WDS'10 Proceedings of Contributed Papers, Part II, 28–32, 2010.
ISBN 978-80-7378-140-8 © MATFYZPRESS
Data Acquisition System and Data Processing
for the New Thomson Scattering System
on the COMPASS Tokamak
M. Aftanas
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.
Academy of Sciences, Institute of Plasma Physics, Czech Republic.
P. Bı́lková, P. Böhm, V. Weinzettl, J. Stöckel, M. Hron, R. Pánek
Academy of Sciences, Institute of Plasma Physics, Czech Republic.
R. Scannell, M. Walsh
Culham Centre for Fusion Energy, Oxfordshire, United Kingdom.
Abstract. The Thomson scattering (TS) will be one of crucial plasma diagnostics
of the COMPASS tokamak. This newly build-up multi-point system consists of two
Nd:YAG lasers (1.6J at 1064nm, 30Hz each) and cascade filter polychromators
with avalanche photodiodes. It will enable measurements of both the electron
temperature Te (20eV − 5000eV ) and density ne (1019 − 1020 m−3 ) profiles with
spatial resolution up to 3mm in the vertical direction in 56 spatial points. The
uniquely designed complex data acquisition system based on fast analog digital
convertors (1GS/s) reflects the need to retrieve/digitize the signal originated from
scattering process of the laser pulse lasting less than 10ns. This paper presents a
detailed review of the architecture of the control and the data acquisition (DAQ)
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system and its features. LabView
will be used as a main layer for the TS
data acquisition. Routines specifically written for controlling the DAQ of TS
on COMPASS are presented. Moreover, control tool developed for calibrating
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procedures by Labview
of the TS is shown.
Introduction
COMPASS (COMPact ASSembly) is a divertor tokamak, reinstaled recently in the Institute of Plasma Physics AS CR, v.v.i. in Prague [Pánek et al. [2006]]. It was operated originally
at UKAEA (now CCFE), Culham, UK. COMPASS (R = 0.56m, r = 0.2m, I = 320kA, Btor
up to 2.1T and pulse length up to 1s), it is the smallest tokamak with a clear H-mode and an
ITER-like plasma geometry. COMPASS is equipped with a unique set of copper saddle coils
for resonant perturbation techniques. Many diagnostics have been designed and are being built
[Weinzettl et al. [2010]]. Neutral beam injectors (NBI) heating and lower hybrid (LH) systems
will be implemented to reach relevant plasma conditions. Proposed COMPASS scientific program is focused on studies of pedestal, H-mode and plasma-wall interactions. The new high
resolution Thomson Scattering (TS) system shall contribute to this aim by detail measurements
of both the electron temperature and the density profiles. Therefore, high priority is given to
the spatial resolution namely at the plasma edge and in the pedestal region. The TS, as a noninvasive technique, is a powerful tool to determine electron temperature Te and density ne from
Doppler-shifted and broadened monochromatic light. Due to the low scattering cross-section,
the diagnostics leads to a complex design and considerable requirements on the construction.
Thus the practical implementation is quite demanding and limits the diagnostic.
For TS on COMPASS, two independent Nd:YAG lasers (1064nm, 30Hz, 1.5J each) were
used to produce highly intense monochromatic light. The light scattered by the plasma is then
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AFTANAS ET AL.: DAQ FOR THE NEW TS SYSTEM ON THE COMPASS
collected, divided into 5 spectral channels by means of filter cascade [Bı́lková et al. [2010]],
and detected by avalanche photodiodes located inside the polychromators. Signals from the
photodiodes are digitized and consequently ne and Te are determined.
Data Acquisition
Requirements for data acquisition are quite tough. Three main parameters has to be fulfil.
Each laser pulse has a duration about 10ns. Proper sampling rate is necessary to fit the time
evolution of low and noisy TS signals with reasonable resolution, moreover signals from all 120
channels (28 polychromators, 4 or 5 channels each) have to be acquired simultaneously. These
signals are synchronously digitized by fast and slow Analog-to-Digital Convertors (ADCs). The
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fast ADCs (NI
PXI-5152) convert data with high throughput of 1GSample/sec, sufficient 8bit
resolution and good inter-channel skew < 300ps. These ADC cards (2 channels in each ADC
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card) have 8M B/channel onboard memory and are housed in 4 chassis (NI
PXI-1045). First
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chassis, so called master chassis, has embedded computer (NI PXI-8110, Intel Core 2 Quad
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Q9100 processor) and triggering and timing cards (NI
PXI-6653 and PXI-6652) to synchronize
remaining 3 chassis (slave chassis). The master chassis is able to store data, perform calculations
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and communicate (NI
PXI-8331) with the slave ones via MXI-4 technology (78M bytes/s) and
with the slow ADC cards and COMPASS control system (CODAC) via ethernet ([Valcárcel et
al. [2009]]). The slow digitizers (D-tAcq ACQ196CPCI) have 16bit ADC per channel for true
simultaneous analog input with sampling rate of 500kSamples/sec. Two slow ADC cards are
used, each 96 channels, 400M Hz RISC processor, 512M B onboard memory.
