Plasma Diagnostic Systems Research Articles

A thorough experimental assessment of THz-TDS plasma diagnostic techniques for nuclear fusion applications.

In this paper, the study of a plasma diagnostic system based on the THz time domain spectroscopy technique is presented. Such a system could potentially probe a large part of the electromagnetic spectrum currently covered by several other diagnostics in a single measurement. This feature, keeping in mind the basic requirements for plasma diagnostics in nuclear fusion experiments, such as robustness and hard environment applicability, as well as durability and low maintenance, makes the diagnostic of great interest. A conceptual design of the THz-TDS diagnostic has been developed, starting from the well-established classical microwave and far infrared plasma diagnostics landscape. The physical constraints and required instrumental characteristics have been studied and are described in detail here, together with the solutions available for each type of plasma measurement. Specific experimental laboratory tests of the different experimental configurations have been carried out, evaluating the capacity and potential of the novel diagnostic, together with the instrumental constraint, within the diagnostic parameter space.

Measurement of plasma parameters in the PBI using the Langmuir probe

When studying the processes of plasma interaction with the surface, there is a need to determine the exposure parameters and the installation operating modes. The goal of this work is to measure the plasma parameters on a plasma beam installation (PBI) using an automated probe diagnostics system developed by us.The plasma diagnostic and monitoring system at PBI is based on the use of a cylindrical Langmuir probe. The software has been developed for measuring, recording and processing the probe voltage characteristic (I-U curve) in real time and a movable probe input for measuring the radial distribution of the plasma parameters (plasma concentration, electronic temperature). The measurements were carried out under different operating modes of the PBI with a variation in the parameters of the accelerating voltage of the electron beam and the pressure of the working gas. The nature of changes in the plasma parameters in the radial direction in a hydrogen medium has been established. The obtained results are in good agreement with the similar work in the study of the plasma parameters in the radial direction at the linear plasma installations.The got dependences make it possible to analyze and establish a correspondence between the parameters of plasma action on materials and the nature of processes on their surface and in volume. The presented engineering idea will be proposed to complement the diagnostic complex of the plasma KTM tokamak.

Verification, validation, and uncertainty quantification for electron-density diagnostics of polarimeter/interferometer system on experimental advanced superconducting tokamak

The verification, validation, and uncertainty quantification (VVUQ) analysis is a systematic method for identifying and quantifying uncertainties in complex engineering projects. This method has been used by the International Atomic Energy Agency as a critical assessment method in the field of high-precision plasma diagnostics in tokamaks. Starting from the basic theory to the final results, it assesses every process to obtain the uncertainties, system confidence level, and similarity between the model and experimental assessments. In this study, we evaluated the electron-density diagnosis of the polarimeter/interferometer system on an experimental advanced superconducting tokamak using a cylindrical simulation model. This study applied the VVUQ to an entire plasma diagnostic system for the first time, coupled with the statistical concepts of confidence and similarity. Through the VVUQ process, the uncertainty from the simulation code, diagnostic code, and engineering errors were identified, quantified, and minimized.

Neutron shielding calculation for DEMO-Prad/SXR measurement system

In plasma fusion devices, the selection of shielding materials is one of the challenges for plasma diagnostic and control systems under high radiation levels during long operation times. This study presents the effect of shielding materials on neutron flux for a radiated power and soft x-ray core intensity measurement system for a DEMOnstration power plant. A calculation model of neutron shielding was used to investigate the neutron shielding performance of borides and carbides for a large distance from plasma to the diagnostic system. The neutron fluxes were characterized for three points close to the measurement system location. The related neutronic calculations were performed with an ADVANTG hybrid code to obtain neutron flux distribution and attenuation rate depending on the thickness of shielding materials. The results indicate that B4C, W2B5, and WB4 are the most effective options to serve as shielding material due to the effect of boron on neutron shielding effectiveness.

MEPhIST-0 integrated control and data acquisition system

The growing fusion research industry faces an extensive need for specialists with practical skills. However, the high complexity of the relevant experiments makes it difficult to maintain laboratories in which students can get their first hands-on experience of interacting with relevant devices. One of the options is using small, simplified machines that are designed for low-qualified personnel. This paper presents the architecture and implementation of a control and data acquisition system for an educational tokamak MEPhIST-0. It integrates different plasma diagnostic systems as well as tokamak's control nodes using the EPICS framework. Different approaches to data archiving and access are discussed, and the overall design is presented from the perspective of a primarily educational installation, as well as the preliminary results.

