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

Printed circuit board 128 pixels (2.5 x 2.5 cm 2 ) To achieve the maximum rate capability, the read-out is divided in 4mm 2 pads that work as independent X-ray counters. Each pad is provided with a whole electronic chain: Pre-amplifier  Amplifier  Discriminator  Gated counter PARALLEL READ-OUT

Detector and front end amplifiers VME acquisition system Threshold Host PC padCharge preamp Pulse shaper discriminatorCounter w memory CPU electrons 20 ns Noise electrons ECL pulses 50 ns width Cables 15 m Unipolar pulses 50 ns width V proportional to x ray energy Energy discrimination logic output 10 ns width Fast counter (up to 100 Mhz) ethernet ELECTRONICS 128 independent channels gatelatching pulser Latch freq up to 1 Mhz

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

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

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

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)

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

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

Detection efficiency (Ne - DME) for three configurations 1)Detector (with ultrathin window) 2)Detector, thin (12 micron) Be window on the machine and He in between 3)Detector, thick (400 micron) Be window on the machine and air in between keV keV keV

Energy range can be preset changing the GEM voltage Ne = 80% DME = 20 % Pulse height (mV)

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

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

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

Magnetic Plasmas are an harsh environment for measurements Strong thermal radiation emissions visible, ultraviolet, X-ray (Tedge=20 eV, Tcenter = 1-10 keV) Suprathermal electrons induced by ECRH keV Suprathermal electrons induced by LH 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 Particle and Radiation

25 ms ms Noise (max) minimum Signal / noise = 1000Effective dynamic range = 300 ab ab ms c c d ms d max Time histories for a few pixels 20

in out down # H H- MODE Click for animation

I p = 0.6 MA frequency = 1 kHz) strongly peaked emissivity during P NBI = 1.6 MW L-mode plasma in # L - MODE

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

t = ms ms # (ohmic, I p ~ 1 MA) in current plateau 800 USXR IRE 1

# khz t = 0.37 – 0.39 ms Click for animation

# khz Click for animation

# Example of sampling at 50 kHz 1 kHz 50 kHz

I p ~ 1 MA, P NBI = 2.0 MW t = 0.33 s H-mode #

Sh 7332 IMAGE REPRODUCE THE CURVATURE OF THE MAGNETIC SURFACES

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

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

View 1View 2

# Sampling rate 1 kHz

Images off axis

To be published in Review of Scientific Instrument, January 2003

Next experimental campaign ( ) New detector layout, compact, light, easy to be moved and shielded

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: 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 SUMMARY