C. Zucca

On the heating mix of ITER

This paper considers the heating mix of ITER for the two main scenarios. Presently, 73 MW of absorbed power are foreseen in the mix 20/33/20 for ECH, NBI and ICH. Given a sufficient edge stability, Q = 10-the goal of scenario 2-can be reached with 40MW power irrespective of the heating method but depends sensitively inter alia on the H-mode pedestal temperature, the density profile shape and on the characteristics of impurity transport. ICH preferentially heats the ions and would contribute specifically with Delta Q 0.5, and strong off-axis current drive (CD). The findings presented here are based on revised CD efficiencies gamma for ECCD and a detailed benchmark of several CD codes. With ECCD alone, the goals of scenario 4 can hardly be reached. Efficient off-axis CD is only possible with NBI. With beams, inductive discharges with f(ni) > 0.8 can be maintained for 3000 s. The conclusion of this study is that the present heating mix of ITER is appropriate. It provides the necessary actuators to induce in a flexible way the best possible scenarios. The development risks of NBI at 1 MeV can be reduced by operation at 0.85 MeV.

Safety factor profile requirements for electron ITB formation in TCV

On the TokamakConfiguration 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 (<τeE) 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 qmin must pass through several low-order rational values. The appearance, in

Current density evolution in electron internal transport barrier discharges in TCV

Simulations of the plasma current density evolution in electron internal transport barrier discharges on the Tokamak a Configuration Variable (TCV) have been performed, in order to determine the relationship between the safety-factor profile and the electron transport improvement. The results show that the formation of the transport barrier is correlated with the shear reversal in all cases studied, regardless of the different heating and current drive schemes. No indications were found of discrete effects related to low-order rational q surfaces. On the contrary, the increase in confinement along with the negative shear is gradual, but constant, indicating that the transition is smooth, although it can be very fast.

Global plasma oscillations in electron internal transport barriers in TCV

In the Tokamak a Configuration Variable (TCV) (Hofmann F et al1994 Plasma Phys. Control. Fusion 36 B277), global plasma oscillations have been discovered in fully non-inductively driven plasmas featuring electron internal transport barriers (ITB) with strong ECRH/ECCD. These oscillations are linked to the destabilization and stabilization of MHD modes near the foot of the ITB and can lead to large oscillations of the total plasma current and line-averaged density, among others. They are intrinsically related to the fact that ITBs have large pressure gradients in a region of low magnetic shear. Therefore, the ideal MHD limit is relatively low and infernal modes can be unstable. Depending on the proximity to the ideal limit, small crashes or resistive modes can appear which affect the time evolution of the discharge. Being near marginal stability, the modes can self-stabilize due to the modification of the pressure gradient and local q-profile. The plasma recovers good confinement, reverses shear and the ITB builds up, until a new MHD mode is destabilized. TCV results show that this cycling behaviour can be controlled by modifying the current density or the pressure profiles, either with Ohmic current density perturbation or by modifying the ECH/ECCD power. It is demonstrated that many observations such as q ≥ 2 sawteeth, beta collapses, minor disruptions and oscillation regimes in ITBs can be assigned to the same physics origin: the proximity to the infernal mode stability limit.

Modulation of electron transport during swing ECCD discharges in TCV

Generation of a swing electron cyclotron current drive (swing ECCD), i.e. driving alternated, symmetric, positive or negative local ECCD, during a single discharge and at constant total input EC power, was performed at the Tokamak a Configuration Variable (TCV). The electron temperature is observed to be modulated inside the deposition radius, implying modulation of the electron transport properties. The modulation of ECCD is the only actuator for the observed modifications in the electron transport properties. These exhibit inverted behaviors depending on the deposition location of the co- and counter-ECCD. At more on-axis depositions, swing ECCD results in a higher electron temperature during the co- ECCD phase, whereas at more off-axis depositions, the electron temperature is higher during the counter-ECCD phase. Transport modeling of these discharges shows that the local electron tranport behavior depends on the value of the modulated magnetic shear. The results are transport model independent, confirming the robustness of the magnetic shear modeling, and indicating that the main contribution is due to the ECCD. Moreover, the results are consistent with predictions from gyrokinetic simulations, that the local electron confinement is proportional to the magnetic shear at low shear and inversely at high shear values, s greater than or similar to 1.

Oblique and correlation ECE in TCV

Keywords: correlation ECE ; oblique ECE ; ECE Reference EPFL-CONF-169385 URL: http://ec16.ipp.ac.cn/ URL: http://crpplocal.epfl.ch/pinboard/papers/102600304.pdf Record created on 2011-10-14, modified on 2017-05-12

Control of the eITB formation and performance in fully non-inductively sustained ECCD discharges in TCV

The X2 ECH antennas on TCV are used to sustain and tailor the plasma current profile, forming either a centrally peaked or hollow profile. During the transition from peaked to hollow profile, an eITB is observed to form rapidly and in a localized region, which correlates with the appearance of a zero shear flux surface off‐axis according to the ASTRA transport code. The barrier position can be controlled via the co‐ECCD off‐axis deposition location, and the barrier strength with central heating or counter‐ECCD (increasing the depth of the hollow current profile), achieving H‐factors of up to ⩽6. In these discharges, the current in the ohmic transformer coil is held constant to avoid an inductively driven current contribution. After the eITB is created, a small amount of ohmically driven counter (co‐) current has been added as a perturbative current source, transfering only a few kW of ohmic power compared to 1.4MW of ECCD. The ohmic current increases (decreases) the eITB performance demonstrating the clea...