1. 1. 1.6 1.8 Time (µs).5 1 1.5.5 3 3.5 Time (µs)...6.8 1 1. 1. 1.6 1.8 Time (µs) ev Products 5 5 mm CZT Energy resolution better than 1% at 6kev Charge transport limits count rates to ~1MHz ev Products ev-593 Preamp High sensitivity ~1mV/MeV τ f =1ms Small size.6 1.5cm Gaussian Shaper Developed in house ~1µs pulse width DC offset Line driver Small size.5 7.6cm DTACQ ACQ16 CPCI form factor 16ch/board at 1MHz 1GB/board memory 1 bit resolution
Shaping Electronics Gaussian pulse shape 3 : No undershoot better energy performance at high count rate Easy to fit reduces pile up and improves rejection 3 Voltage (mv) V3 1 V V1 Vdiff.5 1. 1.5. Time (us) Shaper step response from PSPICE model
Shaping Electronics (cont.) C1 7pF C 18pF C3 33pF C6 6.8uF +5 +5 C1.1uF C7 C11 6.8uF.1uF C 33pF +5 C8 C1 6.8uF.1uF C5 7pF +5 C9 C13 6.8uF.1uF Vdiff PRE_IN C1 18pF LMH67/SOT3_5 3 + R1 1 1 C15 R1 OUT.1uF C19 9.9 - U1 33pF C7.1uF 5 V+ V- R 9.9 R3 137 C 33pF LMH67/SOT3_5 3 + - C3 U 7pF 5 V+ V- OUT 1 C16 C8 R 1.1uF C V1 33pF.1uF R5 9.9 R6 67 C 33pF LMH67/SOT3_5 3 + - C5 U3 18pF 5 V+ V- OUT 1 C17 C9 R7 1.1uF C1 V 33pF.1uF R8 9.9 R9 365 C6 7pF LMH67/SOT3_5 3 + - U 5 V+ V- OUT 1 C18 C3.1uF.1uF DC_LD V3 C31 6.8uF C3 6.8uF -5-5 -5 C33 6.8uF -5 C3 6.8uF R15 R11 37 R16 R1 37 R17 R13 37 R18 R1 37 3.7 37 37 37 Gaussian shaper realized through Sallen- Key filter and surface mount PCB Flexible gain and pulse width DC offset used to adjust signal into digitizer range of ±.5V 5Ω line driver DC_LD V_OFF R19 1 LMH67/SOT3_5 3 + C37 33pF - U5-5 R 37 R 3.7 +5 C35 6.8uF C36.1uF 5 V+ OUT 1 C38.1uF C39.1uF C 6.8uF V- R3 37 R1 1K Q1 MMB39 R 9.9 OUT
Count Rate Count rate estimated through AT target plasma parameters and a crude average over the emissive region Current Density (MA/m ) Safety Factor 1 1 1 8 6 Current Profiles Total 8 ka LH 39 ka OH -7.7 ka BS 3 ka -...6.8 1 Normalized Radius (r/a) 1 8 6 q Profile Electron Density (1 /m 3 ) Temperature (kev) 3 1 Density Profile...6.8 1 Normalized Radius (r/a) 8 6 Temperature Profiles T e T i Count Rate:. N = n e a a d c d k i j( r, hν ) hν l a c = collimator size a d = detector size d = pinhole to detector spacing k i...6.8 1 Normalized Radius (r/a)...6.8 1 Normalized Radius (r/a)
Count Rate (cont.) Spatial, temporal, and energy resolution are interdependent through counting statistics Spatial resolution linked to count rate through aperture size hν is set by bin size, or detector resolution Larger energy bins increase the number of counts per channel at the expense hν Temporal resolution determined by statistical accuracy needed Count Rate (1/s) 1 6 Count Rate vs Energy 1 5 1 1 3 1 1 1 1 5 1 15 5 3 Energy (kev)
Spatial Resolution Plasma a c a d Spatial resolution: Estimation: r ~ a / #chords=1.cm Geometry: r = (1 + D/d)ac ~ 1.7cm D d 1 6 Count Rate and Resolution vs Aperture Size E=-5 kev E=35-5 kev E=5-5 kev.5 Count Rate and Resolution vs Pinhole to Detector Spacing 1 6.8 ac=.5cm a c.6 3.5. Count Rate (1/s) 1 5 3.5 Resolution (cm) Count Rate (1/s) 1 5. Resolution (cm) 1 1.5 1 1.8 1 1.6 Resolution Mapping 1 3.1..3..5.6.7.8.9 1 Aperture size (cm) d=39cm.5 1. E=-5 kev Resolution Mapping E=35-5 kev E=5-5 kev 1 3 3 35 5 5 55 6 65 7 75 8 1. Pinhole to Detector Spacing (cm)
