S. Coda

Electron cyclotron heating (ECH) and current drive (ECCD) experiments have been carried out in the TCV tokamak with three 0.5 MW gyrotrons operating in X-mode at the second harmonic (82.7 GHz). The launching mirrors can be independently rotated both poloidally and toroidally. The hard X-ray (HXR, ~10-150 keV) emission from suprathermal electrons is studied with an 8-channel CdTe pinhole camera

Electron Bernstein waves (EBW) are suitable for heating, current drive, and radiation temperature measurements of overdense plasma, ωpe>>ωce , in spherical torus devices. In NSTX, design of a multi-megawatt EBW current drive system is supported by experimental measurements and computations of obliquely viewing, dual-polarization EBW emission (EBE) radiometry. Efficient EBW coupling, 80±20% at 16.5 GHz, is demonstrated, in agreement with

ABSTRACT The role played by electron density fluctuations near the plasma edge on rf current drive in tokamaks is assessed quantitatively. For this purpose, a general framework for incorporating density fluctuations in existing modelling tools has been developed. It is valid when rf power absorption takes place far from the fluctuating region of the plasma. The ray-tracing formalism is modified in order to take into account time-dependent perturbations of the density, while the Fokker–Planck solver remains unchanged. The evolution of the electron distribution function in time and space under the competing effects of collisions and quasilinear diffusion by rf waves is determined consistently with the time scale of fluctuations described as a statistical process.Using the ray-tracing code C3PO and the 3D linearized relativistic bounce-averaged Fokker–Planck solver LUKE, the effect of electron density fluctuations on the current driven by the lower hybrid (LH) and the electron cyclotron (EC) waves is estimated quantitatively. A thin fluctuating layer characterized by electron drift wave turbulence at the plasma edge is considered. The effect of fluctuations on the LH wave propagation is equivalent to a random scattering process with a broadening of the poloidal mode spectrum proportional to the level of the perturbation. However, in the multipass regime, the LH current density profile remains sensitive to the ray chaotic behaviour, which is not averaged by fluctuations. The effect of large amplitude fluctuations on the EC driven current is found to be similar to an anomalous radial transport of the fast electrons. The resulting lower current drive efficiency and broader current profile are in better agreement with experimental observations. Finally, applied to the ITER ELMy H-mode regime, the model predicts a significant broadening of the EC driven current density profile with the fluctuation level, which can make the stabilization of neoclassical tearing mode potentially more challenging.

The TCV tokamak has the dual mission of supporting ITER and exploring alternative paths to a fusion reactor. Its most unique tools are a 4.5 MW electron cyclotron resonance heating system with seven real-time controllable launchers and a plasma control system with 16 independent shaping coils. Recent upgrades in temperature, density and rotation diagnostics are being followed by new turbulence and suprathermal electron diagnostics, and a new digital real-time network has been commissioned. The shape control flexibility of TCV has enabled the generation and control of the first 'snowflake' divertor, characterized by a null point in which both the poloidal field and its gradient vanish. The predicted increases in flux expansion and edge magnetic shear have been verified experimentally, and stable EC-heated snowflake ELMy H-modes have been obtained and characterized. ECCD modulation techniques have been used to study the role of the current profile in energy transport, and simulations reproduce the results robustly. The relation between impurity and electron density gradients in L-mode is explained in terms of neoclassical and turbulent drives. Studies of torqueless plasma rotation have continued, highlighting the important role of MHD and sawtooth relaxations in determining the rotation profiles. A newly predicted mechanism for turbulent momentum transport associated with up-down plasma asymmetry has been verified in TCV. Sawtooth period control, neoclassical tearing mode control and soft x-ray emission profile control have been demonstrated in TCV using the new digital control hardware, as a step on the way to more complex applications.

ABSTRACT Detailed studies of sawtooth activity in Ohmic plasmas in JET have revealed significant discrepancies both with previously reported observations of the phenomenon and with conventional models of the internal disruption. 'Compound' sawteeth, which display an intermediate collapse during the ramp phase, are observed in the majority of discharges. These usually exhibit no precursor activity, in contrast to observations in smaller tokamaks, but are often accompanied by successor oscillations. Furthermore, the collapse time of such sawteeth is much shorter than expected. These results suggest that the conventional model of the sawtooth is inadequate to explain sawtooth activity in large tokamaks.

