Retsuo Kawakami

A Modified EDDY Code to Simulate Erosion/Redeposition of Carbon Target in an ITER-FEAT Divertor

Modification of a Monte Carlo simulation code, Erosion and Deposition based on DYnamic model (EDDY), for plasma-surface interactions in a designed tokamak, International Thermonuclear Experimental Reactor-Fusion Energy Advanced Tokamak (ITER-FEAT), and its application for erosion and redeposition of a carbon target in the divertor are presented. The modified EDDY code allows us to treat the deposition of plasma impurities and the prompt redeposition of sputtered atoms and molecules on the target surface. At elevated temperatures, furthermore, the impurity diffusion inside the target and chemical sputtering of carbon are taken into account. In the ITER-FEAT, physical sputtering of the divertor target is very small in the scrape-off layer (SOL) region, and chemical sputtering dominates the erosion near the strike point and in the private flux region. Prompt redeposition strongly suppresses the sputtering of the target and plasma carbon impurity deposits on it. As a result, no erosion is calculated in the SOL region and a thick deposition layer is produced near the strike point. A narrow erosion zone remains only in the private flux region. Furthermore, radial distributions of each particle species released in the plasma and their redeposition profiles on the surface are discussed.

Synergy Effect of Particle Radiation and Ultraviolet Radiation from Capacitively Coupled Radio Frequency Argon Plasmas on n-GaN Etching Damage

The change in the morphology of n-GaN surfaces etched by capacitively coupled RF Ar plasmas has been studied from the viewpoint of a synergy effect of particle radiation and UV radiation from the RF plasmas. The particle radiation (in particular, the energy of Ar+ impinging on n-GaN) is intensified with decreasing gas pressure from 200 to 10 mTorr, whereas the intensity of the UV radiation (whose peak wavelength corresponds to the GaN band-gap energy) is significantly weakened. The reverse result occurs when the gas pressure increases. Each type of radiation brings about a smooth surface morphology similar to that of the as-grown surface. However, at 50 or 100 mTorr, at which both types of radiation are expected to coexist, the surface morphology shows various types of pits (defects or dislocations), which seem to be induced by the synergy effect.

Simulation calculations of mutual contamination between tungsten and carbon and its impact on plasma surface interactions

Mutual contamination between C and W, resulting from the simultaneous use of these materials as plasma facing components, is simulated by means of a computer simulation code, Erosion and Deposition based on Dynamic model (EDDY). W deposition on C rapidly increases the reflection coefficient for D and C impurity. In comparison between the calculation and a C-W twin test limiter experiment in TEXTOR-94, C release from the C side of the limiter is dominated by reflection of C impurity from the W deposits, in addition to physical sputtering of C; chemical erosion is strongly suppressed. Due to the dynamic effect which makes C-W mixed layer, C deposition on W gradually changes the reflection coefficient and sputter yields. Formation of a sharp boundary between erosion and C deposition zones on the W side of the limiter is well reproduced by simulation. Local redeposition patterns of C and W on the limiter surface are also calculated.

Dynamic behavior of tungsten surfaces due to simultaneous impact of hydrogen and carbon ion beam

By using a simulation code for ion-solid interactions, EDDY, the dynamical behavior of W surfaces irradiated simultaneously with H+ and C+ impurity has been studied. This code models the fluence evolution of composition changes between C and W at the irradiated surface, which results from sputtering erosion and impurity deposition. The result has been described in terms of C impurity concentration in the irradiation. It has been compared with experimental data obtained by an ion beam irradiation device. In particular, the C impurity concentration has an important role in erosion/deposition at the W surface. As the C impurity concentration increases, the erosion is enhanced. As the C impurity concentration exceeds 3.00%, there is a transition from erosion to deposition which is due to the formation of a C film on the W surface. Regarding the result that the erosion changes to deposition, the simulation qualitatively reproduces the experimental results measured by X-ray photoemission spectroscopy (XPS). There is a different tendency in the fluence dependence for the different C impurity concentrations. For C:0.11%, the erosion rate increases with increasing fluence. This results from a growth of a local peak at around a depth of 20 nm in the depth profile of the deposited C. The growth is in good agreement with the experimental result, which shows that there is a strong contribution from recoil implantation of the deposited C due to a synergetic effect of the H and C impurity. For C:0.84%, there are almost the same tendencies as for C:0.11% in the erosion rate and in the depth profile. However, the growth of the local peak at around a depth of 10 nm is in disagreement with the measured one, which occurs near the surface. The disagreement appears to be attributable to the contribution of the surface segregation.

Computer Simulation Study on Incident Fluence Dependence of Ion Reflection and Sputtering Processes from Layered and Mixed Materials

The fluence dependence of D ion reflection and sputtering from C-layered W material, W-layered C material and WxC1-x mixed material, has been demonstrated using the dynamic Monte Carlo program, EDDY. The fluence-dependent depth profile distributions explain such fluence dependence. For the layered materials, the fluence variations of reflection and sputtering are dependent on layer thickness. In particular, for the C layer thickness parallel to the mean ion range for the impact to pure C, the sputtering of the C layer is enhanced with increasing fluence by C emission due to the reflective scattering collisions of D with W near the surface. This is essentially due to the large target mass difference between W and C, which also brings about the fluence variations for the mixed material. The C sputtering is suppressed due to the dynamic behavior of C in the mixed material, whereas the reflection and W sputtering are enhanced.

