Milan Aftanas - Institute of Plasma Physics AS CR, v.v.i.
Petra Bilkova - Institute of Plasma Physics AS CR, v.v.i.
P. Bohm - Institute of Plasma Physics AS CR, v.v.i.
V. Weinzettl - Institute of Plasma Physics AS CR, v.v.i.
M. Hron - Institute of Plasma Physics AS CR, v.v.i
R. Panek - Institute of Plasma Physics AS CR, v.v.i.
Dr. Daniel Kaminsky - Elcom, a. s.
T. Wittassek - Elcom, a.s.
M. Rumpel - Elcom, a.s.
J. Sima - Elcom, a.s.
Nuclear fusion is the natural power source of stars. It is the process of multiple atomic nuclei merging together to form a single heavier nucleus. Joining light nuclei, such as hydrogen, creates a large emission of energy. Fusion has the potential to be a safe, clean, and virtually limitless energy source for future generations. However, demanding requirements make controlled fusion for civilian purposes very difficult. Magnetic confinement could be a way to overcome the difficulties of nuclear fusion so we can use this process as an energy source. Recently, we identified tokamaks as the most promising devices for magnetic confinement and nowadays tokamaks are closer to the fusion than any other magnetic confinement or inertial fusion device.
A tokamak is a machine that can sustain high-temperature and high-density plasma using a magnetic field. The Institute of Plasma Physics ASCR, v.v.i., member of the European Atomic Energy Community (EURATOM), participates in the worldwide fusion research program. We reinstalled the COMPASS tokamak (Figure 1), originally located at CCFE Culham, United Kingdom, in IPP Prague, Czech Republic . The first plasma was confined in December 2008.
To research and control plasma behavior and sustain its equilibrium, we needed a set of diagnostic tools. One of the most important parameters for fusion plasma research is the plasma temperature and density. Thomson scattering (TS) is a unique diagnostic for this purpose. It is a laser-aided plasma diagnostic  providing highly localized measurements. Some drawbacks of TS are its complex design and the considerable construction required due to its very low scattering efficiency.
The TS system is under construction on COMPASS now . Figure 2 shows the schematic layout of this system. Basically, it consists of high-power lasers, polychromators to measure scattered spectrums, and fast analog-to-digital converters (ADCs). We used two neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers, both with 30 Hz repetition rates and 1.5 J maximum output energy. Laser light goes through the plasma and is partially scattered. Monochromatic light is spectrally broadened by the scattering. Scattered light from 56 spatial points is led by the complex of collection optics and optical fibers to polychromators (designed at CCFE, United Kingdom) where the incoming light is spectrally analyzed by a cascade of spectral filters and avalanche photodiodes (APD). The system uses up to five spectral channels for each polychromator for spectrum determination. Finally, the signal from each APD is digitized by fast ADCs.
Data Acquisition Requirements
The duration of one laser pulse is 8 ns and lasers can operate in different regimes (see Figure 3): both lasers simultaneously, or both lasers separately with tunable time delay (1 μs–16.6 ms). The requirements on fast ADCs reflects the need to digitize such signals with sufficient sampling rate to reconstruct laser pulse time evolution.
We used a high-speed NI PXI-5152 digitizer and slow D-tAcq ACQ196CPCI ADC cards to synchronously digitize signals from all polychromators (120 spectral channels). The fast ADCs convert data with high 1 GS/s throughput, 8-bit resolution, and interchannel skew less than 300 ps. These ADC cards (two channels per card) have 8 MB per channel onboard memory and are housed in four PXI-1045 chassis.
The first chassis, also called the master chassis, houses an embedded quad-core PXI-8110 controller along with triggering and timing cards to synchronize the remaining three slave chassis. The master chassis stores data, performs calculations, and communicates with the slaves via a remote controller and with the slow ADC cards and COMPASS control system (CODAC) via Ethernet. All channels from all chassis are tightly synchronized with the reference clock from the NI PXI-6653. Using NI TClk technology and built-in phase-locked loops (PLLs), we can achieve less than 300 ps interchannel skew, even in this high-channel-count system. The slow digitizers have 16-bit ADC per channel for true simultaneous analog input with a sampling rate of 500 kS/s. We used two slow ADC cards, each with 96 channels, a 400 MHz reduced instruction set computing (RISC) processor, and 512 MB onboard memory.
We used LabVIEW to write the program controlling the digitizers in the TS system. The basic software functionality includes parameter setup, arming the trigger, acquiring and displaying acquired records, and saving data to a file (see Figure 4.). We will include additional features such as analysis, data interfaces, and more in the future as necessary. The software runs on Microsoft Windows. We could use the LabVIEW Real-Time Module in the future for deterministic operation inside the tokamak control loop.
The laser pulses trigger the data acquisition, so laser timing is currently the limiting factor of the real-time TS on COMPASS. Because TS DAQ hardware and software are modular, in the future we can increase the number of digitizers and possibly laser trigger them using the embedded computer in the master chassis. The data will be acquired in segments.
Thanks to the multirecord acquisition feature of the NI PXI-5152 digitizer, the segments can be acquired with as little as 1 µs between them. Each segment represents one laser pulse, or double pulses in the regime when lasers fire simultaneously or with a very small delay (less than 1 µs). A hardware trigger pulse coming from the lasers initiates each segment acquisition without OS intervention. After the experiment (plasma shot), we download all the segments together from the onboard memory of each digitizer to the embedded computer of the master chassis where the raw data is processed. Calibration data is stored there and it has access to the slow sampled background radiation from the slow ADCs and the laser energy data from the energy monitor. The system integrates the scattered signal, while the calculations of temperature and density performed are sent via Ethernet to CODAC.
COMPASS DAQ system for Thomson scattering diagnostic is able to measure evolution of scattered signal and thus gives us information we need to reconstruct the temperature and density profiles. It also let us measure signal from three planned laser timing settings we need for measure during different plasma condition.
Up to now we have tested all the Thomson scattering system and measure Raman scattering signal.
We would like to thank our English colleagues from Culham laboratory, namely Dr. Michael Walsh (ITER Organization, France) and Dr. Rory Scannell, Dr. Graham Naylor and Dr. Martin Dunstan (Culham Center for Fusion Energy, UK) for great help and collaboration on this project. Part of the MAST design was adopted.
 R. Panek, J. Czech Physics 56 (Suppl. B) (2006) B125-B137.
 A. J. H. Donne et al., Fus. Sci. and Technology 53, 397-430 (2008)
 P. Bilkova et al., Nucl. Instr. and Meth. A (2010), doi:10.1016/j.nima.2010.03.121
The work was performed and supported by the grants GA CR no. 202/09/1467, UFP AVCR (#AV0Z20430508), MSMT #7G10072 and Euratom. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
Institute of Plasma Physics AS CR, v.v.i.
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