H.-S. Bosch

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Plans for the first plasma operation of Wendelstein 7-X

Wendelstein 7-X (W7-X) is currently under commissioning in preparation for its initial plasma operation phase, operation phase 1.1 (OP1.1). This first phase serves primarily to provide an integral commissioning of all major systems needed for plasma operation, as well as systems, such as diagnostics, that need plasma operation to verify their foreseen functions. In OP1.1, W7-X will have a reduced set of in-vessel components. In particular, five graphite limiter stripes replace the later foreseen divertor. This paper describes the expected machine capabilities in OP1.1, as well as a selection of physics topics that can be addressed in OP1.1, despite the simplified configuration and the reduced machine capabilities. Physics topics include the verification and adjustment of the magnetic topology, the testing of the foreseen plasma start-up scenarios and the feed-forward control of plasma density and temperature evolution, as well as more advanced topics such as scrape-off layer (SOL) studies at short connection lengths and transport studies. Plasma operation in OP1.1 will primarily be performed in helium, with a hydrogen plasma phase at the end.

Low energy neutral particle fluxes to the walls of ASDEX during He and D2 discharges

Neutral particle fluxes onto the walls of ASDEX have been investigated using a time-of-flight (TOF) method. The energy distributions of the neutrals could be determined in the range of 10–1000 eV/amu. Ohmic divertor and limiter discharges with equal plasma currents and densities have been compared for He and D2. The He0 outflux at ∼2000 eV from He discharges is 110 of the corresponding D0 flux in D2 discharges. At lower energies this difference is much smaller. In all cases many more He neutrals were observed than was anticipated from the CX rate-coefficients for He2+. The impurity fluxes due to sputtering by the CX-neutrals show no significant difference for He and D2 discharges. For divertor discharges CX-sputtering can fully account for the Fe impurity content determined spectroscopically.

Role of divertor geometry on detachment in ASDEX Upgrade

Abstract The change of the divertor plasma behaviour from Div-I to Div-II for ASDEX Upgrade as measured by a set of reconstructed or newly designed divertor diagnostics is presented. The Div-II configuration is characterised – due to the highly inclined target plates – by reflection of neutrals towards the separatrix. Therefore, detachment in Div-II starts rather early localised close to the separatrix. In contrast, the complete global divertor detachment is practically unchanged, because the outer parts of the SOL stay attached even for high densities. The power distribution on the divertor in Div-II is much broader than in Div-I, resulting in a large reduction of peak power loads both in L and H-mode (factor of 2–4) due to larger divertor radiation losses. The larger losses are caused by larger hydrogen losses, enhancement of carbon radiation due to radial transport and convective energy transport into the radiation zone and larger radial energy transport in the divertor. The new Div-II geometry shows larger neutral gas densities in the divertor for the same line averaged density and a much faster helium exhaust rate ( τ ∗ He /τ E ≈4 ) in H-mode. The neon compression is worse compared with helium. No strong effect on impurity compression and overall divertor performance was seen by puff and pump experiments, neither in Div-I nor in Div-II.

2D modelling of the ASDEX-Upgrade scrape-off layer and divertor plasma☆

Abstract Due to the open field lines, the scrape-off layer and divertor region of tokamak plasmas is a complex, two-dimensional system, involving transport parallel and perpendicular to the magnetic field, as well as interaction of the plasma with surfaces and with the neutral gas. Therefore sophisticated two-dimensional codes are required to model the divertor and edge physics. In this paper, the B2-EIRENE code package is used to simulate the ASDEX-Upgrade scrape-off layer plasma and the neutral gas dynamics in a fully self-consistent way. Specific ASDEX-Upgrade discharges are modelled using the actual magnetic configuration and in-vessel components. Single fluid as well as multifluid calculations including self-consistent target and wall erosion of carbon are described. At given input power and bulk plasma radiation, typical divertor plasma profiles from Langmuir probes are fitted by varying the separatrix density and the transport coefficients. On the basis of such multifluid fits, spectroscopic divertor diagnostics are numerically modelled and compared with measured profiles, and reasonable agreement is found.

