Torbjörn Hellsten

Toroidal rotation in RF heated JET plasmas

Observations of bulk plasma rotation in radio frequency (RF) heated JET discharges are reported. This study is concentrated on RF heated L-mode plasmas. In particular, the toroidal rotation profiles in plasmas heated by ion cyclotron resonance frequency (ICRF) waves and lower hybrid (LH) waves have been analysed. It is the first time that rotation profiles in JET plasmas with LH waves have been measured in dedicated discharges. It is found that the toroidal plasma rotation in the outer region of the plasmas is in the co-current direction irrespective of the heating scenario. An interesting feature is that the toroidal rotation profile appears to be hollow in many discharges at low plasma current, but a low current in itself does not seem to be a sufficient condition for finding such profiles. Fast ion transport and finite orbit width effects are mechanisms that could explain hollow rotation profiles. This possibility has been investigated by numerical simulations of the torque on the bulk plasma due to fast ICRF accelerated ions. The obtained torque is used in a transport equation for the toroidal momentum density to estimate the effect on the thermal bulk plasma rotation profile.

Dual sightline measurements of MeV range deuterons with neutron and gamma-ray spectroscopy at JET

Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third harmonic radio-frequency (RF) heating coupled into a deuterium beam are reported. Measurements are ba ...

Theoretical analysis of ICRF heating in JET DT plasmas

A number of experiments with heating of DT plasmas using ICRF waves have been carried out at JET. The results of these experiments have been analysed by comparing experimentally measured quantities with the results of numerical simulations. In particular, four scenarios have been examined: (a) heating of minority (~5-20%) deuterons at the fundamental ion cyclotron frequency, ω = ωcD; (b) second harmonic heating of tritium, ω = 2ωcT; (c) fundamental minority heating of 3He with a few per cent of 3He; (d) second harmonic heating of deuterium, ω = 2ωcD. An important aim of the analysis was to assess whether the present understanding of the ICRF physics is adequate for predicting the performance of ICRF in DT plasmas. In general, good agreement between experimental results and simulations was found which increases the confidence in predictions of the impact of ICRF heating in future reactors. However, when a relatively high deuterium concentration was used in the ω = ωcD scenario, discrepancies were observed. In order to increase confidence in the simulations, the sensitivity of the simulation results to various plasma parameters has been studied.

The influence of finite drift orbit width on ICRF heating in toroidal plasmas

Ion cyclotron resonance heating in a toroidal plasma not only increases the perpendicular energy of the resonating ions but also results in their spatial transport. Depending on the direction of propagation of the waves, the ions will either drift inwards or outwards giving rise to an RF induced rotation with the toroidal torque component in the co-current or counter-current directions, respectively. It is found that the spatial transport induced by the RF field, the topology of the ion drift orbits and a wave field consistent with ion absorption are important for determining the distribution function of the heated species. Studies of ICRF heating with the self-consistent code SELFO reveal new features such as the formation of non-standard passing orbits residing on the low field side of the magnetic axis. For a symmetric spectrum the drift terms will in general not cancel. Some classes of orbit will be subjected only to an inward drift and others only to an outward drift. The lack of cancellation of the drift terms is further enhanced by the self-consistent coupling, increasing the absorption for waves propagating parallel to the plasma current, but not for waves propagating in the antiparallel direction. This results in a strong inward pinch also for symmetric wave spectra as well as for typical experimental spectra, with the dominant peak in the counter-plasma-direction.

A model for calculating ICRH power deposition and velocity distribution

During intense ICRH the velocity distribution of the resonating ions becomes strongly distorted. The power deposition and the velocity distribution depend on each other making it necessary to calculate a power deposition which is consistent with the velocity distribution. A method for calculating such power depositions is presented. A code called PION has been developed based on this method. The code uses simplified models of the power deposition and velocity distribution.

Ion cyclotron range of frequency mode conversion flow drive in D(3He) plasmas on JET

Ion cyclotron range of frequency (ICRF) mode conversion has been shown to drive toroidal flow in JET D(He-3) L-mode plasmas: B-t0 = 3.45 T, n(e0) similar to 3x10(19) m(-3), I-p = 2.8 and 1.8 MA, P-RF l= 3MW at 33MHz and -90 degrees phasing. Central toroidal rotation in the counter-I-p direction, with omega(phi 0) up to 10 krad s(-1) (V-phi 0 similar to 30 km s(-1), central thermal Mach number M-th(0) similar to 0.07 and Alfven Mach number M-A(0) similar to 0.003) has been observed. The flow drive effect is sensitive to the He-3 concentration and the largest rotation is observed in the range X[He-3] = n(He3)/n(e) similar to 10-17%. The rotation profile is peaked near the magnetic axis, and the central rotation scales with the input RF power. The effective torque density profile from the RF power has been calculated and the total torque is estimated to be as high as 50% of the same power from neutral beam injection, and a factor of 5 larger than the direct momentum injection from the RF waves. RF physics modeling using the TORIC code shows that the interaction between the mode converted ion cyclotron wave and the He-3 ions, and associated asymmetry in space and momentum, may be key for flow drive.

