Injected RF Power Research Articles

Evaluations and prospects for the next steps in the low power RF systems of the Tore Supra transmitters

The tokamak Tore Supra (TS) has been operated since 1988 in Cadarache Centre. With the Composants Internes et Limiteur project, it will be the only machine to be able to exploit the Advanced Tokamak Regimes in steady state operation with three heating systems ICRH, LH, ECRH. The injected RF power in the Torus is a very important parameter of the total energy balance of TS, for this reason we need the best possible knowledge of these powers. The analysis and the explanation of the different RF techniques at low power which have been used in TS on the three RF systems will be described. Principles of the different measurement systems are given. The calibration methods of these systems are discussed and the present status of the calibration uncertainties of the RF measurements is analysed. Finally, when the generators are connected to a dummy load, RF measurements are compared with the calorimetric power measurements. This study allows preparation of improvements of these systems which will be used in these next steps: Composants pour l'Injection de Matière et d'Energie Stationnaire project (B. Beaumont, Tore Supra steady state power and particles injection: the CIMES project, in: Proceedings of the 21st SOFT, Madrid, 2000).

First plasma experiments in Tore Supra with a new generation of high heat flux limiters for RF antennas

During the 1997 and 1998 Tore Supra shutdown, a first set of new antenna guard limiters was installed on one of the three ion cycloton resonance heating (ICRH) antennas of Tore Supra. This limiter, which was one of the main technological studies of the 1998 campaign, was widely experimented in real plasma conditions, thus allowing the validation in situ, for the first time, of the technology of active metal casting (AMC) for plasma facing components. The huge improvement in the thermal response of the new limiter generation, compared to the old one, is shown on plasma pulses made identical in terms of antenna position and injected RF power profile. By using the infrared cameras installed inside Tore Supra and viewing the antennas front, the power density fluxes received by the carbon fibre composite (CFC) surface of the limiter were evaluated by correlation with the heat load tests made on the electrons beam facility of CEA/Framatome.

ECR and LH effects on the plasma boundary in DITE

The edge plasma response to Lower Hybrid Current Drive (LHCD) and Electron Cyclotron Heating (ECH) has been studied in DITE with input powers of up to 250 and 300 kW, respectively. During LHCD significant reductions in n e(edge), T e(edge) and the scrape-off layer (SOL) particle flux are observed. These effects, together with an observed decrease in the fluctuation levels E ~ θ and ñ e, and in the H α emission, are strong evidence for an increase in particle confinement. During ECH precisely the opposite occurs and particle confinement decreases. Measurements have also been made during Lower Hybrid Heating (LHH). Global power balance has been studied by comparing the power conducted and convected to the edge, computed using Langmuir probes in the limiter, with the total power input less the radiated power. Reasonably good power accounting is observed in ohmic, ECRH and LHH phases but, during LHCD, virtually none of the injected RF power is observed in that reaching the boundary. This is consistent with a very local power loss and/or power loss via a high-energy component in the electron distribution which would not be registered in the Langmuir probe bias range (—100 to ~ 30 V).

The effect of ICRF on the Alcator C scrape-off layer plasma

This paper presents a characterization of the Alcator C scrape-off layer (SOL) plasma during ICRF second harmonic hydrogen fast wave heating experiments. The SOL plasma parameters were measured using a multifunctional probe, Janus, which is capable of simultaneously measuring the ion and electron parameters both parallel and antiparallel with respect to the toroidal magnetic field. The probe data indicate, at low value of injected RF power, that there is direct edge heating and density increases at radii greater than that of the antenna Faraday shield. Increasing the injected RF power spreads both the temperature and density increases throughout the edge region, flattening the radial profiles. Varying the position of the resonance layer in the main plasma does not significantly change the effect of ICRF on the SOL parameters. Given this single spatial point characterization of the SOL, a crude estimate of power flow into and through the edge plasma indicate that ≈ 20% of the ICRF power launched from the antenna is absorbed directly in the SOL plasma. Additional observation of the impurity source rates confirms the conclusions of an earlier paper, which attributed increasing central densities of high-Z impurities to the increase in physical sputtering rate at both the ICRF antenna's Faraday shield.

Plasma Current Sustainment and Ramp-Up by Electron Cyclotron Waves in the WT-2 Tokamak

By injecting RF power in the electron cyclotron(EC) range of frequency, the plasma current is sustained and ramped up after OH power is shut off. Here EC driven current is generated by EC heating of suprathermal electrons in the OH plasma. The plasma current is also generated in the RF discharge plasma at EC resonance (ECR plasma) in WT-2. Both in OH-produced and ECR-produced plasmas, the electron velocity distribution function is asymmetric with respect to the magnetic field. This asymmetry leads to an asymmetric EC heating generating the toroidal plasma current. The efficiency of EC current drive is obtained as η∼1×10 -4 (10 20 m -2 kA/kW), which is small in comparison with the theoretical prediction.

Toroidal Plasma Current Sustainment by Lower Hybrid Waves in the WT-2 Tokamak

By injecting rf power ${P}_{\mathrm{rf}}$ near the lower hybrid frequency into an Ohmically heated plasma near the end of tokamak discharge in the WT-2, an rf-driven current-sustained plasma is produced and continues to exist during the rf injection period, the toroidal current (${I}_{p}\ensuremath{\sim}10$ kA) being generated by the lower hybrid wave only without the Ohmic power. The efficiency of rf-driven current generation is high and attains $\frac{{I}_{p}}{{P}_{\mathrm{rf}}}\ensuremath{\approx}0.9$ kA/kW at the plasma density ${n}_{e}\ensuremath{\simeq}1.4\ifmmode\times\else\texttimes\fi{}{10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, with somewhat lower efficiency at higher density.

Stochastic ion heating by a lower hybrid wave

The motion of an ion in a lower hybrid wave in a tokamak type plasma is studied. For ions with ν⊥≳ω/k⊥ the motion is stochastic for fields satisfying E/B0≳ (1/4)(Ωi/ω)1/3(ω/k⊥). Provided that the perpendicular phase velocity, ω/k⊥, can be slowed down to a few times the ion thermal speed, this stochastic ion motion may be an important mechanism by which injected rf power near the lower hybrid frequency can directly heat the ions.