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In neutral beam injection and ion cyclotron resonance frequency (NBI+ICRF) heated high-temperature Joint European Torus (JET) plasmas, electron cyclotron emission (ECE) spectra measured by the Michelson interferometer are inconsistent with the electron bulk distribution being Maxwellian. This conclusion follows from new, accurate modeling of the ECE spectra over the full measured frequency range, covering the first four harmonics of the cyclotron frequency. A model distribution function obtained by fitting a measured ECE spectrum reveals a distortion of the Maxwellian that is sharply localized at ∼1.5×thermal momentum. A consequence deduced from this distribution is that the electron temperature measured by JET’s Thomson scattering diagnostic, which weighs the bulk electrons differently, should be lower than the ECE one in the spatial region where this distortion exists. This prediction appears to be confirmed by temperature profile comparisons showing that core Thomson scattering electron temperatures on JET are up to 20% lower than ECE values for plasmas with strong NBI+ICRF heating. The possibility of addressing experimentally the question of whether the plasma has a Maxwellian bulk is also discussed. This goal can be accomplished by observing the emission at different angles with respect to the magnetic field (oblique ECE). Simulations of oblique ECE spectra using the model distribution function obtained previously show an angular variation of the emission that is characteristic of the distorted bulk.

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R=1.5 m, a

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, coupling of the full ECRH power (PECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

In April 1994 first ECRH plasmas (28 GHz, 2nd harmonic) have been produced in the TJ-I U torsatron (R=0.6m, [a]=0.1m, B(0)=0.52 T, E(0)=0.21). In November 1994 a second experimental campaign using a 37.5 GHz gyrotron (B(o)=0.69 T) has started. The main parameters of the device, the basic characteristics of the heating system and some results concerning magnetic surface mapping and first plasma discharges, are reported. 4 refs., 4 figs.

This paper addresses the problem of diagnosing the electron distribution function by measuring Electron Cyclotron Emission (ECE) at different angles with respect to the magnetic field. This diagnostic set-up, that is often referred to as oblique ECE, provides a unique method for measuring the electron distribution function with good energy and space resolution at once. First oblique ECE measurements have been performed on FTU during onaxis Electron Cyclotron Resonance Heating (ECRH) applied on the current ramp-up phase of the discharge. The experimental results appear to be consistent with theoretical FokkerPlanck calculations, showing the existence of a deformation of the bulk of the electron distribution function.

A method for investigating the non-equilibrium kinetic processes which determine the formation of a non-Maxwellian electron distribution in tokamak plasmas is discussed. The case of a non-Maxwellian fast electron tail is considered in detail. It is shown that the study of the transient phase after modification of the tail by a short pulse of electron cyclotron wave energy yields valuable information on the mechanisms of tail formation.

In order to study the absorption and emission properties of a magnetized plasma in the electron cyclotron range of frequencies, the weakly relativistic (Shkarofsky) plasma dispersion functions are simply and exactly expressed in terms of the Z function. This gives a useful working form to the dielectric tensor, for any wave vector and harmonic number, covering also the case of electron Maxwellian distributions drifting along the magnetic field.

Electron cyclotron resonance current drive in tokamak plasmas in the presence of a lower hybrid tail is investigated using a two-dimensional Fokker-Planck code. For an extraordinary mode at oblique propagation and down-shifted frequency it is shown that the efficiency of electron cyclotron current drive becomes (i) substantially greater than the corresponding efficiency of a Maxwellian plasma at the same bulk temperature, (ii) equal to or greater than that of lower hybrid waves, and (iii) comparable to the efficiency of a Maxwellian plasma at much higher temperatures. This enhancement of the efficiency results from a beneficial cross-effect of the two waves on the formation of the current carrying electron tail. The use of the ordinary mode for outside launching in the equatorial plane is also discussed and it is shown that the O-mode is generally not suited for current drive via the lower hybrid tail and that in a hot Maxwellian plasma the efficiency at down-shifted frequencies is comparable to that at up-shifted frequencies.

Wave damping near the electron gyrofrequency in a Tokamak plasma with the energetic tail generated by the electric field is investigated. The electron tail is computed by a Fokker-Planck initial value code as a function of the relevant parameter E/sub ////Ec=E/sub ///Te/(2 pi nee3 Lambda ). It is shown that strong damping of the e-mode occurs for oblique propagation. The results are of relevance for studies of ECRH in present-day Tokamaks and in future reactors where a mildly relativistic electron tail is naturally present for large Te. Special emphasis is therefore given to wave absorption for frequencies of significantly below the central electron gyrofrequency, and to the associated r.f.-driven current.

