Progress and scientific results in the TCV tokamak

Abstract

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.

Experimental study of high power mm-waves scattering by plasma turbulence in TCV plasmas

“…is characterized by its high shaping capabilities enabled by a plasma control system made of 16 independent shaping coils. TCV possesses a unique 4.5MW electron cyclotron resonance heating (ECRH) system distributed over two frequencies (second and third harmonic X-mode, respectively X2 at 82.7GHz and X3 at 117.8 GHz) [7]. The path length of the X3 beam to the X3 transmission diagnostic (see section 2.3 for details) in TCV is about 1.5m, which is comparable to the path length of the edge-to-resonance path in ITER.…”

“…During this phase, electron densities are typically between 0.7 and 4 x10 18 m −3 and electron temperatures between 30 and 50eV. The duration of the plasma was only limited by the gyrotron pulse length, and could in principle be extended to 4s using back-to-back pulses, as previously done for fully Electron Cyclotron Current Driven (ECCD) plasmas [7].…”

Low-frequency linear-mode regimes in the tokamak scrape-off layer

“…We focus our attention on the following sets of parameters: first, a parameter set with R=L n ¼ 10, L y ¼ 1000, q ¼ 4, ¼ 0:1, and m e =m i ¼ 2:72 Â 10 À4 , called "low-gradient;" second, a "high-gradient" parameter set, with R=L n ¼ 90, ¼ 0:01, being the other parameters the same as in the first set; third, we apply our analysis to a TCV tokamak 45 L-mode discharge, where the plasma with approximately circular flux surfaces is created close to the high-field side of the machine, creating a scenario that reproduces the toroidal limiter configuration considered here:…”

Advanced divertor configurations with large flux expansion

“…In both tokamaks, thanks to the open divertor geometry and a flexible plasma control system, ideal SF, SF-plus and SF-minus configurations were obtained [19,20,22,21]. In TCV, SF-plus configurations with σ 0.5 were used to study H-mode access, pedestal stability, ELMs and divertor properties.…”

“…TCV is a medium-size conventional aspect ratio tokamak with R = 0.88 m, a = 0.25 m, and B t ≤ 1.5 T [21]. It has an open, up-down symmetric divertor plate geometry and graphite PFCs.…”