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The fully non-inductive sustainment of high normalized beta plasmas (βN) is a crucial challenge for the steady-state operation of a tokamak reactor. In order to assess the difficulties facing such scenarios, steady-state regimes have been explored on the tokamak à configuration variable (TCV) using the newly available 1 MW neutral beam injection (NBI) system. The operating space is extended towards plasmas that are closer to those expected in JT-60SA and ITER, i.e. with significant NBI and electron cyclotron resonance heating and current drive (ECRH/CD), bootstrap current and fast ion (FI) fraction. βN values up to 1.4 and 1.7 are obtained in lower single null L-mode (H98(y, 2) ∼ 0.8) and H-mode (H98(y, 2) ∼ 1) plasmas, respectively, at zero time averaged loop voltage and q95 ∼ 6. Fully non-inductive operation is not achieved with NBI alone, whose injection can even increase the loop voltage in the presence of EC waves. A strong contribution to the total plasma pressure of thermal and FIs from NBI is experimentally evidenced and confirmed by interpretative ASTRA and NUBEAM modeling, which further predicts that FI charge-exchange reactions are the main loss channel for NBH/CD efficiency. Internal transport barriers, which are expected to maximize the bootstrap current fraction, are not formed in either the electron or the ion channel in the plasmas explored to date, despite a significant increase in the toroidal rotation and FI fraction with NBI, which are known to reduce turbulence. First results on scenario development of high-βN fully non-inductive H-mode plasmas are also presented.

Many tokamaks have observed that sawteeth of sufficient duration may trigger neoclassical tearing modes (NTMs) that lead to plasma performance degradation. In this paper, TCV's ability to accurately control the period of individual sawteeth is exploited, using localized electron cyclotron resonance heating and current drive (ECRH and ECCD), to trigger NTMs under controlled conditions, providing an excellent environment for the study of the seeding of NTMs by sawtooth crashes. The TCV experiments show evidence of a fast formation of seed islands with poloidal/toroidal mode numbers m/n = 3/2 and 2/1 within a few tens of microseconds following the sawtooth crash. Crashes of sawteeth with a longer period duration are observed to generate larger seed islands but also increase the plasma stability to conventional tearing modes. While these two effects compete, the NTM stability is found to decrease with increasing sawtooth period. The seed island size can be reduced and, thereby, the NTM stability improved, by increasing the value of the safety factor q(95). Alternatively, NTM stability can be increased by application of preemptive ECRH at the resonant surface of the NTM. Preemptive ECRH is found to enlarge the plasma operational domain by improving the conventional tearing stability and by reducing the coupling between the driving (m/n = 1/1 or 2/2) and the driven modes (m/n = 2/1 or 3/2), resulting in smaller sawtooth generated seed islands. The efficiency of preemptive ECRH increases when sufficient ECRH power is applied in a short time interval prior to the sawtooth crash.

Experiments and simulations show that the sawtooth period can be influenced and precisely tuned by modulated ECCD power.

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TP9.00126 submitted for the DPP11 Meeting of The American Physical Society.

TP9.127 submitted for the DPP11 Meeting of The American Physical Society.

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Articolo numero ITR/P1-10

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The Tokamak á Configuration Variable, TCV, is equipped with a moveable ECE receiving antenna, identical to the 6 second harmonic (X2) ECH/ECCD launchers, which can track the plasma at any position within the vacuum vessel in real time. In contrast to this poloidal plane viewing, the receiver can be rotated to obtain an oblique view. In this configuration, measurements of the asymmetry of the electron distribution function, EDF, are performed during co/counter ECCD sweeps at constant input power. Direct evidence of the driven current is provided by the ratio of radiation temperatures from co and counter views of identical plasmas. The results are simulated by the NOTEC/TCV radiation-transfer code in which current profile broadening can be included to reproduce the measurements. Studies of core broadband temperature uctuations, induced by plasma turbulence, are performed with a high-resolution X2 correlation ECE diagnostic. ECE correlation is measured by two frequency-tunable YIG filters that can be placed between r/a = 0 - 0.9 using both their wide tuning range, and plasma geometry changes from shot to shot. Evidence of broadband (20{150 kHz) uctuations with peak frequencies ranging from 20 kHz up to 90 KHz at r/a = 0.3 - 0.8 in Ohmic sawtooth-free discharges was obtained. The amplitude of the temperature uctuations decreases with increasing density and thus, increasing collisionality, which is in qualitative agreement with predictions from quasi-linear gyrokinetic calculations performed with the gyrokinetic code GS2. The mixing length heat diffusivity calculated from GS2 decreases with increasing collisionality, as does that obtained from a power balance analysis. The real frequency of the broadband turbulence stays positive over the range of collisionalities explored, indicating dominant TEM turbulence.

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Submitted for the DPP08 Meeting of The American Physical Society.

Using a new LFS receiving antenna in the oblique ECE configuration, we measure the emission during central co-ECCD and counter-ECCD discharges. The asymmetry of the electron distribution function (EDF) resulting from ECCD is put into evidence by performing 4 successive discharges measuring quasi-X mode and quasi-O mode in 2 separate discharges; then, repeating these measurements while viewing in the opposite toroidal direction. The plasma configuration and EC injection are identical in the 4 cases. The ratios of the X-mode signals are compared to simulations of the ECE by the NOTECTCV code and are found to be generally consistent with emission from a small, drifting, suprathermal, electron population carrying the ECCD current of the order calculated by the TORAY-GA code.

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Recently, there has been increased interest in avoiding the destabilization of neoclassical tearing modes (NTMs) by controlling the sawtooth instability which frequently provides a large enough seed island to trigger the growth of the NTM. Electron cyclotron heating (ECH) and current drive (ECCD) are prime candidates for such control as the all important deposition location can be adjusted using external control parameters alone. Sawtooth control studies have been carried out on the Tokamak à Configuration Variable (TCV) and ASDEX Upgrade tokamaks. The experiments and subsequent sawtooth period modeling help to determine the optimum locations for sawtooth period control and understand the mechanism by which this control is attained.