Neutronics Simulations for DEMO Diagnostics

One of the main challenges in the development of a plasma diagnostic and control system for DEMO is the need to cope with unprecedented radiation levels in a tokamak during long operation periods. A list of diagnostics required for plasma control has been developed during the pre-conceptual design phase. Different approaches are proposed for the integration of these diagnostics in DEMO: in equatorial and upper ports, in the divertor cassette, on the inner and outer surfaces of the vacuum vessel and in diagnostic slim cassettes, a modular approach developed for diagnostics requiring access to the plasma from several poloidal positions. According to each integration approach, diagnostics will be exposed to different radiation levels, with a considerable impact on their design. This paper provides a broad overview of the radiation environment that diagnostics in DEMO are expected to face. Using the water-cooled lithium lead blanket configuration as a reference, neutronics simulations were performed for pre-conceptual designs of in-vessel, ex-vessel and equatorial port diagnostics representative of each integration approach. Flux and nuclear load calculations are provided for several sub-systems, along with estimations of radiation streaming to the ex-vessel for alternative design configurations. The results can be used as a reference by diagnostic designers.

Evaluation of optical transmission across the ITER hard x-ray monitor system designed for the first plasma scenarios.

Hard x-ray (HXR) spectroscopy is applied for diagnostics of runaway electrons in nuclear fusion reactors. The scintillation counter is one of the most commonly used types of detectors for HXR spectroscopy. It consists of a detector that emits light when excited by HXR radiation (scintillator) directly coupled to a PMT (Photomultiplier Tube) that converts light pulses into an electrical signal. This type of detector is commonly used in existing tokamaks, such as Joint European Torus (JET),Experimental Advanced Superconducting Tokamak (EAST),Compact Assembly (COMPASS), andAxially Symmetric Divertor Experiment (ASDEX-U). In all these cases, the scintillator is directly coupled to the PMT to provide the best possible light transmission efficiency. The Hard X-ray Monitor (HXRM) is one among the first plasma diagnostic systems at ITER that provides information about the energy distribution of runaway electrons inside a tokamak by HXR spectroscopy. This system also uses a scintillator and a PMT as a detector. Due to the heavy shielding of the blanket modules, vacuum vessel, and port-plugs, it is not possible to assemble the scintillator outside the tokamak vacuum vessel. The PMT detector cannot be installed in the close vicinity of the tokamak due to either the significant magnetic field or temperature. A possible solution is to decouple the scintillator from the PMT and place the PMT inside the port-cell. Light pulses will be transmitted to the PMT via a 12m long optical fiber bundle. Evaluation of the optical transmission was carried out to assess the performance of the HXR monitor and verify possible problems related to the PMT pulse discrimination under low light conditions.

High-precision digital Faraday cups for FEEP thrusters

The number of scientific missions that require an extremely precise alignment of spacecraft achieved by electric thrusters has increased significantly in recent years. This requires a highly accurate knowledge of the thrust vector misalignment and the thrust intensity variation over time. In order to be able to resolve this high accuracy of the thruster beam, a high-precision plasma diagnostics system is required. For example, for upcoming missions like NGGM, it is required to get a spatial resolution of the ion beam current measurements <0.5° in polar and azimuthal direction. FOTEC's plasma diagnostics system based on Faraday cup analysis was optimised in order to be able to analyse the thrust vector misalignment with high spatial resolution. On the one hand, the geometry was adapted to keep the reduction of the signal due to secondary electron emission at a minimum. On the other hand, the entire measuring electronics were built into the head of each Faraday cup. This completely eliminates signal interference due to the cable length and movement of the diagnostics arm. Measurements of the digital Faraday cup diagnostics system will be presented, which were carried out at an IFM Nano Thruster. In particular, the signal-to-noise ratio could be significantly improved.

