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Fast 2-D Soft X-ray Imaging Device Based on MPGD for Magnetic Fusion Plasmas

8th Topical Seminar on Innovative and Radiation Detectors. Siena , 24/10/2002. Fast 2-D Soft X-ray Imaging Device Based on MPGD for Magnetic Fusion Plasmas. D. Pacella. Permanent address : ENEA, C.R. Frascati, Italy Present address (since 2001): Johns Hopkins University, Baltimore, MD, USA.

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Fast 2-D Soft X-ray Imaging Device Based on MPGD for Magnetic Fusion Plasmas

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  1. 8th Topical Seminar on Innovative and Radiation Detectors Siena , 24/10/2002 Fast 2-D Soft X-ray Imaging Device Based on MPGD for Magnetic Fusion Plasmas D. Pacella Permanent address : ENEA, C.R. Frascati, Italy Present address (since 2001): Johns Hopkins University, Baltimore, MD, USA

  2. Plasma imaging with MicroPattern Gas Detectors. D. Pacella, ENEA-Frascati (Italy) – Visiting Scientist at JHU (MD, USA) R. Bellazzini INFN - Pisa (Italy) An innovative fast system for X-ray imaging has been developed at ENEA Frascati and INFN Pisa (Italy). It is based on a pinhole camera coupled to a Micro Pattern Gas Detector (MPGD) having a Gas Electron Multiplier (GEM) as amplifying stage. This detector (2.5cm X 2.5cm active area) is equipped with a 2-D read-out board with 144 pixels with high speed asynchronous individual counting capability. The system has been successfully tested on the Frascati Tokamak Upgrade (FTU, Italy)) and on the National Spherical Tokamak eXperiment (NSTX) at Princetown (US). Time resolved, two-dimensional, high rate (up to 50KHZ framing rate) X-ray images of the NSTX plasma core will be presented.

  3. PARALLEL READ-OUT To achieve the maximum rate capability, the read-out is divided in 4mm2 pads that work as independent X-ray counters. Each pad is provided with a whole electronic chain: Pre-amplifier  Amplifier  Discriminator  Gated counter Printed circuit board 128 pixels (2.5 x 2.5 cm2)

  4. ELECTRONICS 128 independent channels Detector and front end VME acquisition system amplifiers Host PC pulser gate latching Cables 15 m pad Charge preamp Threshold Pulse shaper discriminator Counter w memory CPU ethernet Unipolar pulses 50 ns width 0.01 - 1 V proportional to x ray energy 104 -106 electrons 20 ns ECL pulses 50 ns width Energy discrimination logic output 10 ns width Fast counter (up to 100 Mhz) Noise 5 103 electrons Latch freq up to 1 Mhz

  5. VME Crate (CPU, timer pulser, discriminators and Scalers for 128 channels ) by CAEN amplifiers NIM crate ( High Voltage Power supplies ) CD power supplies for Preamplifiers and amplifiers

  6. plasma diagnostic Danilo Pacella MPGD detector Ronaldo Bellazzini - Pisa data acquisition CAEN - Viareggio set-up FTU - Frascati

  7. First tests at the Enea Laboratories in Frascati 2000-2001 with 1-D view Bt = 8 T, Ip = 1.6 MA R = 0.93 m a = 0.3 m 1-D view The inner toroid The Frascati TOKAMAK (FTU)

  8. Linearity of GEM current at very high counting rates. Counting rate linear up to 2 MHz/pixel (limited by electronics dead-time) D. Pacella et al., Review of Scientific Instrument, Vol.72 No.2 (2001)

  9. Imaging capabilities at high rates (> 10e6 ph/s mm2) X-ray source Pinholes (1 and 1.3 mm) wrench Lead screen 2.5 e6 ph/s Exposuretime 20 ms Detector (read-out) Pixel 2*2 mm

  10. All the pixels have the same spectral response (by 2%) as shown , with an X ray source of 1- 10 keV (128 low resolution spectrometers) 10 keV 3.5 keV