Schematic layout of the the DAQ system is shown in the Figure 1. The fast digitizers have
been tested and are ready to be tightly synchronized with the reference clock by the phaselocked-loop technology. The data acquisition will be triggered by the laser pulses and thus the
laser timing is for now the limiting factor of the real-time TS on COMPASS. Modular design of
TS DAQ allows future expansion of the digitizers and possible laser triggering by the embedded
computer in the master chassis of fast digitizers.
The data will be acquired in segments. Each segment represents one laser pulse or double
pulses in case of firing lasers simultaneously or with very small delay (< 1us). All segments
together will be downloaded after the experiment (plasma shot) from the onboard memory
of each digitizer to the embedded computer of the master chassis where the raw data will be
processed. Spectral and Raman calibration data will be stored there and it will have access to
Figure 1. Scheme of the TS on COMPASS with the layout of the triggers and data flow.
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AFTANAS ET AL.: DAQ FOR THE NEW TS SYSTEM ON THE COMPASS
the slow sampled background radiation from the slow ADCs and the laser energy data from
energy monitor. The scattered signal will be integrated, the calculations of Te and ne performed
(relativistic formula for TS Naito [1993]) and results send via ethernet to CODAC.
Control and calibration routines
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LabView
is used as programming language for controlling the ADC system and polychromator calibration procedures. Basic ADC control program was provided by manufacturer.
These routines were modified with respect to system layout and triggering was implemented.
Two main calibrations are necessary to perform - spectral and intensity calibration [Scannell
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[2009]]. LabView
routine was written to automate the spectral calibration. These calibrations
are being carried out (Figure 2).
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Figure 2. Spectral calibration in progress. Routine written in LabView
.
Figure 3. Parameters of collection optics: F/# numbers and back image of the fibers for the
edge TS spatial points.
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AFTANAS ET AL.: DAQ FOR THE NEW TS SYSTEM ON THE COMPASS
Error analysis
During the design process several calculations were performed while keeping ne , scattering
angle, F number and length of scattering volume constant [Aftanas et al. [2009]] to obtain
relevant information about expected parameters of the COMPASS TS system. These analysis
were done with the MAST team and using MAST IDL codes [Scannell et al. [2008]]. As the
last built part of the COMPASS TS system, designed parameters of collection optics [Bı́lková
et al. [2010]] were taken (Figure 3) and simulations were performed to evaluate qualities of
the design by estimating expected Te and ne errors. For such purpose simulations for varying
input parameters have been modified. Angular effect, different spatial resolution, F number
and expected electron temperature and density profiles were taken into account. The error
on scattered signal was modeled as contribution of Poisson distribution, background light and
amplifier noise error. In the Figure 4 input profiles of Te and ne (continuous line) are shown with
the simulated values (the dots and the error bars). Finite length of scattering volume (spatial
resolution) caused small distortion of the fitted pedestal width (become wider). Relative error
of fitted pedestal width for given profiles is below 5%.
Conclusion
The data acquisition system is being built. Unique synchronization of the channels was
implemented. Fast digitizers were tested and the layout of the DAQ was designed. It allows
flexible measurements in the different laser firing modes and the system can be synchronized
with the central timing unit. New simulations were performed with respect to real parameters
of the system and expected plasma profiles. Estimated errors are sufficiently low to determine
pedestal shape and position. Existing routines are being modified and extended. New routine
was tested and is ready to use.
Figure 4. Theoretical (continuous line) and simulated (dots and error bars) ne and Te profiles.
Finite spatial resolution caused broadening of the fitted pedestal.
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AFTANAS ET AL.: DAQ FOR THE NEW TS SYSTEM ON THE COMPASS
Acknowledgments. This work was supported by Czech Science Foundation, grant 202/08/H057,
the Academy of Sciences of the Czech Republic IRP #AV0Z20430508, the Ministry of Education, Youth
and Sports CR #7G09042 and European Communities under the contract of Association between EURATOM/IPP.CR No. FU07-CT-2007-00060. The views and opinions expressed herein do not necessarily
reflect those of the European Commission.
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