Toroidal mode conversion in the ICRF

Mode conversion is studied in the ion cyclotron range of frequencies (ICRF), taking into account the toroidal geometry relevant for tokamaks. The global wavefields obtained using the gyrokinetic toroidal PENN code illustrate how the fast wave propagates to the neighbourhood of the ion-ion hybrid resonance, where it is converted to a slow wave that deposits the wave energy through resonant Landau and cyclotron interactions with the particles. The power deposition profiles obtained are dramatically different from the toroidal resonance absorption, showing that Buddens fluid model is not a good approximation in the torus. Radially and poloidally localized wavefield structures characteristic of slow wave eigenmodes are predicted, which could be used in experiments to form transport barriers and to interact with fast particles.

Minority and mode conversion heating in (3He)–H JET plasmas

Radio frequency (RF) heating experiments have recently been conducted in JET (He-3)-H plasmas. This type of plasmas will be used in ITERs non-activated operation phase. Whereas a companion paper in this same PPCF issue will discuss the RF heating scenarios at half the nominal magnetic field, this paper documents the heating performance in (He-3)-H plasmas at full field, with fundamental cyclotron heating of He-3 as the only possible ion heating scheme in view of the foreseen ITER antenna frequency bandwidth. Dominant electron heating with global heating efficiencies between 30% and 70% depending on the He-3 concentration were observed and mode conversion (MC) heating proved to be as efficient as He-3 minority heating. The unwanted presence of both He-4 and D in the discharges gave rise to 2 MC layers rather than a single one. This together with the fact that the location of the high-field side fast wave (FW) cutoff is a sensitive function of the parallel wave number and that one of the locations of the wave confluences critically depends on the He-3 concentration made the interpretation of the results, although more complex, very interesting: three regimes could be distinguished as a function of X[He-3]: (i) a regime at low concentration (X[He-3] < 1.8%) at which ion cyclotron resonance frequency (ICRF) heating is efficient, (ii) a regime at intermediate concentrations (1.8 < X[He-3] < 5%) in which the RF performance is degrading and ultimately becoming very poor, and finally (iii) a good heating regime at He-3 concentrations beyond 6%. In this latter regime, the heating efficiency did not critically depend on the actual concentration while at lower concentrations (X[He-3] < 4%) a bigger excursion in heating efficiency is observed and the estimates differ somewhat from shot to shot, also depending on whether local or global signals are chosen for the analysis. The different dynamics at the various concentrations can be traced back to the presence of 2 MC layers and their associated FW cutoffs residing inside the plasma at low He-3 concentration. One of these layers is approaching and crossing the low-field side plasma edge when 1.8 < X[He-3] < 5%. Adopting a minimization procedure to correlate the MC positions with the plasma composition reveals that the different behaviors observed are due to contamination of the plasma. Wave modeling not only supports this interpretation but also shows that moderate concentrations of D-like species significantly alter the overall wave behavior in He-3-H plasmas. Whereas numerical modeling yields quantitative information on the heating efficiency, analytical work gives a good description of the dominant underlying wave interaction physics.

On the orbit-averaged Monte Carlo operator describing ion cyclotron resonance frequency wave–particle interaction in a tokamak

In a toroidal plasma the distribution function of ions interacting resonantly with waves in the ion cyclotron range of frequencies (ICRF) can be described with a three-dimensional orbit-averaged Fokker–Planck equation. This equation can be solved with a Monte Carlo method. Explicit expressions for the Monte Carlo operator describing wave–particle interaction, within the framework of quasilinear theory, are given. Furthermore, properties of the operator are discussed.

Fast wave absorption at the Alfvén resonance during ion cyclotron resonance heating

For ICRH scenarii where the majority cyclotron resonance intersects the plasma core, mode conversion of the fast magnetosonic wave to an Alfven wave takes place at the plasma boundary on the high field side. Simple analytical estimates of the converted power for this mode conversion process are derived and compared with numerical calculations including finite electron inertia and kinetic effects. The converted power is found to depend on the local value of the wave field as well as on plasma parameters at the Alfven wave resonance. The interference with the reflected wave will therefore modify the mode conversion. If the conversion layer is localized near the wall, the conversion will be strongly reduced. The conversion coefficient is found to be strongest for small density gradients and high density and it is sensitive to the value of the parallel wave number. Whether it increases or decreases with the latter depends on the ion composition. Analysis of this problem for ICRH in JET predicts that a large fraction of the power is mode converted at the plasma boundary for first harmonic heating of tritium in a deuterium-tritium plasma.