Abstract. Measurements of electron cyclotron emission from the high field side of the TCV tokamak have been made on plasmas heated by second and third harmonic X-mode Electron Cyclotron Heating (ECH) and Electron Cyclotron Current Drive (ECCD). Suprathermal Electron Cyclotron Emission (ECE), up to a factor of 6 in excess of thermal emission, is detected in the presence of second harmonic X-mode (X2) ECCD and of third harmonic X-mode (X3) ECH. The measured ECE spectra are modelled using a bi-Maxwellian describing the bulk and the suprathermal electron populations. Suprathermal temperatures between 10-50keV and densities in the range 1 · 10 17 − 6 · 10 18 m −3 are obtained, and correspond to 3 -15 bulk temperatures and 1% -20% bulk densities. Good agreement between ECE suprathermal temperatures and energetic photon temperatures, measured by a hard X-ray camera, is found. For optically thin X3 Low Field Side (LFS) injection in presence of X2 CO-ECCD, the suprathermal population partly explains the discrepancy between global and first pass absorption measurements.
On the Tokamakà Configuration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs rapidly (<τ eE) and locally and, according to ASTRA modelling, is consistent with the appearance of a local minimum in the safety factor (q) profile. The eITB is sustained by non-inductively driven currents (including the off-axis bootstrap current) for many current redistribution times while the current in the tokamak transformer is held constant. The maximum duration is limited by the pulse length of the gyrotrons. The transformer coil can be used as a counter (or co-) current source with negligible accompanying input power. In established eITBs the performance can be enhanced (degraded) by altering solely the central current or q-profile. New experiments show that the same stationary eITB performance can be reached starting from discharges with centrally peaked current. A fine scan in surface voltage shows a smooth increase in performance and no sudden improvement with voltage despite the fact that q min must pass through several low-order rational values.
Electron cyclotron resonance heating (ECRH) is a mature technology that has progressed constantly over a period of forty years, particularly as a tool in magnetic confinement fusion. As with other heating methods, this technique has seen a steady increase in the sophistication of its applications, from bulk heating through profile tailoring and finally to distribution function engineering. By comparison with other techniques, ECRH presents the significant advantages of good coupling, localized power deposition, easy launching and precise directionality. This paper reviews some recent applications related to third harmonic ECRH and highlights the role of the relaxation dynamics of suprathermal electrons, both in real space and in velocity space, in regulating the overall effect of ECRH on fusion plasmas. A technique for direct visualization of these relaxation phenomena, using modulated ECRH, is described and demonstrated.
The acceleration of electrons during a sawtooth crash is demonstrated in various regimes of the TCV tokamak plasma. Simultaneous measurements of electron cyclotron emission from the low field side and from the high field side of the tokamak provide information on the fast electrons dynamics with a high resolution. The density, energy and effective radial diffusion coefficient of the suprathermal population are quantitatively reconstructed. The total kinetic energy of fast electrons generated during a sawtooth crash is comparable to the magnetic energy released during the crash.
Soft β limiting phenomena have been observed in T-10 in ECRH heated
plasmas. Neoclassical tearing modes are supposed to be responsible for the β
limitation. MHD onset was observed at high βp values but low βN values.
The critical β has been found to be almost independent of the collisionality parameter
νe*. Sawtooth stabilization by ECCD does not result in an increase of critical
beta. A dependence of the critical β on the q(r) profile (modified by ECCD)
has been observed.
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