  11. Detection efficiency (Ne - DME) for three configurations • Detector (with ultrathin window) • Detector, thin (12 micron) Be window on the machine and He in between • Detector, thick (400 micron) Be window on the machine and air in between 10 2002 2.5 - 8 keV 1 2003 0.8 - 8 keV 0.1 1 0.2 10 keV

  12. Energy range can be preset changing the GEM voltage 7 6 Ne = 80% DME = 20 % 5 4 3 2 250 0 Pulse height (mV)

  13. National Spherical Tokamak Experiment (NSTX) Princeton Plasma Physics Laboratory (PPPL) Plasma Volume 12 m3 Image on plasma 80 cm*80 cm Central electron temperature = 1.2 (2) keV Central electron density = 8 1019 m-3

  14. NSTX 3- D source (plasma), whose depth is comparable with the ‘focal length’ Tangential view Imaging capability + Energy discrimination Tune up on the X-ray energy of the emitting zone of interest Pinhole Detector

  15. Magneto Hydrodynamic instabilities “macroscopic” Electrodynamical forces J*B, pressure gradient, viscosity, etc.. R, a , aspect ratio, curvatue radius, bending of B lines (elicoidal) etc Spatial scale : many cm Frequencies: 100 Hz - 100 KHz “microscopic” Microturbulence and kinetic instabilities B, E fluctuating fileds and non maxwellian particle energy distribution Larmor radius, Debye length, free mean path for collisions etc.. Spatial scale : up to 1 cm Frequencies: 100 kHz - 1 MHz

  16. Magnetic Plasmas are an harsh environment for measurements Particle and Radiation • Strong thermal radiation emissions visible, ultraviolet, X-ray (Tedge=20 eV , Tcenter = 1-10 keV) • Suprathermal electrons induced by ECRH 20 - 50 keV • Suprathermal electrons induced by LH 80 - 300 keV • Runaways electrons produce  - rays • Fusion processes produce high neutron yield • Electrical disturbs • Strong RF power (many MW) from 30 Mhz to 144 Ghz (a few percent is not absorbed) • Electrical disturbs commutation of high currents and magnetic fields

  17. Time histories for a few pixels d c a b Noise (max) max minimum 20 6 20 97 ms 150 ms 5000 500 420 ms 25 ms d a b c Signal / noise = 1000 Effective dynamic range = 300

  18. H- MODE H # 107314 out Click for animation in down

  19. # 107352 L - MODE Ip = 0.6 MA frequency = 1 kHz)strongly peaked emissivity during PNBI = 1.6 MW L-mode plasma 4000 in

  20. The plasma center is affected by strong oscillations in soft X-ray emission. This effect disappears at r ~ 20 cm # 107352 GEM Click for animation USXR_VTOP USXR_HUP Click for animation

  21. 800 800 IRE # 107320 (ohmic, Ip ~ 1 MA) in current plateau t = 0.237 ms USXR 500 0.239 ms 1

  22. # 107316 Click for animation 10 khz t = 0.37 – 0.39 ms

  23. # 107316 10 khz Click for animation

  24. Example of sampling at 50 kHz 1 kHz # 107356 50 kHz

  25. # 107332 Ip ~ 1 MA, PNBI = 2.0 MW t = 0.33 s H-mode

  26. IMAGE REPRODUCE THE CURVATURE OF THE MAGNETIC SURFACES Sh 7332

  27. ARE THESE SPATIAL MODULATIONS REAL ? 5000 330 ms 280 ms # 107332 Time histories

  28. IT IS A PINHOLE CAMERA : ZOOM ON THE CORE ! View 2 View 1

  29. View 1 View 2

  30. Sampling rate 1 kHz # 107390

  31. Images off axis

  32. To be published in Review of Scientific Instrument, January 2003

  33. Next experimental campaign (2002-2003) New detector layout , compact, light, easy to be moved and shielded

  34. SUMMARY • a novel X-ray system has been set up and validated for fusion magnetic plasmas • it combines spatial imaging capability, at high rates, with energy discrimination • adjustable energy range: 1- 10 keV • high time resolution (up to 100 Khz), extremely high dynamic range (300) • large flexibility in imaging: zoom and changes of the spot • new concept of plasma diagnostic and adapt for real time measurement

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