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Hard Xray Diagnostic for Lower Hybrid Current Drive on Alcator C-Mod

Hard Xray Diagnostic for Lower Hybrid Current Drive on Alcator C-Mod. John Liptac. Advisor: Prof. R. Parker Readers: Prof. I. Hutchinson and Dr. P. Bonoli. NED Joint Seminar December 10, 2003. Motivation.

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Hard Xray Diagnostic for Lower Hybrid Current Drive on Alcator C-Mod

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  1. Hard Xray Diagnostic for Lower Hybrid Current Drive on Alcator C-Mod John Liptac Advisor: Prof. R. Parker Readers: Prof. I. Hutchinson and Dr. P. Bonoli NED Joint Seminar December 10, 2003

  2. Motivation • Tokamaks are pulsed devices limited by the plasma current; however, steady state operation is very desirable for any reactor Current drive is needed • Current profile can be controlled using lower hybrid waves • Location of current is critical to performance and can be measured through Bremsstrahlung emission

  3. LH Waves • LH frequency→ x-mode (kB) root of cold plasma dispersion relation, 2lh= 2ce/(1+ 2pe/ 2ce) • LH limit:2ce » 2 » 2ci • Numbers for 5T, 1020 m-3: fce=140, fpe=90, fpi=1.4, flh=1.2, fci= 0.04 GHz • Pure LH wave is cutoff Plot conditions: Parabolic density profile with peak of 3 1020 m-3, B  1/R with B0 of 5.3 T

  4. LH Waves LHCD is calculated based on • Ray tracing • Damping calculation • k|| =(krBr+m/rB+n/RB)/|B| Experimental verification of LHCD location is needed LHCD works through electron Landau damping around v~3vth v /k • Creation of non-thermal Bremsstrahlung radiation from high energy tail Bonoli ACCOME Simulation

  5. Non-Thermal Bremsstrahlung Bremsstrahlung radiation is continuous: an electron of energy E can radiate any h < E Eelectron = 200 keV • No direct measure of distribution function, need model • e-i Bremsstrahlung is much larger than e-e for E < 500 keV • Radiation is anisotropic, peaked in the forward direction

  6. Experimental Layout C-Mod top view LH Antenna Wave Guides Klystrons HXR Diagnostic • Located on mezzanine • 12 klystrons at 250kW each →3MW • f = 4.6GHz • Located on C-port • Grill composed of 96 waveguides, 4 rows of 24 • Dynamic phase change possible, ~ms, allowing for pre-program spectra or feedback control • Located on B-port • 32 channel pinhole camera • 20-250keV energy range • ~1.5cm spatial resolution • ~10ms temporal resolution

  7. HXR Camera Design 32 channel pinhole camera using CZT detectors and fast digitization techniques C-Mod cross section with HXR camera

  8. HXR Camera Design (cont.) CdZnTe detectors: • High Z 49.1, 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 2GB/shot raw data • ~150 shot scratch disk • Local data processing • Processed data archived

  9. Pulse Processing Photons Detector Amplifier and Shaper Fast Digitizer Preamplifier eV Products 552mm CZT eV Products eV-5093 Preamp Gaussian Shaper DTACQ ACQ216 • Energy resolution better than 10% at 60kev • Charge transport limits count rates to ~1MHz • High sensitivity ~120mV/MeV • f =1ms • Small size 2.61.5cm • Developed in house • ~1s pulse width • DC offset • Line driver • Small size 2.57.6cm • CPCI form factor • 16ch/board at 10MHz • 1GB/board memory • 14 bit resolution

  10. Shaping Electronics Gaussian pulse shape: • No undershoot → better energy performance at high count rate • Easy to fit → reduces pile up and improves rejection 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 ± 2.5V • 50 line driver Shaper step response from PSPICE model

  11. Shielding Measured background Expected signal Neutron Rate 7.5e13 (1/s) Background measurements indicate gamma shielding is required Background measurement using pixilated detector and ASIC shot 1030605017 from t = 0.975-0.985

  12. Gamma Production in Shield MCNP using neutron background used to estimate gamma production from shield materials Estimation valid for relative contributions only from simple source model- absolute levels need correct source spatial distribution

  13. Summary • Lower hybrid waves are a tool used to study steady state operation through current profile control • A 32 channel CZT detector pinhole camera using fast digitization has been developed to measure the 20-250keV electron population resulting from LHCD • Emissivity profile reconstruction gives the LHCD location • Compact design for spatial constraints • Camera will be ready for operation when LH is installed

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