G. Sergienko

Tokamak operation with high-Z plasma facing components

Plasma operation with high-Z plasma facing components is investigated with regard to sputtering, core impurity contamination and scenario restrictions. A simple model based on dimensionless quantities for fuel and high-Z ion sources and transport to describe the high-Z concentration in the plasma core is introduced. The impurity release and further transport is factorized into the sputtering yield, the relative pedestal penetration probability and a core confinement enhancement factor. Since there are quite large uncertainties, in particular, in the sputtering source and the edge transport of high-Z impurities, very different scenarios covering a wide parameter range are taken into account in order to resolve the experimental trends. Sputtering of tungsten by charge exchange neutrals in the energy range 0.5–2 keV is comparable to the effect of impurity ion sputtering, while the impact of thermal fuel ions is negligible. Fast ions produced by neutral beam injection as well as sheath acceleration during ICR heating may cause considerable high-Z sources if the limiters on the lowfield side have high-Z surfaces. The critical behaviour of the central high-Z concentration in some experimental scenarios could be attributed to edge and core transport parameters in the concentration model. The improved H-mode with off-central heating turns out to be the most critical one, since a hot edge is combined with peaked density profiles. (Some figures in this article are in colour only in the electronic version)

0D model of magnetized hydrogen helium wall conditioning plasmas

In this paper the 0D description of magnetized toroidal hydrogen?helium RF discharges is presented. The model has been developed to obtain insight into the ICRF plasma parameters, particle fluxes to the walls and the main collisional processes, which is especially relevant for the comprehension of RF wall conditioning discharges. The 0D plasma description is based on the energy and particle balance equations for nine principal species: H, H+, H2, , , He, He+, He2+ and e?. It takes into account (1) elementary atomic and molecular collision processes, such as excitation/radiation, ionization, dissociation, recombination and charge exchange, and elastic collisions, (2) particle losses due to the finite dimensions of the plasma volume and confinement properties of the magnetic configuration, and particle recycling, (3) active pumping and gas injection, (4) RF heating of electrons (and protons) and (5) a qualitative description of plasma impurities. The model reproduces experimental plasma density dependences on discharge pressure and coupled RF power, both for hydrogen RF discharges (ne ? (1?5) ? 1010?cm?3) and for helium discharges (ne ? (1?5) ? 1011?cm?3). The modeled wall fluxes of hydrogen discharges are in the range of what is estimated experimentally: ~1019?1020?m?2?s?1 for H atoms, and ~1017?1018?m?2?s?1 for H+ ions. It is found that experimentally evidenced impurity concentrations have an important impact on the plasma parameters, and that wall desorbed particles contribute largely to the total wall flux.

Experimental investigation of ion cyclotron range of frequencies heating scenarios for ITER's half-field hydrogen phase performed in JET

Two ion cyclotron range of frequencies ( ICRF) heating schemes proposed for the half-field operation phase of ITER in hydrogen plasmas-fundamental H majority and second harmonic He-3 ICRF heating-were recently investigated in JET. Although the same magnetic field and RF frequencies (f approximate to 42 MHz and f approximate to 52 MHz, respectively) were used, the density and particularly the plasma temperature were lower than those expected in the initial phase of ITER. Unlike for the well-performing H minority heating scheme to be used in He-4 plasmas, modest heating efficiencies (n = P-absorbed/P-launched < 40%) with dominant electron heating were found in both H plasma scenarios studied, and enhanced plasma-wall interaction manifested by high radiation losses and relatively large impurity content in the plasma was observed. This effect was stronger in the He-3 ICRF heating case than in the H majority heating experiments and it was verified that concentrations as high as similar to 20% are necessary to observe significant ion heating in this case. The RF acceleration of the heated ions was modest in both cases, although a small fraction of the 3He ions reached about 260 keV in the second harmonic He-3 heating experiments when 5MW of ICRF power was applied. Considerable RF acceleration of deuterium beam ions was also observed in some discharges of the He-3 heating experiments (where both the second and third harmonic ion cyclotron resonance layers of the D ions are inside the plasma) whilst it was practically absent in the majority hydrogen heating scenario. While hints of improved RF heating efficiency as a function of the plasma temperature and plasma dilution (with He-4) were confirmed in the H majority case, the He-3 concentration was the main handle on the heating efficiency in the second harmonic He-3 heating scenario.

Operational issues at high lower hybrid power density in JET: waveguide conditioning and arc detection

The power handling capability of the JET Lower Hybrid Current Drive (LHCD) system is examined using the long-term database. The limitations, in particular in H-mode plasmas, are discussed and the performance compared with other LHCD experiments using multijunctions as power dividers. Although the power density of 25 MW m−2 has been exceeded in L-mode and almost obtained in ELMy H-mode (on 1/6th of the antenna), it is concluded that the RF conditioning performed on JET does not allow us to exceed an electric field of ~5.5 kV cm−1 which is generally not sufficient under the rather weak coupling conditions of the JET H-mode. Modelling of an arc occurring in a waveguide indicates that rather small variations of the reflected wave (amplitude and phase) may occur, rendering arc detection based on RF measurements difficult in some cases. The JET bolometry diagnostic with four chords viewing the antenna front is found to be an efficient tool to detect an arc. In L-mode plasmas, a very good correlation between the amplitude of the bolometry signals and the iron spectroscopic lines is found. In H-mode the arc detection is clearly more difficult with enhanced radiation during the ELM but is still possible when the bolometry signals are properly processed.

Study of TEXTOR ICRF Antenna Coupling in the ICWC Mode of Operation

Ion Cyclotron Wall Conditioning (ICWC) discharges, in pulsed‐mode operation, were carried out in TEXTOR to simulate a scenario of ITER wall conditioning at half‐field. The ICWC discharges were performed in a continuous Helium flow with adjunction of molecular Hydrogen, puffed after the ICRF ignition thus achieving better antenna coupling in the Mode Conversion scenario with improved ICWC performance. Reproducible generation of ICRF plasmas and reliable wall conditioning, after proper wall preloading with Deuterium glow, were achieved by coupling the RF power at 29 MHz from one or two ICRF antennas at different toroidal magnetic fields (different cyclotron harmonics). The antenna coupling properties were analyzed in relation to wall conditioning output (removal rates) for selected marker masses. Present study of ICWC at low and high power density per particles (at different gas pressures) emphasizes on the efficiency of the application of stationary, oscillating or rotating poloidal magnetic field. These e...

Impurity Radiation for Detecting Arcs during High Lower Hybrid Power Transmission at JET

During high power commissioning of the JET LH launcher, the radiation and impurity release has been analyzed from various diagnostics: VUV and visible spectroscopy, bolometry. These two last diagnostics have lines‐of‐sight viewing the launcher and can provide information about the electron and/or impurity source localisation. Using a database of 800 plasmas, it is concluded that the iron contamination (FeXV and FeXXIII) is very low for 94% of the pulses and increases linearly with LH power. During arcs, a strong and fast increase of the radiation along the line‐of‐sight viewing the launcher is observed. This diagnostic could provide a tool for arc detection complementary to the RF measurements aiming at reducing the metal contamination in the plasma.

Doppler Line Shapes, Turbulence and Neutral Transport in Tokamak Edge Plasmas

First investigations of the effect of ion temperature fluctuations on Doppler spectral line shapes emitted in tokamak edge plasmas are reported on. It is shown that hot charge exchange neutrals have to be taken into account in the analysis.