Plasma Facility Research Articles

Study of the behavior of fine tungsten crystallic and amorphous particles in varying conditions

Formation of dust particles and their agglomerates with sizes ranging from tenths to hundreds of microns is observed in many modern plasma facilities. Dust accumulation can significantly affect discharge parameters in future thermonuclear facilities and lead to the accumulation of dangerously high amounts of tritium. Experiments on the generation of tungsten and tungsten-aluminium dust made of flakes formed during exfoliation of coating deposited on chamber walls from tungsten and aluminium targets sputtered by ions are described in this paper. A comparative analysis of the behavior of such dust and dust (figure 1), formed by crystal particles with simple geometric shapes (“crystal particles”) in electric field, their degassing and change occurring in temperatures of up to 2000 K is conducted.

Development of a compact electromagnetic undulator for linac-based coherent THz radiation source in Thailand

A linac-based accelerator system at the Plasma and Beam Physics Research Facility of Chiang Mai University in Thailand can produce relativistic femtosecond electron bunches for generating coherent THz radiation. Originally, this accelerator system was built to produce high peak power THz/FIR radiation via transition radiation technique. To extend the system’s capability in production of higher average power and tunable coherent radiation, a project to add an undulator magnet as a source of coherent THz radiation was started. Construction of a compact 40-period electromagnetic undulator with short period length is in progress. Low electricity consumption, inexpensive materials and uncomplicated fabrication process are aimed. An undulator prototype was designed and built to investigate the magnetic field properties and to estimate the possibility on manufacturing of a future undulator magnet. A computer program RADIA was used to model a three-dimensional undulator prototype based on the fabricated magnet material and dimensions. This prototype has 4.5 periods consisting of 8 main-poles and 2 end-poles for magnetic field correction. The simulated peak magnetic field was 0.1858 T or equivalents to an undulator parameter of 0.98. This led to a calculated undulator radiation wavelength at the first harmonic of 109 μm for electron beam with energy of 10 MeV. This radiation is in the forward direction. After manufacturing, the magnetic field of the undulator prototype was measured and analyzed. The measurement result at an optimal excitation current of 5 A showed that an average period length of 54.8 mm and an average peak magnetic field of 0.1855 T were achieved. The measured undulator parameter was 0.97, which resulted in the undulator radiation wavelength of about 107 μm. A future electromagnetic undulator with 40 periods was designed and will be built based on the study results of this undulator prototype. The estimated angular flux density of the radiation emitted from 40 period undulator magnet is dominated at the first harmonic radiation wavelength of 109 μm, which is equivalent to the radiation frequency of 2.8 THz.

Coherent transition radiation from femtosecond electron bunches at the accelerator-based THz light source in Thailand

A THz radiation source based on electron linear accelerator (linac) has been built and commissioned at the Plasma and Beam Physics (PBP) Research Facility, Chiang Mai University (CMU) in Thailand since 2005. The accelerator system consists of a thermionic cathode RF electron gun, a magnetic bunch compressor in a form of an alpha-magnet, an S-band travelling-wave linac structure, beam steering and focusing magnets, beam and radiation diagnostic instruments, control system and other support components. Electron bunches with an average kinetic energy of about 8 MeV, a bunch charge of 100 pC, and a bunch length of 300 fs can be produced from the PBP-CMU electron linac system. Coherent THz radiation generated via transition radiation technique is used as a source of the THz spectroscopy and THz imaging applications. The measured power spectrum indicates that the radiation obtained from short electron bunches covers the frequency range of 0.15–1.80 THz or the wavenumber range of 5–60 cm−1. Experimental results reveal that a peak radiation power as high as 0.2 MW per micropulse and an average radiation power of 2.5 mW can be obtained. Generation and characterization of the coherent THz transition radiation as well as some examples of experiments to achieve electron bunch length and radiation power spectrum are reported and discussed in this paper.

A Study on Deuterium Retention Behavior of Plasma-Facing Materials for EAST

To investigate the deuterium (D) retention behavior of plasma-facing materials (PFMs) in the Experimental Advanced Superconducting Tokamak (EAST), SiC-coated graphite, bulk W, and W coating were exposed to D plasmas employing the material and plasma evaluation system in EAST. A comparative study was performed in an electron cyclotron resonance (ECR) linear plasma facility as well. For the EAST experiments, the local electron temperature and density were $T_{e}\approx$ 10–15 eV and $n_{e}\approx$ (1–3) $\times$ 1018 m−3, respectively. In the ECR facility, the plasma parameters were measured to be $T_{e} \approx 2.4$ eV and $n_{e} \approx 1.5 \times 10^{17}$ m−3. The surface morphology was examined by scanning electron microscopy, and D retention properties of various PFMs were examined by thermal desorption spectroscopy. No significant modification of the surface morphology was observed after tokamak exposure experiments. Bulk W and W coating exhibit similar D retention behavior, while D inventory in SiC-coated graphite has been found to be much larger than that in W materials.

