Ohmic H-modes in the TCV tokamak

Abstract

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion ∇B drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors (β tor aB/I p ) up to 2 and line average densities up to 2.2×10 20 m −3 , corresponding to the Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary Hmodes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of ELMs have been found to depend on configurational parameters such as the position of the 'active' X-point in unbalanced double-null discharges and the plasma-wall separation in single-null discharges. These dependencies have permitted active control of ELM behaviour in TCV. A continuous spectrum of ELM amplitudes and frequencies has been observed, ranging from clearly identifiable type III ELMs to large, low-frequency ELMs which expel up to 12% of the stored energy and up to 7% of the particle content and are reminiscent of type I ELMs. A previously unknown, benign kind of ELM, with a maximum amplitude in the divertor region, has also been observed.

Turbulent particle transport in magnetized fusion plasma

“…In some cases the observed pinch is found to be in agreement with V = V ware , and D proportional to the heat diffusivity (χ eff ) as found for JET high density H-modes [11,30] and in ASDEX-U at high density plasmas [31,32]. Earlier results from a perturbative particle transport study in TFTR [12] had also shown that a neoclassical pinch could explain the observed density profile relaxation. But in most other cases, the density profile cannot be explained by a neoclassical pinch alone.…”

“…Cross-field heat and particle transport in fusion plasmas is only partly caused by the collisional mechanisms described by neoclassical transport theory. The measured heat diffusivity is much higher than expected from neoclassical prediction, the difference being referred as "anomalous" transport (see for example [12]), believed to be driven by microturbulence. .…”

“…This anomalous particle pinch is predicted by a quasilinear theory of particle transport [2], and confirmed by non-linear turbulence simulations [3] and general considerations based on the conservation of motion invariants [4]. Experimentally, the particle pinch is found to be sensitive to the magnetic field gradient in many cases [5,6,7,8], to the temperature profile [5,9] and also to the collisionality that changes the nature of the microturbulence [10,11,12]. The consistency of some of the observed dependences with the theoretical predictions gives us a clearer understanding of the particle pinch in tokamaks, allowing us to predict more accurately the density profile in ITER.…”

Transition from order to chaos, and density limit, in magnetized plasmas

“…For example, it is obvious that σ v ⊥ and σ v 2 ⊥ should be expressed in terms of temperature, albeit with coefficients which depend on the assumptions made for the velocity distribution. Analogously, the statistical properties of [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]) along with the original ones of Murakami (see [28]), stellarator machines [45,46,47,48], and spherical tokamaks [49,50,51]. Dotted line is the theoretical density limit (11).…”

Ignition Physics in Multiscale Plasma Turbulence

“…Indeed, H-modes have been observed in limiter configurations [Weisen, et al (1996)]. There is evidence from Alcator C-MOD of an enhanced L-mode with the magnetic X-point on the vacuum vessel wall [Greenwald, et al (1995)].…”