Monolithic Integration of Graphene in SiC Radiation Sensors for Harsh-Environment Applications

Due to their low leakage current, low noise levels, high thermal conductivity, and potential radiation hardness, SiC devices offer various advantages over Si devices in certain applications. As a result, they are being considered for operation in harsh environments, such as plasma diagnostic systems in future nuclear fusion reactors or in high energy physics applications. We report on relevant results of the GRACE project, which seeks to deliver a new generation of SiC sensors with graphene-enhanced contacts. Such devices are aimed to be radiation-hard and functional at high temperatures. The work presented in this paper focuses on the optimisation of the electrical contacts, along with the electrical characterisation and radiation-tolerance assessment of the first sensor prototypes produced.

Time-resolved ion energy distribution in pulsed inductively coupled argon plasma with/without DC bias

Pulsed plasmas have emerged as promising candidates as a means for precise control of ion energy/angle dependent surface processes and surface chemistry during the plasma process, which are key to 3 nm and beyond device fabrication. The ion energy distribution functions (IEDFs) and ion fluxes over a pulsed period are important to understand as they directly influence the feature profile, damage, and selectivity. We have developed an advanced plasma diagnostics (APD) system with advanced pulsing capability, including source, bias, and synchronous pulsing. It is a compact inductively coupled plasma system with a RF source frequency of 13.56 MHz intended to diagnose the general behavior of biased high density plasmas. We report the effect of the pulse frequency (2–10 kHz), RF duty cycle (25%–75%), DC duty cycle (5%–50%), phase lag (50–60 μs), RF power (120–180 W), DC bias voltage (0–150 V), and discharge pressure (20–80 mTorr) on the IEDFs and ion flux over a pulse period on the APD system. The time-resolved IEDFs and ion flux were measured using a retarding field energy analyzer. The ion energy transitions in a pulsed period from a plasma ignition stage to a stable stage and from plasma in a glow period to an afterglow period are studied. The results indicate that the ion energy and ion flux are tailored by RF pulsing and RF-DC pulsing. The time-resolved IEDF demonstrates the merits of pulsing to precisely control ion energy and flux, and the ion energy spread was narrowed by the pulsed plasma.

Development of a concept and basis for the DEMO diagnostic and control system

An initial concept for the plasma diagnostic and control (D&C) system has been developed as part of European studies towards the development of a demonstration tokamak fusion reactor (DEMO). The main objective is to develop a feasible, integrated concept design of the DEMO D&C system that can provide reliable plasma control and high performance (electricity output) over extended periods of operation. While the fusion power is maximized when operating near to the operational limits of the tokamak, the reliability of operation typically improves when choosing parameters significantly distant from these limits. In addition to these conflicting requirements, the D&C development has to cope with strong adverse effects acting on all in vessel components on DEMO (harsh neutron environment, particle fluxes, temperatures, electromagnetic forces, etc.). Moreover, space allocation and plasma access are constrained by the needs for first wall integrity and optimization of tritium breeding. Taking into account these boundary conditions, the main DEMO plasma control issues have been formulated, and a list of diagnostic systems and channels needed for plasma control has been developed, which were selected for their robustness and the required coverage of control issues. For a validation and refinement of this concept, simulation tools are being refined and applied for equilibrium, kinetic and mode control studies.

Plasma diagnostic systems and methods used on the stellarator TJ-II

The TJ-II is a heliac-type stellarator device with major radius of 1.5 m and averaged minor radius ⩽0.22 m that has been operated at CIEMAT, Madrid since 1998. Its full magnetic field is created by a system of poloidal, central, toroidal and vertical field coils, thus it possesses a fully 3-dimensional plasma structure and a bean-shaped plasma cross-section. Although this results in a complicated vacuum-vessel layout, it has excellent port access for diagnostics (96 portholes). During its initial operational phase, it was equipped with a limited set of essential diagnostics. Since then, a broad variety and large number of both passive and active diagnostics have been installed. The former include Hα monitors, light spectrometers, an electron cyclotron emission radiometer, X-ray filter monitors, neutral particle analysers, magnetic diagnostics, as well as cameras, among others, while the latter include various laser, atomic and ion beam based diagnostics, microwave probe beams, Langmuir probes plus impurity injection techniques. In this paper, after describing the TJ-II stellarator, its heating and fuelling systems, the diagnostic systems employed are outlined and discussed briefly here. Finally, results obtained with selected systems are highlighted.

