[en] The first ecperiments to study plasma heating at lower hybrid frequencies and current drive phenomena were performed with klystrons derived from tubes developed for other purposes (Radar, TV, ISM...). To increase the energy transmitted to the plasma and for extensive use of the RF current drive, the development of a specific klystron was necessary with the following requirements: RF frequency in upper S-band or above; large RF unit power per source; quasi-steady-state or CW operation. The TH 2103 responds to these objectives with the following characteristics: 3.7 GHz - 500kW - 60 sec. or CW - 45% typical efficiency and 50 dB gain - VSWR of 1.3:1 - 60 kV x 18.5 A. (author). 8 figs

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[en] The impact of the presence of trapped electrons on noninductive current drive by RF waves in tokamak plasmas is investigated. The appropriate response function, allowing to express the current drive efficiency J/P by a simple analytical formula, has been derived. The approach displays the reasons for the degradation of the current drive efficiency away from the plasma axis in the case of methods relying on the diffusion of electrons in the velocity component perpendicular to the confining magnetic field. It is shown that this degradation is appreciable even for large resonant parallel velocities. (author)

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[en] General formulas for the efficiency of current drive in toroidal plasmas are derived using entropy arguments. The highest possible efficiency for current drive in which a high-energy electron tail is formed is shown to be p = E/sub r/j, with p and j the power and current densities and E/sub r/≅0.09n₁₄ V/m with n₁₄ the electron density in units of 10¹⁴/cm.³ The electric field required to maintain the current in a runaway discharge is also shown to equal E/sub r/. If the plasma current is carried by near-Maxwellian electrons, waves that have a low phase velocity, compared to the energy of the electrons with which they interact, can drive a current with Ohmic efficiency, p = etaj². Such waves were first discussed in the context of current drive by Fisch [Rev. Mod. Phys. 59, 175 (1987)]

[en] To reach ignition in a Tokamak plasma, large additional power P_aux has to be injected in the device on top of the Joule heating P_OH=VI_r, V the plasma loop voltage, I_r the resistive port of plasma current. Typi-cally < T >_JH ∼ 1 KeV, whereas ignition would requi- re < T >_IG ∼ 7-10 KeV. To gain this factor 7, one at least should inject additional power P_aux ∼ 7P_OH, supposing that nothing, especially the heat transport, is modified. This is by far not the case, with the so-called energy lifetime degradation, largely observed in oil experiments (but less dramatic with divertors), where energy lifetime tau_E behaves like P_tot^-b with b∼1/2. In large machines where ignition temperature is the target to be imperiously reached, this implies to inject a very large power, typically P_aux∼ 50 to 100 MW, depending on size and parameters and on actual transport. So it is of importance with such figures, or even larger ones owing to uncertain ties, to optimize at best injected power by increasing its efficiency, both with respect to possible transport laws, and to physical phenomena governing heat flow in the system from the sources. This leads to the concept of scenarios, as time sequences of power input, where physical properties of the plasma system are used to build up ion temperature so that ignition is reached with minimum P_tot = P_OH + P_aux and with fixed Q = Q_o > 1. Elements for this study are given. The method is outlined. The resulting system of equations describing the evolution of a thermonuclear plasma is given

[en] With the advent of the new large tokamaks JET, JT-60 and TFTR important advances in magnetic confinement have been made. These include the exploitation of radio frequency and neutral beam heating on a much larger scale than previously, the demonstration of regimes of improved confinement and the demonstration of current drive at the Megamp level. A number of small and medium sized tokamaks have also come into operation recently such as WT-3 in Japan with an emphasis on radio frequency current drive and HL-1 a medium sized tokamak in China. Each of these new tokamaks is addressing specific problems which remain for the future development of the system. Of these particular problems: β, density and q limits remain important issues for the future development of the tokamak. β limits are being addressed on the DIII-D device in the USA. The anomalous confinement that the tokamak displays is being explored in detail on the TEXT device in the USA. Two other problems are impurity control and current drive. There is significant emphasis on divertor configurations at the present time with their enhanced confinement in the so called H mode. Due to improved discharge cleaning techniques and the ability to repetitively refuel using pellets, purer plasmas can be obtained even without divertors. Current drive remains a crucial issue for quasi of near steady state operation of the tokamak in the future and many current drive schemes are being investigated. (author)

[en] Lower hybrid heating and current drive experiments at 1.3 GHz with up to 800 kW/1.5 sec pulses have been performed in ASDEX in a wide parameter range, anti nsub(e)=0.2-5x10¹³ cm^-3, Bsub(t)=2-2.5 T, and Isub(p)=140-400 kA. For the current drive experiments reported here, the eight waveguide grill is operated with a relative phase of π/2 between successive waveguides. These experiments have been performed in two different modes of operation. (orig./GG)