Simultaneous Calculation of Reflection, Physical Sputtering and Secondary Electron Emission from a Metal Surface due to Impact of Low-Energy Ions

A computer simulation code which treats elastic and inelastic collision processes of low-energy ions in solids is presented. In the code, the direct excitation of electrons by a penetrating ion and recoiling atoms is simulated using the Monte Carlo technique, in addition to the simulation of elastic collisions of the moving particles with solid atoms. Electron cascades of the excited electrons and collision cascades of the recoil atoms are also taken into account, and as a result, the code allows us to simulate ion-solid interactions such as ion reflection, physical sputtering and secondary electron emission from the solids. This code is applied to calculations of the energy and angular distributions of emitted particles and the total particle yields of aluminum by impact of ions with the atomic numbers Z1 of 1 to 17 and energies Ei of 10 eV to 10 keV at normal incidence. The calculated sputtering yield and ion reflection coefficient are in reasonable agreement with empirical formulae which have been recently presented. The calculated electron yield shows the clear dependence on Z1 and Ei, but the Ei-dependence is different from that of the electronic stopping power at such low impact energies. The energy and angular distributions of emitted particles indicate the similarities of the secondary electron emission and the physical sputtering, as observed in recent experiments.

Effects of Capacitively Coupled Radio Frequency Krypton and Argon Plasmas on Gallium Nitride Etching Damage

GaN etching damage characteristics by capacitively coupled radio frequency Kr and Ar plasmas have been found to differ significantly, on the basis of experimental and simulation results. The morphology of a GaN surface etched by Kr plasma is as smooth as that of the as-grown surface, and is independent of gas pressure and etching time. The agreement between the experimental and simulated etching depths for the Kr plasma, which are lower than those for the Ar plasma, indicates a significant contribution to the GaN damage of the physical etching effect. In contrast, Ar plasma etching produces a rough surface, which is dependent on gas pressure and etching time, and appears to be due to a chemical effect. The difference in the GaN surface morphologies etched by the Kr and Ar plasmas may be attributed to the different depths etched for these two plasmas. Moreover, the simulation shows that, for the Kr plasma, Ga is preferentially etched from GaN, whereas the preferential etching of N occurs for the Ar plasma. The difference in preferential etching between the Kr and Ar plasmas may be related to the difference between the GaN surface morphologies etched by these two plasmas.

Damage Analysis of Plasma-Etched n-GaN Crystal Surface by Nitrogen K Near-Edge X-ray Absorption Fine Structure Spectroscopy

The surface of an n-GaN crystal etched with an Ar, Kr, or Xe plasma was analyzed by nitrogen K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectroscopy was carried out with the total electron yield (TEY) method in a sample current mode and the total fluorescence yield (TFY) method by measuring the amount of fluorescence using a photodiode. The shapes of the spectra of Ar plasma-etched samples obtained by the TEY method became smooth (blunt) with increasing Ar pressure from 10 to 200 mTorr. However, those obtained by the TFY method did not change with pressure. These results indicate that the etching damage was restricted in the shallow region of less than a few nm from the surface. A change in the NEXAFS spectral shape of Kr or Xe plasma-etched samples was not observed even when measured by the TEY method. This result indicates that the surface damage in Kr or Xe plasma-etched samples was less pronounced than that in Ar plasma-etched samples.

Ion Transport Properties in Model Gases Based on A·r-n Type Single Potentials

Transport properties of ions in model gases at temperatures T g of 50 and 300 [K] are analysed using extended FTI method with the assumption of isotropic scattering in the center of mass frame. Single potentials inversely proportional to the powers of internuclear separation r as A · r - n which gives the collision frequency proportional to the powers of relative speed as v ( n -4)/ n r are assumed for n values above 4. Variation of reduced thermal mobilities K 0th with T g in a low range of reduced electric field E / N and the E / N dependence of transport properties in a high E / N range are mainly discussed in relation with the power values n .

Damage Characteristics of TiO2 Thin Film Surfaces Etched by Capacitively Coupled Radio Frequency Helium Plasmas

Damage characteristics of TiO2 thin film surfaces etched by capacitively coupled RF He plasmas are found to be dependent on gas pressure and etch time. At a low gas pressure (10 mTorr), the morphology of TiO2 surface etched for 5 min is smooth like the as-grown surface. When the etch time lengthens to 60 min, the surface morphology is smoother. However, the atomic O concentration at the surface is lower than that of the as-grown surface. On the other hand, at a high gas pressure (50–100 mTorr), the He plasma etch causes a rough surface morphology (surface defects) when the etch time lengthens to 60 min.