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

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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

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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)

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NSTX

0.2 1 10 keV

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10

2003

Energy range can be preset changing the GEM voltage

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0 50 100 150 200 250

Vgem=470Vgem=500Vgem=450

Threshold (mV)

Ne = 80%

DME = 20 %

0 250Pulse height (mV)

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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