Hydrogen glow discharge cleaning (H-GDC) is a routine conditioning procedure for the modern tokamaks and the future fusion machines, including ITER. Due to the low energy of hydrogen ions in glow discharge plasmas, the probability of any considerable damage to the plasma-facing components was mainly ignored. In this work, Tungsten, as the primary candidate for the plasma-facing materials in tokamaks, is considered for studies regarding effects of the H-GDC procedure on the material during a routine vacuum vessel conditioning in Damavand tokamak. After performing routine H-GDC using pure hydrogen, the formation of loosely attached nanostructured bundles (NSBs) on the surface of the tungsten samples was observed. The presence of the NSBs, which can be a source of dust, would be significant due to their possible effects on the functionality of the future fusion plasma devices. The NSBs are observed on the tungsten samples with the surface temperature of <370 K after 2.5–4 hours of hydrogen ion bombardment with incident energy of ≈120 eV and fluence of ≈(2–3.5) × 1022 m–2. The surface modifications of specimens exposed to H‑GDC were examined using a material probe experiment and several surface analysis techniques such as SEM, EDX, XRD, and ERDA. Therefore, the formation of loose nanostructures on the wall of plasma confinement vessels due to H-GDC draws attention to probable damaging effects of this phenomenon upon functionality and outcomes of tokamaks.

Investigation of Hydrogen Glow Discharge Cleaning Side Effects on Tungsten

The effects of high-energy protons produced in the plasma focus device on molybdenum and copper metals were studied. Molybdenum and copper samples were simultaneously irradiated with 20 hydrogen ion shots after the initial preparation and polishing of their surface. Molybdenum and copper samples were analyzed before and after irradiation by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lee code was used to characterize the ion beam generated in the plasma focus device. The XRD results showed that the size of crystals in molybdenum samples increased from 33.1 to 41.4 nm and for copper samples from 28 to 37.2 nm. Also, the location of peaks in molybdenum and copper samples was shifted to larger angles by 0.041 and 0.174, respectively. The SEM micrographs revealed that cracks and cavities were formed on the surface of molybdenum due to the radiation of high-energy protons, while dense blisters were formed on the surface of copper, and in some areas on the surface of copper, melting and physical sputtering occurred due to the collision of high-energy ions with copper surface atoms. Lee code results indicated that each shot of the plasma focus device emits 7.9 × 1014 ions from the plasma column.

The investigation of the destructive effects of high-energy hydrogen ions on molybdenum and copper

Graphite samples, used in first-wall and divertor of nuclear fusion reactors like tokamaks, were irradiated using hydrogen and argon ions produced in an MTPF-2 plasma focus device in 20 shots to study radiation damages. Ion energy spectra are $dN_{\text {H}}\left ({E_{\text {H}} }\right)/ dE_{\text {H}}\approx E_{\text {H}}^{-2.8}$ with a minimum energy of 120 keV to a maximum of 1 MeV for hydrogen ions and ${dN_{\text {Ar}}\left ({E_{\text {Ar}} }\right)} / {dE_{\text {Ar}}}\approx E_{\text {Ar}}^{-3.5}$ with a minimum energy of 200 keV and a maximum energy of 2 MeV for argon ions. Based on the results of the SRIM code, the maximum destruction of hydrogen ions is at a depth of 900 nm, and a rate of 0.025 DPA/shot and highest hydrogen density is 0.5% at 920 nm. Conversely, the highest degradation of argon ions is at a depth of 120 nm and 0.23 DPA/shot, and the highest density of argon ions at a depth of 200 nm is 0.07%. Different maximum degradation locations and the maximum density of argon ions are due to their heavy mass compared to hydrogen. SEM micrographs show holes with high population density and some sublimated areas on hydrogen-irradiated surfaces. The predominant phenomenon due to argon ion irradiation is physical sputtering, leading to cavities with a depth of several micrometers. Based on the X-ray diffraction (XRD) spectrum, the location and intensity of peaks are changed, and irradiation reduces graphitization and increases the size of the graphite crystal.

Radiation Damage of Graphite Surface, Used in First-Wall and Divertor of Tokamaks, Irradiated by Hydrogen and Argon in Plasma Focus Device

We report results of measurements on the evolution of the surface morphology of a hot tungsten surface due to impacting low-energy (80 ? 12,000 eV) He ions, performed at the ORNL Multicharged Ion Research Facility (MIRF). Surface-morphology changes were investigated over a broad range of fluences, energies and temperatures for both virgin and pre-damaged W-targets. At low fluences, ordered coral-like and ridge-like surface structures are observed, with great grain-to-grain variability. At the largest fluences, individual grain characteristics disappear in FIB/SEM scans, and the entire surface is covered by a multitude of near-surface bubbles with a broad range of sizes, and disordered whisker growth, while in top-down SEM imaging the surface is virtually indistinguishable from the nanofuzz produced on linear plasma devices. These features are evident at progressively lower fluences as the He-ion energy is increased. In addition, simulations were carried out of damage caused by cumulative bombardment of 1 keV W self-atoms, using LAMMPS at the Kraken supercomputing facility of the University of Tennessee. The simulations show strong defect-recombination effects that lead to a saturation of the total defect number after a few hundred impacts, while sputtering and implantation lead to an imbalance of the vacancy and interstitial numbers.

http://absimage.aps.org/image/DPP13/MWS_DPP13-2013-020120.pdf

Surface morphology changes and damage in hot tungsten by impact of 80 eV - 12 keV He-ions and keV-energy self-atoms

Abstract In this study, nanoparticles of neodymium oxide were deposited on NaX zeolite powder using a 5.3 kJ Mather‒type plasma focus device. The NaX zeolite samples were exposed to the 10, 15, and 20 shot numbers of the plasma focus device at a distance of 14 cm from the anode tip and 0 angular positions with respect to the anode axis. Parent NaX zeolite and NaX:Nd composites with different Nd 2 O 3 percent weights were characterized using different techniques such as; XRF, XRD, SEM, FT‒IR, and UV‒vis. It was shown that the amount of Nd 2 O 3 in the samples increases with increase of the shot number. It was also found from the results that variations in amount of Nd 2 O 3 can have significant effects on the degree of the crystallinity and optical properties of final products such as refractive index, band gap energy, dielectric constant, and absorption coefficients. 

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Investigation of optical properties and chemical structure of Nd2O3 nanoparticles deposited on NaX zeolite powder using plasma focus device

Radiation damage of tungsten surface irradiated with high-energy hydrogen and helium beams of plasma focus device

Damage study and comparison the effects of high-energy pulsed-protons of plasma focus device with low-energy protons of glow discharge plasma of tokamak

Farid Sedighi

Papers

Radiation damage of tungsten surface irradiated with high-energy hydrogen and helium beams of plasma focus device

Damage study and comparison the effects of high-energy pulsed-protons of plasma focus device with low-energy protons of glow discharge plasma of tokamak

Investigation of Hydrogen Glow Discharge Cleaning Side Effects on Tungsten