Current drive by electron cyclotron heating of the far end of a lower-hybrid-sustained electron tail is investigated. The current density profile is determined by means of the kinetic theory and the ray-tracing code for the PLT and the Alcator C parameters. It is shown that the value of the ratio W/I for the combined effects of lower hybrid and electron cyclotron wave absorption can be improved compared to that of the present lower hybrid experiments. Suitable shaping of the current density profiles can be achieved by adjusting the wave-launching parameters.

The absorption is considered of electron cyclotron waves (ECW) propagating away from normal to the magnetic field in hot tokamak plasmas. The relativistic dispersion relation including Larmor radius effects is presented in a relatively simple and instructive form. It is shown that for the X-mode at down-shifted frequency and Te 20 keV, the resonant frequency lies in the subharmonic range of frequencies and the parallel velocity of the absorbing electrons is close to the speed of light. Also pointed out is the relevance of the parallel change in the kinetic energy and the momentum induced by the ECW of relativistic resonant electrons to the quasi-linear evolution of the electron distribution.

Plasma heating in existing tokamak devices by electron cyclotron waves with frequency (f) significantly smaller than the electron gyrofrequency (fc) is investigated for the case of Maxwellian plasmas. It is shown that for central electron temperatures larger than 3 keV, strong absorption of extraordinary waves can occur at values of the toroidal field for which the condition f = fc is not satisfied in the plasma region. The cases of f = 60 GHz and f = 100 GHz are discussed for the PLT and TFTR devices as being representative of medium (≈30 kG) and high (≈50 kG) magnetic field tokamaks, respectively. Numerical calculations with a ray tracing code indicate that most of the RF energy is absorbed in a central plasma region. These results are of practical interest because they significantly simplify the main technical problem of ECH in a tokamak reactor, i.e. the development of high-frequency and high-power microwave sources.

We present the solution of the kinetic equation describing the quasi-linear evolution of the electron momentum distribution function under the influence of the electron cyclotron wave absorption, Coulomb collisions and the dc electric field in a tokamak plasma. The solution of the quasi-linear equation is obtained numerically using a two-dimensional initial value code following an ADI scheme. Most emphasis is given to the full non-linear and self-consistent problem, namely, the wave amplitude is evaluated at any instant and any point in space according to the actual damping. This is necessary since wave damping is a very sensitive function of the slope of the local momentum distribution function because the resonance condition relates the electron momentum to the location of wave energy deposition.

Electron cyclotron wave absorption at down-shifted frequencies in the JET device is investigated. It is shown that strong damping of the extraordinary mode for top-side launching is found at B(o)=32 kG, f=70 GHz and Te(o) approximately=4 keV. It is shown that the method is suited for body heating and assist of lower hybrid current drive.

The ordinary mode damping near the electron‐cyclotron frequency is investigated using the relativistic dispersion relation. It is shown that the Doppler splitting of the absorption spectrum claimed by Arunasalam et al. [Phys. Rev. A 36, 3909 (1987); 37, 2063 (1988)] is an erroneous result obtained from an oversimplified dispersion relation and assuming an infinite speed of light in the resonance condition.

Electron cyclotron resonance current drive in a tokamak plasma in the presence of a lower hybrid tail is investigated using a 2D Fokker‐Planck code. For an extraordinary mode at oblique propagation and down‐shifted frequency, it is shown that the efficiency of electron cyclotron current drive becomes equal or greater than that of the lower hybrid waves and much greater than the corresponding efficiency of a Maxwellian target plasma at the same bulk temperature.

It is shown that the ECH at down shifted frequencies can be tested in present day totamaks with the existing microwave sources (GoGHz) or those under development (100GHz).(AIP)

The first experimental campaign of the TJ-II stellarator has been conducted using electron cyclotron resonance heating (f = 53.2 GHz, P-ECRH approximate to 250 kW) with a pulse length of Delta t approximate to (80-200) ms. The flexibility of the device has been used to study five different configurations varying plasma volume and rotational transform. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma-wall interaction phenomena on TJ-II confinement is briefly discussed.

ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a < 0.22 m, B = 1 T, f = 53.2 GHz, P-ECRH = 300 kW, power density = 1-25 W cm(-3)). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.

The power dependence of the wave damping in a tokamak plasma for arbitrary direction of propagation, mode of polarization, and wave frequency is investigated. Using a 2‐D Fokker–Planck code, it is shown that the wave damping increases or decreases with the rf power depending on the velocity range of the absorbing electrons.

Electron‐cyclotron resonance absorption for oblique propagation at frequency significantly below the central electron gyrofrequency is considered. It is shown that the extraordinary mode for external launching is suited for additional heating in a tokamak reactor. The properties of wave damping at downshifted frequencies are discussed and are found to differ from those at f≊fc. Using the ray‐tracing code, the wave damping of the e mode is investigated at high densities for which strong refraction of the wave beam occurs.