Maryna Chernyshova

Development of GEM gas detectors for X-ray crystal spectrometry

Two Triple Gas Electron Multiplier (Triple-GEM) detectors were developed for high-resolution X-ray spectroscopy measurements for tokamak plasma to serve as plasma evolution monitoring in soft X-ray region (SXR). They provide energy resolved fast dynamic plasma radiation imaging in the SXR with 0.1 kHz frequency. Detectors were designed and constructed for continuous data-flow precise energy and position measurement of plasma radiation emitted by metal impurities, W46+ and Ni26+ ions, at 2.4 keV and 7.8 keV photon energies, respectively. High counting rate capability of the detecting units has been achieved with good position resolution. This article presents results of the laboratory and tokamak experiments together with the system performance under irradiation by photon flux from the plasma core.

FPGA based charge fast histogramming for GEM detector

This article presents a fast charge histogramming method for the position sensitive X-ray GEM detector. The energy resolved measurements are carried out simultaneously for 256 channels of the GEM detector. The whole process of histogramming is performed in 21 FPGA chips (Spartan-6 series from Xilinx) . The results of the histogramming process are stored in an external DDR3 memory. The structure of an electronic measuring equipment and a firmware functionality implemented in the FPGAs is described. Examples of test measurements are presented.

Serial data acquisition for the X-ray plasma diagnostics with selected GEM detector structures

The measurement system based on GEM—Gas Electron Multiplier detector is developed for X-ray diagnostics of magnetic confinement tokamak plasmas. The paper is focused on the measurement subject and describes the fundamental data processing to obtain reliable characteristics (histograms) useful for physicists. The required data processing have two steps: 1—processing in the time domain, i.e. events selections for bunches of coinciding clusters, 2—processing in the planar space domain, i.e. cluster identification for the given detector structure. So, it is the software part of the project between the electronic hardware and physics applications. The whole project is original and it was developed by the paper authors. The previous version based on 1-D GEM detector was applied for the high-resolution X-ray crystal spectrometer KX1 in the JET tokamak. The current version considers 2-D detector structures for the new data acquisition system. The fast and accurate mode of data acquisition implemented in the hardware in real time can be applied for the dynamic plasma diagnostics. Several detector structures with single-pixel sensors and multi-pixel (directional) sensors are considered for two-dimensional X-ray imaging. Final data processing is presented by histograms for selected range of position, time interval and cluster charge values. Exemplary radiation source properties are measured by the basic cumulative characteristics: the cluster position distribution and cluster charge value distribution corresponding to the energy spectra. A shorter version of this contribution is due to be published in PoS at: 1st EPS conference on Plasma Diagnostics

X-ray crystal spectrometer upgrade for ITER-like wall experiments at JET

The high resolution X-Ray crystal spectrometer at the JET tokamak has been upgraded with the main goal of measuring the tungsten impurity concentration. This is important for understanding impurity accumulation in the plasma after installation of the JET ITER-like wall (main chamber: Be, divertor: W). This contribution provides details of the upgraded spectrometer with a focus on the aspects important for spectral analysis and plasma parameter calculation. In particular, we describe the determination of the spectrometer sensitivity: important for impurity concentration determination.

Conceptual design and development of GEM based detecting system for tomographic tungsten focused transport monitoring

Implementing tungsten as a plasma facing material in ITER and future fusion reactors will require effective monitoring of not just its level in the plasma but also its distribution. That can be successfully achieved using detectors based on Gas Electron Multiplier (GEM) technology. This work presents the conceptual design of the detecting unit for poloidal tomography to be tested at the WEST project tokamak. The current stage of the development is discussed covering aspects which include detectors spatial dimensions, gas mixtures, window materials and arrangements inside and outside the tokamak ports, details of detectors structure itself and details of the detecting module electronics. It is expected that the detecting unit under development, when implemented, will add to the safe operation of tokamak bringing the creation of sustainable nuclear fusion reactors a step closer. A shorter version of this contribution is due to be published in PoS at: 1st EPS conference on Plasma Diagnostics

Determination of tungsten and molybdenum concentrations from an x-ray range spectrum in JET with the ITER-like wall configuration