Effects of double-layer tungsten coatings on hydrogen isotopes plasma-driven and gas-driven permeation through F82H

The behavior of hydrogen isotopes plasma-driven permeation (PDP) and gas-driven permeation (GDP) through tungsten coated F82H membranes has been studied in the temperature range from 200 °C to 550 °C, using a laboratory-scale steady-state plasma facility. Tungsten coatings have a double-layer structure: the first layer on F82H prepared by sputter-deposition (SP) which is then overlaid with the second layer prepared by vacuum plasma-spraying (VPS). It has been found that VPS/SP double-layer coatings can suppress by orders of magnitude both GDP and PDP of hydrogen isotopes under the plasma-surface interaction conditions relevant to those to be seen nearby the first wall in a DEMO reactor.

A Study of the Fast Neutron Response of a Single-Crystal Diamond Detector at High Temperatures

The fast neutron response of a single-crystal diamond (SCD) detector was studied up to 300 °C. The SCD detector was fabricated using a $5\,\,\text {mm}\times 5\,\,\text {mm}\times 0.4\,\,\text {mm}$ substrate with premetallization treatments for obtaining oxygen terminated surfaces. The electrical contacts to the detector were provided by Cr/Au metallization. The performance of the fabricated detector was investigated using leakage current versus bias voltage (I–V) measurements and using pulse height spectra measurements with 5.5 MeV alphas and 14 MeV fast neutrons. The leakage current was observed to be about 400–600 pA at a field of 1 V/ $\mu \text{m}$ . From the full-width half-maximum of the alpha peak at 5.5 MeV, an energy resolution of about 2.1 ± 0.1% was estimated. The fast neutron response measurements were carried out using a 14 MeV deuterium–tritium neutron source. No measurable degradation of the detector performance was observed up to the maximum studied temperature of 300 °C. The neutron spectrum obtained at 300 °C showed a well-defined peak at 8.5 MeV due to the 12C(n, $\alpha)^{9}$ Be reaction which was the same as that observed at room temperature. Considering the total integrated counts, the detector sensitivity was observed to be $10^{-2}$ count per neutron. The results presented in this paper demonstrate for the first time the high temperature operation of an SCD detector up to 300 °C. These results also suggest that such SCD detectors could be used at higher temperatures for neutron diagnostics in the future fusion plasma facilities such as International Thermonuclear Experimental Reactor.

Characteristics of the Plasma Source for Ground Ionosphere Simulation Surveyed by Disk-Type Langmuir Probe

A space plasma facility has been operated with a back-diffusion-type plasma source installed in a mid-sized vacuum chamber with a diameter of ~1.5 m located in Satellite Technology Research Center (SaTReC), Korea Advanced Institute of Science and Technology (KAIST). To generate plasma with a temperature and density similar to the ionospheric plasma, nickel wires coated with carbonate solution were used as filaments that emit thermal electrons, and the accelerated thermal electrons emitted from the heated wires collide with the neutral gas to form plasma inside the chamber. By using a disk-type Langmuir probe installed inside the vacuum chamber, the generation of plasma similar to the space environment was validated. The characteristics of the plasma according to the grid and plate anode voltages were investigated. The grid voltage of the plasma source is realized as a suitable parameter for manipulating the electron density, while the plate voltage is suitable for adjusting the electron temperature. A simple physical model based on the collision cross-section of electron impact on nitrogen molecule was established to explain the plasma generation mechanism.