The far-field plasma characteristics of LHT40 low-power Hall thruster for commercial aerospace applications

To achieve a better insight into the far-field plasma spatial distribution and evolution characteristics of the 300 W class low-power Hall thruster (LHT) for commercial aerospace applications, a dedicated and integrated plasma diagnostic system composed of seventeen Faraday cups (FC) and two triple Langmuir probes (TLP) is established to investigate the time-averaged in situ spatial distribution characteristics of far-field ions and electrons. The ion current density (ICD), plasma potential, plasma density, and electron temperature at 1000 mm downstream of 300 W class LHT for commercial aerospace applications in the azimuthal angle range of −90° to 90° were investigated under the conditions of different anode mass flow rates and discharge voltages. The results demonstrated that ICD, beam divergence angle, and mass utilization efficiency increased with increasing anode mass rate. The double-wings phenomenon was observed in the spatial distribution of ICD at large angles from the thruster axis, which is attributed to charge exchange collisions at increasing vacuum backpressure. The plasma electron temperature, electron density, and plasma potential parameters derived from the TLP decreased rapidly in the angle range from 0° to 30° and did not exhibit significant variations above 30°, which was also in good agreement with the results of the measured divergence angle of the FC. The discrepancy of average ion speed was calculated. The maximum error is better than 31.5% which checks the consistency between the TLP’s results and that of FC to some extent.

Compact, portable, laser induced fluorescence diagnostic for laboratory plasma sources.

As diagnostic groups are increasingly called upon to participate in experimental campaigns at remote facilities, there is a need to develop portable versions of plasma diagnostic systems. One such diagnostic is laser induced fluorescence (LIF). Here, we describe a portable LIF apparatus that eliminates the need for an optical table, beam splitters, and an optical chopper. All of the light exiting the laser system is coupled through optical fibers to the experiment and housekeeping diagnostics. The collected light is coupled through an optical fiber as well. A key feature is modulation of the tapered amplifier current instead of physical modulation of the laser output. Using this portable LIF system, measurements of ion temperature, ion flow, and relative metastable ion density are reported for two different remote experiments.

Electron, Neutron, and Proton Irradiation Effects on SiC Radiation Detectors

Owing to their low dark current, high transparency, high thermal conductivity, and potential radiation hardness, there is a special interest in silicon carbide (SiC) devices for radiation monitoring in radiation harsh environments and with elevated temperatures and, especially, for the plasma diagnostic systems in future nuclear fusion reactors. In this work, four-quadrant p-n junction diodes produced on epitaxial 4H-SiC substrates are studied. The impact of electron, neutron, and proton irradiations (up to fluences of $1 \times 10^{16}$ electrons (e)/cm2, $2 \times 10^{15}$ neutrons (n)/cm2, and $2.5\times 10^{15}$ protons (p)/cm2, respectively) on the electrical characteristics is studied by means of current–voltage ( $I$ – $V$ ) and capacitance–voltage ( $C$ – $V$ ) techniques. Regardless of the particle type and applied fluences, the results show similar low reverse currents for irradiated SiC devices, which are at least about four orders of magnitude lower than comparable Si devices. The effects of irradiation on interquadrant resistance and charge build-up in the interquadrant isolation are assessed. Furthermore, device performance as a radiation detector is investigated upon exposure to a collimated 239Pu–241Am–244Cm tri-alpha source. The performance at room temperature is preserved even for the highest irradiation fluences, despite the fact that the rectification character in electrical characteristics is lost. From the results, advantages of using SiC devices in alpha particle detection in harsh environments can be envisaged.

Evaluation of the ITER Real-Time Framework for Data Acquisition and Processing from Pulsed Gigasample Digitizers

Plasma diagnostics systems are becoming progressively more advanced. Contemporarily, researchers strive to achieve longer plasma pulses, and therefore, appropriate hardware is required. Analogue-to-Digital Converters are applied for data acquisition in many plasma diagnostic systems. Some diagnostic systems need data acquisition with gigahertz sampling frequency. However, gigasample digitizers working in continuous mode generate an enormous stream of data that requires suitable, high-performance processing systems. This becomes even more complicated and expensive for complex multi-channel systems. Nonetheless, numerous plasma diagnostic systems operate in a pulse mode. Thomson scattering (TS) diagnostics is a good example of a multi-channel system that does not require continuous data acquisition. Taking this into consideration, the authors decided to evaluate the CAEN DT5742 gigasample digitizer as a more cost-effective solution that would utilize the pulsed nature of the TS diagnostic system. The paper presents a complete data acquisition and processing system dedicated for plasma diagnostics based on the ITER real-time framework (RTF). Integration of RTF with real hardware is discussed. The authors of the paper have developed software including RTF function block for the CAEN DT5742 digitizer, example data processing algorithms, data archiving and publishing for plasma control system.

