Advantages of Al based GEM detector aimed at plasma soft-semi hard X-ray radiation imaging

Development of gaseous detectors, more specifically Gas Electron Multiplier (GEM) based detectors, for application at tokamak plasma radiation monitoring/imaging in Soft−Semi Hard X-ray (S−SH) region is an ongoing research activity aiming to deliver valuable information on plasma shape, magnetic configuration, non-axisymmetry phenomena of the plasma, etc. Wide radiation range and brightness of plasma radiation impose some restrictions on choice of materials in the detecting chamber, as their interaction with the incident radiation may disrupt original signals. This work proposes usage of aluminum as GEM foils electrodes for the first time. The detector based on these foils was constructed and examined. The operational characteristics and spectral capabilities of such detector were compared with the ones based on the standard (commonly used) copper GEM foils. The laboratory tests were performed using X-ray tube and 55Fe sources to examine detectors’ capabilities in energy-resolved imaging. Additionally, simulations of origin and number of the generated electrons, which determine the detector signal, were performed for Al and Cu GEM foils for a wide energy range of incident photons. The experimental and modelling data demonstrated that Cu based GEM detector produces higher parasitic signal than Al one necessitating total elimination of copper from detector’s chamber

Study of tungsten surface interaction with plasma streams at DPF-1000U

In this note experimental studies of tungsten (W) samples irradiated by intense plasma-ion streams are reported. Measurements were performed using the modified plasma focus device DPF-1000U equipped with an axial gas-puffing system. The main diagnostic tool was a Mechelle®900 optical spectrometer. The electron density of a freely propagating plasma stream (i.e., the plasma stream observed without any target inside the vacuum chamber) was estimated on the basis of the half-width of the Dβ spectral line, taking into account the linear Stark effect. For a freely propagating plasma stream the maximum electron density amounted to about 1.3 × 1017 cm−3 and was reached during the maximum plasma compression. The plasma electron density depends on the initial conditions of the experiments. It was thus important to determine first the plasma flow characteristics before attempting any target irradiation. These data were needed for comparison with plasma characteristics after an irradiation of the investigated target. In fact, spectroscopic measurements performed during interactions of plasma streams with the investigated W samples showed many WI and WII spectral lines. The surface erosion was determined from mass losses of the irradiated samples. Changes on the surfaces of the irradiated samples were also investigated with an optical microscope and some sputtering and melting zones were observed

Studies of plasma interactions with tungsten targets in PF-1000U facility

This paper presents results of experimental studies of tungsten samples of 99.95% purity, which were irradiated by intense plasma-ion streams. The behaviour of tungsten, and particularly its structural change induced by high plasma loads, is of great importance for fusion technology. The reported measurements were performed within a modified PF-1000U plasma-focus facility operated at the IFPiLM in Warsaw, Poland. The working gas was pure deuterium. In order to determine the main plasma parameters and to study the behaviour of impurities at different instants of the plasma discharge, the optical emission spectroscopy was used. The dependence of plasma parameters on the initial charging voltage (16, 19 and 21 kV) was studied. Detailed optical measurements were performed during interactions of a plasma stream with the tungsten samples placed at the z-axis of the facility, at a distance of 6 cm from the electrode outlets. The recorded spectra showed distinct WI and WII spectral lines. Investigation of a target surface morphology, after its irradiation by intense plasma streams, was performed by means of an optical microscope. The observations revealed that some amounts of the electrodes material (mainly copper) were deposited upon the irradiated sample surface. In all the cases, melted zones were observed upon the irradiated target surface, and in experiments performed at the highest charging voltage there were formed some cracks

Synchronization between computation and acquisition parts in the GEM detector-based measurement system

The presented system is used for monitoring of the plasma impurities in the tokamak. It is done by measuring radiation in Soft X-Ray range with the use of a GEM-based detector. Acquired data is transferred through the whole system with low latency. Presented system can be divided into many parts - detector, analog electronics, FPGA, PCIe transmission line and computer system with high-performance CPU. This work will concentrate on synchronization between FPGA, which write data to the memory on the CPU side and computational part, which is executed in the computer. In long-running measurements, there is a synchronization problems which can arise. There is a difference in variables based on which the execution time of both parts is dependent. Working on measurements of radiation of plasma impurities requires limits in terms of latency. This paper presents reasons, descriptions and solutions for such problems

Recent progress in L-H transition studies at JET: Tritium, Helium, Hydrogen and Deuterium

We present an overview of results from a series of L-II transition experiments undertaken at JET since the installation of the ITER-like-wall (JET-ILW), with beryllium wall tiles and a tungsten divertor. Tritium, helium and deuterium plasmas have been investigated. Initial results in tritium show ohmic L-H transitions at low density and the power threshold for the L-H transition (P-LH) is lower in tritium plasmas than in deuterium ones at low densities, while we still lack contrasted data to provide a scaling at high densities. In helium plasmas there is a notable shift of the density at which the power threshold is minimum ((n) over bar (e,min)) to higher values relative to deuterium and hydrogen references. Above (n) over bar (e,min) (He) the L-H power threshold at high densities is similar for D and He plasmas. Transport modelling in slab geometry shows that in helium neoclassical transport competes with interchange-driven transport, unlike in hydrogen isotopes. Measurements of the radial electric field in deuterium plasmas show that E-r shear is not a good indicator of proximity to the L-H transition. Transport analysis of ion heat flux in deuterium plasmas show a non-linearity as density is decreased below (n) over bar (e,min). Lastly, a regression of the JET-ILW deuterium data is compared to the 2008 ITPA scaling law