Impact of carrier gas flow rate on the synthesis of nanodiamonds via microplasma technique

Nanodiamonds (NDs) have been fabricated utilizing micro plasma facility. The mixture of argon as carrier gas and ethanol as precursor has been dissociated in micro plasma. The argon flow rate was varied from 4 to 9l/min. The influence of argon gas flow rates on structural, surface and optical properties of NDs was compared. An increased amount of diamond has been reported with the increase in flow rate, obtaining the maximum value at 6l/min. Further increase in flow rate starts decreasing the quality of NDs. Raman and XRD confirm that the smallest size and stress free NDs are obtained at 6l/min argon flow rate. AFM micrograph also reveals the smallest size of NDs ~27nm for 6l/min argon flow rate. The hydrophobicity of NDs increases with increasing argon flow rate as confirmed by FTIR. Photoluminescence indicates the presence of nitrogen vacancy defect centers such as N3, H3 and NV for all the flow rates. The smallest PL peak intensity at 6l/min argon flow rate indicates the lowest density of vacancy defects in the diamond. UV–Visible spectroscopy indicates the presence of nitrogen-hydrogen defect and nitrogen vacancy defect centers (N4 and N5). The largest band gap at 6l/min argon flow rate also confirms the highest quality of NDs. The centers offer promising applications as, active laser material, fluorescent markers, quantum information, nanoscale magnetic and electric filed sensors.

Innovative potential of plasma technology

The review summarizes recent experimental observations of materials exposed to extreme hot plasma loads in fusion devices and plasma facilities with high-temperature plasma. Plasma load on the material in such devices lead to the stochastic clustering and fractal growth of the surface on scales from tens of nanometers to hundreds of micrometers forming statistical self-similarity of the surface roughness with extremely high specific area. Statistical characteristics of hierarchical granularity and scale invariance of such materials surface qualitatively differ from the properties of the roughness of the ordinary Brownian surface which provides a potential of innovative plasma technologies for synthesis of new nanostructured materials with programmed roughness properties, for hypersonic technologies, for biotechnology and biomedical applications.

Determination of active nitridation reaction efficiency of graphite in inductively coupled plasma flows

Accurate prediction of the gas-surface interaction during spacecraft reentry remains a challenging problem for thermal protection system design. Attempts to model the surface chemistry of ablating materials focus on oxidation and sublimation of the carbon surface, but usually neglect nitridation processes. Although nitridation may only lead to a minor increase in overall mass loss through ablation, it can highly impact other surface chemical processes through consumption of available atomic nitrogen, affecting the energy balance. We present experiments on graphite ablation in nitrogen plasmas, aiming at the determination of carbon nitridation reaction efficiencies in a relevant reentry environment reproduced in an inductively coupled plasma facility. The actual carbon nitridation reaction efficiencies are extracted using a numerical model with ablative boundary condition coupled to the flowfield. Emission spectroscopy of the CN radiation provides local CN species densities used for validation of the numerical model. The data are in-line with values reported in literature and extend the investigated temperature range. Our computed CN species densities agree well with the experimental values, providing confidence in our method. Based on our findings and additional literature data, we propose an updated Arrhenius form for evaluation of nitridation reaction efficiencies as a function of the wall temperature.

Design and Development of Emittance Measurement Device by Using the Pepper-pot Technique

Transverse emittance of a charged particle beam is one of the most important properties that reveals the quality of the beam. It is related to charge density, transvers size and angular displacement of the beam in transverse phase space. There are several techniques to measure the transverse emittance value. One of practical methods is the pepper-pot technique, which can measure both horizontal and vertical emittance value in a single measurement. This research concentrates on development of a pepper-pot device to measure the transverse emittance of electron beam produced from an accelerator injector system, which consists of a thermionic cathode RF electron gun and an alpha magnet, at the Plasma and Beam Physics Research Facility, Chiang Mai University. Simulation of beam dynamics was conducted with programs PARMELA, ELEGANT and self-developed codes using C and MATLAB. The geometry, dimensions and location of the pepper-pot as well as its corresponding screen station position were included in the simulation. The result from this study will be used to design and develop a practical pepper-pot experimental station.

Vacuum System and Modeling for the Materials Plasma Exposure Experiment

Understanding the science of plasma-material interactions (PMI) is essential for the future development of fusion facilities. The design of divertors and first walls for the next generation of long-pulse fusion facilities, such as a Fusion Nuclear Science Facility (FNSF) or a DEMO, requires significant PMI research and development. In order to meet this need, a new linear plasma facility, the Materials Plasma Exposure Experiment (MPEX) is proposed, which will produce divertor relevant plasma conditions for these next generation facilities. The device will be capable of handling low activation irradiated samples and be able to remove and replace samples without breaking vacuum. A Target Exchange Chamber (TEC) which can be disconnected from the high field environment in order to perform in-situ diagnostics is planned for the facility as well. The vacuum system for MPEX must be carefully designed in order to meet the requirements of the different heating systems, and to provide conditions at the target similar to those expected in a divertor. An automated coupling-decoupling (“autocoupler”) system is designed to create a high vacuum seal, and will allow the TEC to be disconnected without breaking vacuum in either the TEC or the primary plasma materials interaction chamber. This autocoupler, which can be actuated remotely in the presence of the high magnetic fields, has been designed and prototyped, and shows robustness in a variety of conditions. The vacuum system has been modeled using a simplified finite element analysis, and indicates that the design goals for the pressures in key regions of the facility are achievable.