IBIC analysis of SiC detectors developed for fusion applications

In this work, we consider a 4H-SiC detector as a plasma diagnostic system for the detection of fusion-born alpha particles in future nuclear fusion reactors. A nuclear microprobe was used to locally irradiate micrometer-sized regions of the detector with 3.5 MeV He ions to fluences from 5 × 109 to 5 × 1011 cm-2. Ion Beam Induced Charge (IBIC) microscopy was employed to study its degradation in Charge Collection Efficiency (CCE) and energy resolution after irradiation. At high reverse-bias voltages, both parameters remain practically unaffected for fluences up to 1 × 1011 cm-2, while a significant deterioration of the spectroscopic performance was observed above 3 × 1011 cm-2. A theoretical drift-diffusion model, in combination with Shockley-Read-Hall recombination statistics, was used to obtain the holes lifetime from the fitting of the experimental CCE values measured at different reverse voltages. Holes lifetime was found to strongly decrease with increasing particle fluence, changing from 57 ns in pristine detectors to 0.2 ns after irradiation with a fluence of 1 × 1011 cm-2.

The plume diagnostics of 30 cm ion thruster with an advanced plasma diagnostics system

In order to achieve a better understanding of plume characteristics of LIPS-300 ion thruster, the beam current density, ion energy and electron number density of LIPS-300 ion thruster plume are studied with an Advanced Plasma Diagnostics System (APDS) which allows for simultaneous in situ measurements of various properties characterizing ion thruster, such as plasma density, plasma potential, plasma temperature and ion beam current densities, ion energy distribution and so on. The results show that the beam current density distribution has a double ‘wing’ shape. The high energy ions were found in small scan angle, while low energy ions were found in greater scan angle. Electron number density has a similar shape with the beam current density distribution.

Deposition and Characterization of a Rhodium Thin Films with a Niobium Interlayer on a Molybdenum Mirror for ITER First-Mirror Application

Metallic mirrors are now essential components of the (International Thermonuclear Experiment Reactor) ITER plasma diagnostics system. The optical properties of the first mirror (FM) in the hostile ITER environment have adverse effects due to erosion and deposition owing to high energy charge exchange of neutral (CXN) particles, UV, X-ray, and gamma radiation. Molybdenum (Mo) and rhodium (Rh) are attractive metals for the FM(s) as they can withstand the harsh conditions of the plasma environment and have better reflectivity. However, the high cost of Rh limits its use as a single material in the FM; as a result, it is deposited as a thin film over a metallic substrate. Moreover, due to the different thermal expansion of the coated materials on FM leads to weak adhesion of the thin layers thereby reducing the life of the coated mirror; this eventually leads to the deterioration of the optical properties of the diagnostic mirror. Furthermore, this phenomenon is becoming a problem concerning the stainability and reflectivity of the diagnostic mirror. This study aims to show the performance of niobium (Nb) as an interlayer between Mo and an Rh thin films deposited via DC magnetron sputtering at the deposition power of 40 W. The Mo mirrors were exposed to the high temperature and were left to cool naturally to room temperature. Rougher surfaces, larger grain sizes, and excellent adhesion were observed. The overall reflectance of the mirrors exposed to high temperature was reduced by 10–15%.

Parallel computing in soft X‐rays plasma diagnostic systems for thermal fusion reactors—feasibility studies for GPUs

SummaryThis paper presents feasibility studies in utilizing graphics processing units (GPUs) as high‐performance computing hardware with front‐end electronics in high‐scale magnetic confinement thermal fusion experiments. The objective of the research is to provide scalable, high‐throughput, and low‐latency measurements for the runtime tokamak metallic impurities X‐ray diagnostic for the Tungsten Environment in Steady‐State Tokamak (WEST) reactor. The heterogeneous system of front‐end with field‐programmable gate arrays and the back‐end server was introduced to decompose workloads efficiently. It allows the comprehensive evaluation of CPUs and accelerators. In particular, a novel implementation of the back‐end algorithm for GPU with the performance analysis are presented.