Shielding Count Rate (1/s) 1 6 Background Count Rate vs Channel 1 5 1 1 3 1 1 1 1 6 8 1 1 1 16 18 Channel Number Neutron Rate 7.5e13 (1/s) Background measurement using pixilated detector and ASIC shot 136517 from t =.975-.985 Background measurements indicate gamma shielding is required cm minimum Pb for gammas Al RF shield Diagnostics on B-port redesigned giving more room for shielding Neutron shielding may now be considered MCNP will be used to investigate tradeoffs in shield design
Pulse Height Analysis Count Rate (1/s) 1.8 1.6 1. 1. 1.8.6.. x 1 5 Count Rate vs Channel 6 8 1 1 1 16 18 Channel Number No Shielding Neutron Rate 7.5e13 (1/s) Software analysis: Variable time and energy binning adjust depending on count rate giving more data points for acceptable statistics Gaussian fitting double peaks fitted to reduce pile up and increase effective count rate 1.8 Digitized Signal 1.6 Digitization Rate = 6MHz 1. Voltage ) (V) 1. 1.8.6.. Background measurement using pixilated detector and ASIC shot 136517 from t =.975-.985 5 1 15 5 3 Time (us)
Reconstruction HXR measures the line integrated emissivity and a reconstruction is needed to recover the radial profile MATLAB GUI developed for Tore Supra HXR analysis is being adapted for C-Mod Reconstruction methods include: Abel inversion Minimum Fisher Maximum entropy And others
Distribution Function Model LH distribution function model 5 coupled with a FP calculation gets at the underlying physics while including collision and the Ohmic field LH model Local plasma parameters T, n, I p, Z eff LH parameters D ql, v min, v max FP Code f e (p,p ) pperp/pth-ref 16 1 1 1 8 6 -D steady state electron distribution function Physical interpretation D ql LH power level - -5 5 1 15 ppar/pth-ref v max Wave accessibility v min Landau damping Code written by J. Decker and Y. Peysson
HXR Calculation Once the distribution function is known then the local emissivity may be found given the Bremsstrahlung cross sections and the viewing angle 1-5 Local Emissivity 1-6 f e (p,p ) θ v HXR Code Local j(hν) Emissivity (m 3 s -1 sr -1 kev -1 ) 1-7 1-8 1-9 1-3 T ph T ph is the characteristic slope of the emission and is related to the count rate by: dn = dtdωdhν A exp( hν hν T ph ) 1-31 Thermal emission 1-3 5 1 15 5 Energy (kev)
Summary A 3 channel CZT detector pinhole camera using fast digitization has been developed to measure the -5keV electron population resulting from LHCD Emissivity profile reconstruction gives the LHCD location Measurements coupled to a FP model give information about the electron distribution function and are directly related to wave accessibility and power level Camera will be ready for operation when LH is installed
References 1 Y. Peysson and F. Imbeaux Rev. Sci. Instrum. 7(1) 3987 (1999) R. O Connell et. al. Rev. Sci. Instrum. 7(3) 1 (3) 3 Ohkawa et. al. Nuc. Instrum. Meth. 138 85 (1976) Anton et. al. Plasma Phys. Control. Fusion 38 189 (1976) 5 J. Decker and Y. Peysson 9 th EPS Montreax 6B(P-.5) ()