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 board128 pixels (2.5 x 2.5 cm2)
To achieve the maximum rate capability, the read-out is divided in4mm2 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
pad Charge preamp
Pulse shaper
discriminator Counterw memory
CPU
104 -106
electrons20 ns
Noise 5 103 electrons
ECL pulses50 ns width
Cables 15 m
Unipolar pulses50 ns width
0.01 - 1 V proportionalto x ray energy
Energydiscriminationlogic output10 ns width
Fast counter(up to 100 Mhz)
ethernet
ELECTRONICS
128 independent channels
gate latching
pulser
Latch freq up to 1 Mhz
VME Crate (CPU, timerpulser, discriminators andScalers 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 Frascati2000-2001 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)
1 2 3 4 5 6 7 8 9 10 11 12
S1
S3
S5
S7
S9
S11
0
0
210100 150
60
700
2250
650
2620
70
270
1320
700
0
0
500
1000
1500
2000
2500
3000
Cont / ms
X (pixel)
Y (pixel)
Imaging capabilities at high rates (> 10e6 ph/s mm2)
Lead screen
X-raysourcePinholes (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 1- 10 keV
0
500
1000
1500
2000
2500
3000
3500
4000
20 40 60 80 100 120 140
Peak amplitude (ch)
3.5 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 between3) Detector, thick (400 micron) Be window on the machine and air in between
20022.5 - 8 keV
0.8 - 8 keV
0,01
0,1
1
1 10
Eff detectEff.LEEffHE
E (keV)
0.2
NSTX
0.2 1 10 keV
0.1
1
10
2003
Energy range can be preset changing the GEM voltage
2
3
4
5
6
7
0 50 100 150 200 250
Vgem=470Vgem=500Vgem=450
Threshold (mV)
Ne = 80%
DME = 20 %
0 250Pulse height (mV)
2
7
3
4
5
6
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 = 8 1019 m-3
Plasma Volume 12 m3
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 energyof the emitting zone of interest
Magneto Hydrodynamic instabilities
Spatial scale : many cm
“macroscopic”
Frequencies: 100 Hz - 100 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 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
Particle and Radiation
25 ms6 20 97 ms
Noise (max) minimum
Signal / noise = 1000 Effective dynamic range = 300
a b
a b
500 150 ms
c
c
d
5000 420 ms
d
max
Time histories for a few pixels
20
in
out
down
# 107314
HH- MODE
Click for animation
Ip = 0.6 MA frequency = 1 kHz)
strongly peaked emissivity during
PNBI = 1.6 MW L-mode plasma
in
# 107352
4000
L - MODE
The plasma center is affected by strong oscillations in soft X-ray emission. This effect disappears at r ~ 20 cm
USXR_VTOP USXR_HUP
GEM# 107352
Click for animation
Click for animation
800
500
t = 0.237 ms
0.239 ms
# 107320 (ohmic, Ip ~ 1 MA) in current plateau
800
USXR
IRE
1
# 107316
10 khz
t = 0.37 – 0.39 ms
Click for animation
# 107316
10 khz
Click for animation
# 107356
Example of sampling at 50 kHz1 kHz
50 kHz
Ip ~ 1 MA, PNBI = 2.0 MW t = 0.33 s H-mode# 107332
Sh 7332
IMAGE REPRODUCE THE CURVATURE OF THE MAGNETIC SURFACES
280 ms330 ms
# 107332
ARE THESE SPATIAL MODULATIONS REAL ?
5000
Timehistories
IT IS A PINHOLE CAMERA : ZOOM ON THE CORE !
View 1 View 2
View 1 View 2
# 107390 Sampling rate 1 kHz
Images off axis
Fast X-ray imaging of the NSTX plasma with a Micro Pattern Gas Detector
based on GEM amplifier
D. Pacella*, R. Bellazzini &, A. Brez & , G. Pizzicaroli*, M. Leigheb*, M. Finkenthal °,
D. Stutman°, B. Blagodjevic °, R. Vero °, R. Kaita ç , D. Johnson ç
* Associazione ENEA-EURATOM sulla Fusione, ENEA – Frascati, It
& Istituto Nazionale di Fisica Nucleare – Pisa, It
° Johns Hopkins University – Baltimore, MD, USA
ç Princeton Plasma Physics Laboratory – Princeton, NJ, USA
To be published in Review of Scientific Instrument, January 2003
Next experimental campaign (2002-2003)
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: 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
SUMMARY
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