Sputtering in a high-flux plasma environment

Sputtering yield measurements of metals using ion beam devices and high-flux plasma facilities are compared. During light inert gas bombardment, a general decrease in the sputtering yield is observed as the flux of energetic particles to the target increases. However, during heavy inert gas bombardment, the decrease is not observed and measurements agree with ion beams and binary collision approximation calculations. A potential cause, accumulation of gas atoms within the implantation zone, is hypothesized to be the culprit and various in-situ surface diagnostic techniques to measure the gas composition in the near surface of the metal targets are discussed.

Progress in magnet design activities for the material plasma exposure experiment

One of the critical challenges for the development of next generation fusion facilities, such as a Fusion Nuclear Science Facility (FNSF) or DEMO, is the understanding of plasma material interactions (PMI). Making progress in PMI research will require integrated facilities that can provide the types of conditions that will be seen in the first wall and divertor regions of future fusion facilities. To meet this need, a new linear plasma facility, the Materials Plasma Exposure Experiment (MPEX), is proposed. In order to generate high ion fluence to simulate fusion divertor conditions, a steady-state plasma will be generated and confined with superconducting magnets. The on-axis fields will range from 1 to 2.5T in order to meet the requirements of the various plasma source and heating systems. Details on the pre-conceptual design of the magnets and cryogenic system are presented.

Irradiation effects of hydrogen and helium plasma on different grade tungsten materials

Fine-grain tungsten alloys could be one of the solutions for the plasma facing materials of future DEMO reactors. In order to evaluate the service performances of the newly developed W alloys under edge plasma irradiation and the synergetic effect of fusion plasma together with high heat flux, both low energy He ions and high energy H, H/He mixed neutral beam irradiation on W-ZrC, W-K, W-Y2O3, W-La2O3 and CVD-W coating were performed respectively at a liner plasma facility (Dalian Nationality University, China) and the neutral beam facility GLADIS (IPP, Germany). Surface damages were characterized, and the crack formation and extension behaviors under ELM-like transient loading after H and H/He mixed beam irradiation were also investigated in the 60kW EMS-60 facility (Electron beam Materials testing Scenario) at SWIP (Southwestern Institute of Physics, China). The experimental results indicated that surface damages induced by low or high energy H/He ion/neutral beam didn't closely correlate with the type of tungsten materials. However, H/He (6at% He concentration) neutral beam induced more significant surface damages of the tested W materials than only H neutral beam irradiation under the similar irradiation conditions. Similarly, the mixed H/He pre-exposure remarkably reduced the critical power of crack initiation compared with the un-irradiated samples under 100 repetitive loads of 1ms pulse, while no significant degeneration for the case of only H beam irradiation was observed.

A new concept of fusion neutron monitoring for PF-1000 device

Abstract The power output of plasma experiments and fusion reactors is a crucial parameter. It is determined by neutron yields that are proportional and directly related to the fusion yield. The number of emitted neutrons should be known for safety reasons and for neutron budget management. The PF-1000 is the large plasma facility based on the plasma focus phenomenon. PF-1000 is operating in the Institute of Plasma Physics and Laser Microfusion in Warsaw. Neutron yield changes during subsequent pulses, which is immanent part of this type device and so it must be monitored in terms of neutron emission. The reference diagnostic intended for this purpose is the silver activation counter (SAC) used for many years. Our previous studies demonstrated the applicability of radio-yttrium for neutron yield measurements during the deuterium campaign on the PF-1000 facility. The obtained results were compared with data from silver activation counter and shown linear dependence but with some protuberances in local scale. Correlation between results for both neutron monitors was maintained. But the yttrium monitor registered the fast energy neutron that reached measurement apparatus directly from the plasma pinch. Based on the preliminary experiences, the yttrium monitor was designed to automatically register neutron-induced yttrium activity. The MCNP geometrical model of PF-1000 and yttrium monitor were both used for calculation of the activation coefficient for yttrium. The yttrium monitor has been established as the permanent diagnostic for monitoring fusion reactions in the PF-1000 device.