ABSTRACT The Tokamak a Configuration Variable (TCV) has a two channel correlation electron cyclotron emission (CECE) radiometer with a line of sight perpendicular to the magnetic field. The antenna pattern of this radiometer limits resolution to kθ < 112 m-1 or kθρs < 0.3 at 500 kHz. It can access the region with minor radius ρvol < 0.7. A series of measurements has been made of the turbulence spectra at positive and negative triangularity and as a function of collisionality. Also, a series of measurements has been made as a function of poloidal angle, by varying the plasma vertical position with respect to the antenna, the measurements being made on the same flux surface. It is planned to extensively upgrade the diagnostic by integrating four more channels and acquiring a new front-end for the radiometer. This upgrade should reduce the required number of shots for a radial scan by a factor four and improve the signal-to-noise ratio. It is also planned to use a high gain antenna that will extend access to kρ < 174m-1 or kθρs < 0.5 at 500 kHz. The present system, measurements and the upgrade are described in this paper.

ECRH and ECCD, disruptive events, and sawtooth activity have been demonstrated to produce suprathermal electrons in fusion devices, motivating increasingly detailed studies of the generation and dynamics of this suprathermal population. Past hard-X-ray (HXR) and ECE measurement in the TCV tokamak, which is equipped by a 4.5-MW ECRH system, have led to the identification of the crucial role of spatial

Intense electron cyclotron resonance heating (ECRH) is employed on the TCV tokamak both in second- and third-harmonic X-mode (X2 and X3). The plasma behaviour under such conditions is driven largely by the electron dynamics, motivating extensive studies on TCV of the heating and relaxation phenomena governing both the thermal and suprathermal electron populations. The dynamics of suprathermal electrons is intimately

ABSTRACT Methods of modifying sawtooth oscillations using waves in the ion cyclotron range of frequencies (ICRF) in the JET tokamak are presented. Examples of sawtooth stabilization by ICRF-accelerated high-energy ions are shown, including experiments with ICRF-acceleration of {sup 4}He-beam ions to simulate the effects of fusion born alpha particles. With high power ICRF heating in low-density plasmas, fast ion stabilization of sawteeth is lost and a new type of small-period and small-amplitude sawteeth appears. ICRF-induced radial pinch with toroidally asymmetric waves is found to be useful in affecting the radial profile of the ICRF-driven fast ion populations and thereby their influence on sawteeth. Ion cyclotron current drive (ICCD) applied close to the sawtooth inversion radius is effective in modifying the sawtooth period. The latest achievements include the successful application of ICCD to shorten the fast-ion-induced long-period sawteeth and thereby avoid triggering of neoclassical tearing modes (NTMs)

ABSTRACT Real time control of heating systems is essential to maximize plasma performance and avoid or neutralize instabilities under changing plasma conditions. Several feedback control algorithms have been developed on the Tokamak a Configuration Variable (TCV) tokamak that use the electron cyclotron (ECRH/ECCD) system to control a wide range of plasma properties, including the plasma current, shape, profiles as well as the sawtooth instability. Controllers have been developed to obtain sawteeth of a pre-determined period, to maximize the sawtooth period using an extremum seeking control algorithm and finally to provide simultaneous control of the plasma emission profile peak and width using multiple independent EC actuators.

ABSTRACT TCV experiments demonstrate the basic power exhaust properties of the snowflake (SF) plus and SF minus divertor configurations by measuring the heat fluxes at each of their four divertor legs. The measurements indicate an enhanced transport into the private flux region and a reduction of peak heat fluxes compared to a similar single null configuration. There are indications that this enhanced transport cannot be explained by the modified field line geometry alone and likely requires an additional or enhanced cross-field transport channel. The measurements, however, do not show a broadening of the scrape-off layer (SOL) and, hence, no increased cross-field transport in the common flux region. The observations are consistent with the spatial limitation of several characteristic SF properties, such as a low poloidal magnetic field in the divertor region and a long connection length to the inner part of the SOL closest to the separatrix. Although this limitation is typical in a medium sized tokamak like TCV, it does not apply to significantly larger devices where the SF properties are enhanced across the entire expected extent of the SOL.

Clear evidence is reported for the first time of a rapid localized reduction of core electron energy diffusivity during the formation of an electron internal-transport barrier. The transition occurs rapidly (approximately = 3 ms), during a slow (approximately = 200 ms) self-inductive evolution of the magnetic shear. This crucial observation, and the correlation of the transition with the time and location of the magnetic shear reversal, lend support to models attributing the reduced transport to the local properties of a zero-shear region, in contrast to models predicting a gradual reduction due to a weak or negative shear.