Thomson scattering is an important technique for plasma diagnostics in nuclear fusion facilities such as tokamaks and stellarators. It is used to measure both the electron temperature and density at many locations inside the plasma.
The measurement is conducted using a pulsed laser, typically a Ruby laser or Nd:YAG, that generate repeated pulses that are shone into the plasma. Some of the laser light (photons) is scattered of free electrons in the plasma resulting in spectral expansion due to the Doppler effect.
Avalanche photo diodes (APDs) detect the scattered photons and high-performance digitizers capture the resulting waveforms. The spectral expansion helps assess the plasma temperature whereas the number of scattered photons determine the plasma density.
- Thomson scattering systems often consist of several hundred channels commissioned over several stages. It is therefore beneficial to use modular instruments to create a compact yet scalable solution that easily can be expanded over time to host more measurement channels.
- High dynamic range is required to accurately measure pulse characteristics and determine plasma density
- High analog bandwidth and sampling rate is required to match the performance of the detectors and to provide high spatial resolution
- High board-level channel density helps keep the overall size down and total number of required boards reasonable
- A flexible and accurate trigger input is crucial to ensure that it shares a common time reference with the laser
- Synchronization capabilities is required for accurate multi-channel data capture and form factors such as MTCA and PXIe helps keep external cabling reasonable since triggers and clocks are distributed in the backplane
- The on-board memory need to be sufficient to store the entire scattered waveform
- Data throughput needs to be high enough so that waveforms from all channels can be transferred to a central processing unit in the time slot between consecutive triggers
- An open on-board FPGA offers a future-proof solution where calculations can be performed in real-time to achieve data reduction and therefore lower the data throughput requirements
Our Thomson scattering customers typically use ADQ14DC-4C in MTCA or PXIe form factor. It is a quad-channel, 14-bit, 1 GS/s digitizer with 700 MHz analog input bandwidth that provide an excellent match with commonly used detectors. It offers highly accurate backplane trigger and synchronization capabilities in order to support massive multi-channel data capture while keeping external cabling to a minimum.
Furthermore, it hosts 2 Gbyte on-board data memory capable of storing very long pulses and supports a native data transfer rate of 3.2 Gbyte/s which can be extended even more through optional firmware upgrades. The customer can also gain access to the on-board Xilinx Kintex 7 K325T FPGA if custom real-time processing is required. The optional firmware and development kits can be purchased separately at any time and upgrades can be done in place without any need for system disassembly or significant downtime.