Diffusion effects on the determination of surface catalysis in Inductively Coupled Plasma facility

Atomic recombination is an important process to consider when computing the heat flux transferred to the wall of a re-entry vehicle. Two chemical processes are influencing the species diffusion in the boundary layer surrounding a re-usable Thermal Protection System: gas phase reactions and catalytic recombination at the surface. The coupling between them is not normally taken into account when determining the catalytic recombination coefficient (γ) in plasma facilities. This work aims to provide evidence of such coupling based on both a theoretical analysis and an experimental campaign in the VKI-Plasmatron facility. Recombination coefficient measurements at off-stagnation point configuration on a linear copper calorimeter are provided. An evolution from a high-catalytic to a low-catalytic condition due to the boundary layer growth along the probe is observed. This result is consistent with a parametric analysis carried out using the in-house non-equilibrium boundary layer solver, which shows how the experimentally determined catalysis could be influenced by the amount of gas-phase recombination inside the boundary layer.

Ablation properties of plasma facing materials using thermal plasmas

We investigated the ablation characteristics of carbon/carbon composites and tungsten coated plasma facing materials (PFMs) using a 400kW plasma wind tunnel (PWT) and a 55kW vacuum plasma spraying system. These thermal plasma facilities allow a particle flux greater than 1024/(m2s) and a heat flux greater than 10MW/m2, which are the levels relevant for testing PFMs under fusion reactor conditions. We identified the ablation properties through measurement of the ablation rate and investigation of the microstructures of the PFMs before and after the ablation test.

Electron linear accelerator system for natural rubber vulcanization

Development of an electron accelerator system, beam diagnostic instruments, an irradiation apparatus and electron beam processing methodology for natural rubber vulcanization is underway at the Plasma and Beam Physics Research Facility, Chiang Mai University, Thailand. The project is carried out with the aims to improve the qualities of natural rubber products. The system consists of a DC thermionic electron gun, 5-cell standing-wave radio-frequency (RF) linear accelerator (linac) with side-coupling cavities and an electron beam irradiation apparatus. This system is used to produce electron beams with an adjustable energy between 0.5 and 4MeV and a pulse current of 10–100mA at a pulse repetition rate of 20–400Hz. An average absorbed dose between 160 and 640Gy is expected to be archived for 4MeV electron beam when the accelerator is operated at 400Hz. The research activities focus firstly on assembling of the accelerator system, study on accelerator properties and electron beam dynamic simulations. The resonant frequency of the RF linac in π/2 operating mode is 2996.82MHz for the operating temperature of 35°C. The beam dynamic simulations were conducted by using the code ASTRA. Simulation results suggest that electron beams with an average energy of 4.002MeV can be obtained when the linac accelerating gradient is 41.7MV/m. The rms transverse beam size and normalized rms transverse emittance at the linac exit are 0.91mm and 10.48πmm·mrad, respectively. This information can then be used as the input data for Monte Carlo simulations to estimate the electron beam penetration depth and dose distribution in the natural rubber latex. The study results from this research will be used to define optimal conditions for natural rubber vulcanization with different electron beam energies and doses. This is very useful for development of future practical industrial accelerator units.

Surface Catalyzed Reaction Efficiencies in Oxygen Plasmas from Laser-Induced Fluorescence Measurements

Recent advancements in laser techniques for plasma facility diagnostic applications have enabled measurements of temperature and species gradients in the reacting zone above a high-temperature material. The laser measurements, which are done with submillimeter spatial resolution, provide a direct, qualitative illustration of surface-catalyzed reaction efficiency when reactive species distributions are compared for different materials. Further analysis of the measured quantities balancing the diffusive reactant flux with the consumption of atoms by surface reactions yields quantitative values for surface-catalyzed reaction rates and catalytic efficiencies. In this work, measurements of oxygen atom distributions in plasmas are presented for reference materials of high and low catalytic efficiency. The laser measurements are then used to determine values for atomic oxygen recombination to molecular oxygen. Comparisons of the present values of the recombination efficiency ( cold copper, cold quartz, and hot quartz) with previously published data suggest that this technique provides a viable method for characterizing the catalytic behavior of high-temperature materials in plasma test facilities.