Neutron calibration techniques for comparison of tokamak results

A workshop on 1–3 August 1989 reviewed the techniques, uncertainties, and experiences of neutron calibration on PLT, TFTR, JET, Tore Supra, JT‐60, JIPPT‐IIU, Alcator C‐Mod, ATF, FT, ASDEX, Textor, and DIII‐D. In the summary session, the workshop participants discussed possible consensus neutron calibration techniques appropriate to D‐D plasmas in tokamaks. The application of such techniques would facilitate a more accurate comparison of neutron yields from different devices, and also allow new calibration techniques to relate their precision to a reference value. General agreement was reached on the suitability of two techniques: (1) a 252Cf source calibration of epithermal neutron detectors, and (2) threshold neutron activation of Ni foils placed vertically above or below the plasma. This paper will present details on detector positioning, neutron transport calculations, and interlab normalization needed to accomplish the standardized calibration using a Cf neutron source.

B2-Eirene modelling of ASDEX Upgrade

Abstract The extension of the computational region of the coupled fluid plasma, Monte-Carlo neutrals code, B2-Eirene, to the plasma center is discussed. The simulation of completely detached H-mode plasma is presented, as is the modelling of He and Ne compression.

Radiative boundary discharges with impurity injection and the H-L transition in ASDEX Upgrade

The influence of impurity radiation on the and transitions is investigated for highly radiative divertor discharges in the ASDEX Upgrade tokamak. The transitions between H- and L-mode depend on the net heat flow across the separatrix, , calculated from the heating power and bolometric radiation profiles. For typical radiative boundary conditions in ASDEX Upgrade, the radiation distribution is dominated by line emission in the vacuum-ultraviolet region and peaks near the separatrix. For the case of neon used as seed impurity, about 2/3 of the main chamber radiation is emitted inside, but close to the separatrix. Argon seed results in a higher fraction of core radiation, while the line emission is shifted further outside for nitrogen. The radiation-corrected threshold is not affected by gas puffing and is described by [MW, , T, amu]. The threshold power, which is typically lower by a factor of two without strong deuterium puffing, is increased by heavy gas puffing leading to . In the vicinity of the radiation-induced transition, a general alignment of H and L mode is observed with regard to global energy confinement time and edge density and temperature profiles. The transition itself exhibits a smooth evolution in time. Reduction of target plate power load down to about 10% of the total heating power is easily achieved by edge radiation in the CDH-mode for low conditions. However, this reduction is attributed mainly to radiation from inside the separatrix and is connected to relatively high values of the core . These results emphasize the importance of the development of more closed divertor concepts, allowing for higher divertor radiation levels in connection with lower core .

TFTR epithermal neutron detector system: Recalibration and effect of nonisotropic neutron emission

The primary TFTR neutron source strength measurement system consists of seven fission detectors previously calibrated with D–D and D–T neutron generators and a 252Cf neutron source inside the TFTR vacuum vessel. A recalibration became desirable because of the addition of major components to the tokamak. The new calibration with the D–D neutron generator in situ is consistent with the detection efficiencies measured in the previous calibrations, within the uncertainties. Effects of the anisotropic emission of the neutron generator, due both to the variation of the differential D–D yield with angle (similar to that from beam–target and beam–beam reactions in the beam‐driven TFTR plasma) and to scattering and absorption by the generator heads have been observed.

The fishbone instability in ASDEX upgrade

In ASDEX Upgrade the excitation of the fishbone instability has been experimentally investigated. The initial frequency of the fishbone oscillation scales with the toroidal precession frequency of the fast trapped ions. A new feature, the coupling of the fishbone mode with the edge localized mode (ELM) instability during high-β discharges, is found. Fishbones occur in ASDEX Upgrade only for βfast = βtor/(1+τE /τsd) above a certain threshold. It is thus possible to calculate their destabilization regime in terms of global plasma parameters, such as plasma current, plasma density, toroidal magnetic field and central electron temperature.