The W and W 3p-4d inner shell excitation lines in addition to Mo 2p3s lines have been identified from the spectrum taken by an upgraded high-resolution X-ray spectrometer. It is found from analysis of the absolute intensities of the W and Mo lines that W and Mo concentrations are in the range of ∼ 10−5 and ∼ 10−7, respectively. Comparison of the W concentration from the X-ray spectrometer with those from a vacuum-ultra-violet spectrometer and from soft X-ray cameras indicates that the W concentration from the X-ray spectrometer is lower by a factor of ∼ 2 and > 7, respectively. In contrast, comparison of a plasma effective charge determined from the X-ray spectrometer with that from a visible range continuum intensity shows that the plasma effective charge from the X-ray spectrometer is higher by a factor of ∼ 3. Hence it is probable that the W concentration from the X-ray spectrometer is valid within a factor of ∼ 3, while the W concentrations from the vacuum-ultra-violet spectrometer and from the soft X-ray cameras are further higher. Determination of tungsten and molybdenum concentrations from an X-ray range spectrum in JET2 Submitted to: J. Phys. B: At. Mol. Phys. Determination of tungsten and molybdenum concentrations from an X-ray range spectrum in JET3

Serial data acquisition for GEM-2D detector

This article debates about data fast acquisition and histogramming method for the X-ray GEM detector. The whole process of histogramming is performed by FPGA chips (Spartan-6 series from Xilinx). The results of the histogramming process are stored in an internal FPGA memory and then sent to PC. In PC data is merged and processed by MATLAB. The structure of firmware functionality implemented in the FPGAs is described. Examples of test measurements and results are presented.

FPGA-based GEM detector signal acquisition for SXR spectroscopy system

The presented work is related to the Gas Electron Multiplier (GEM) detector soft X-ray spectroscopy system for tokamak applications. The used GEM detector has one-dimensional, 128 channel readout structure. The channels are connected to the radiation-hard electronics with configurable analog stage and fast ADCs, supporting speeds of 125 MSPS for each channel. The digitalized data is sent directly to the FPGAs using fast serial links. The preprocessing algorithms are implemented in the FPGAs, with the data buffering made in the on-board 2Gb DDR3 memory chips. After the algorithmic stage, the data is sent to the Intel Xeon-based PC for further postprocessing using PCI-Express link Gen 2. For connection of multiple FPGAs, PCI-Express switch 8-to-1 was designed. The whole system can support up to 2048 analog channels. The scope of the work is an FPGA-based implementation of the recorder of the raw signal from GEM detector. Since the system will work in a very challenging environment (neutron radiation, intense electro-magnetic fields), the registered signals from the GEM detector can be corrupted. In the case of the very intense hot plasma radiation (e.g. laser generated plasma), the registered signals can overlap. Therefore, it is valuable to register the raw signals from the GEM detector with high number of events during soft X-ray radiation. The signal analysis will have the direct impact on the implementation of photon energy computation algorithms. As the result, the system will produce energy spectra and topological distribution of soft X-ray radiation. The advanced software was developed in order to perform complex system startup and monitoring of hardware units. Using the array of two one-dimensional GEM detectors it will be possible to perform tomographic reconstruction of plasma impurities radiation in the SXR region.

Fast modular data acquisition system for GEM-2D detector

A novel approach to two dimensional Gas Electron Multiplier (GEM) detector readout is presented. Unlike commonly used methods, based on discriminators and analogue FIFOs, the method developed uses simulta- neously sampling high speed ADCs with fast hybrid integrator and advanced FPGA-based processing logic to estimate the energy of every single photon. Such a method is applied to every GEM strip / pixel signal. It is especially useful in case of crystal-based spectrometers for soft X-rays, 2D imaging for plasma tomography and all these applications where energy resolution of every single photon is required. For the purpose of the detector readout, a novel, highly modular and extendable conception of the measurement platform was developed. It is evolution of already deployed measurement system for JET Spectrometer.

Optimization of FPGA processing of GEM detector signal

This paper presents analysis of processing method of the signal from Gas Electron Multiplier (GEM) detector acquired in our Field-Programmable Gate Array (FPGA) based readout system. We have found that simple processing of GEM signal received from the charge amplifier, sampled at 100MHz with 10-bit resolution, after low-pass filtering with 15 MHz cut-off frequency, provides accuracy similar to obtained by processing of the raw GEM signal sampled at 2.5 GHz frequency with 8-bit resolution. Even when 3 bits are lost due to long term instability of the detector and analog part of the system - resulting in 7-bit effective resolution, the reasonable accuracy is still preserved. Additionally we have analyzed computational power required to perform the real-time analysis of the GEM signal, taking into consideration resources offered by the FPGA chip used in the prototype platform.