Effect of plasma shape on confinement and MHD behaviour in the TCV tokamak

Abstract

ABSTRACT. The energy confinement time of TCV ohmic L mode discharges depends strongly on plasma shape. For fixed average current and electron densities, confinement times increase with plasma elongation and decrease with (positive) plasma triangularity. This dependence can be explained by the geometrical effects of flux surface expansion and compression on the temperature gradients together with the effect of power degradation, without the need to invoke a shape dependence of the transport coefficients. A global factor of merit, the shape enhancement factor H,, is introduced to quantify this geometrical effect. The shape enhancement factor also has the potential to improve the description of the shape dependence in existing interdevice scaling laws. Modified versions of Nec-Alcator scaling and of Rebut-Lallia-Watkins scaling provide successful descriptions of ohmic L mode confinement for a large variety of plasma shapes in TCV by making use of H,. Magnetohydrodynamic activity is also strongly dependent on plasma shape. Sawtooth amplitudes are largest at positive triangularity and sometimes vanish at negative triangularity, where the amplitude of MHD modes is highest. It is shown that the changes in MHD behaviour are to a large extent consequences of the confinement changes produced in these shaping experiments. EXPERIMENTAL CONDITIONSThe distinctive feature of TCV is a vacuum vessel with a height to width ratio of 3 surrounded by 16 poloidal field shaping coils (Fig. 1). This construction permits the creation of highly elongated and strongly shaped plasmas [l, 21. The machine therefore offers a unique capability to extend the world confinement database and to improve our understanding of transport in shaped plasmas. The basic parameters in the experiments reported are Ro = 0.88 m, a = 0.25 m for the major and minor radius, respectively, and BT = 1.43 T for the toroidal magnetic field.In the work presented the influence of plasma shape on the energy confinement and MHD behaviour has been investigated by means of a large variation of the plasma shape, from circular to highly elongated equilibria and from strongly D shaped (positive triangularity) to inverse D shaped equilibria (negative triangularity). The discharges investigated consist of 232 limited ohmic L mode plasmas in which elongation triangularity 6, and edge safety factor * Present ufiliution: Max-Planck-Institut fur Plasmaphysik, Garching, Germany. qa at the last closed flux surface and line averaged density ii, were systematically scanned in the range and 2.85 x lo1' m-3 < ii, < 8.5 x m-3. Plasmas were created with all possible combinations from four values of each of these parameters, with the exception of some 20 combinations that were inaccessible. All data shown and discussed in this article are from this dataset of 232 discharge conditions. Unless otherwise indicated, the figures show all available data for which the relevant quantities could be obtained.The TCV shape control system was programmed to produce equilibria such that the shape of the l...

Understanding and suppressing the near scrape-off layer heat flux feature in inboard-limited plasmas in TCV

“…The decrease of q ⊥0 with plasma density is consistent with a cooling of the SOL at the limiter at fixed power into the SOL, P SOL , due to the conservation of total pressure along the field lines. The decrease of q ⊥0 with the elongation κ is consistent with the observed decrease of P Ω , due to the increase of confinement with elongation [23,24] for constant plasma current (I p ) and density, resulting in a lower power into the SOL, P SOL . In the following we compare the perpendicular heat flux at the contact point q ⊥0 with the heat flux crossing the LCFS, q LCF S .…”

Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

“…This is due to the limited repetition rate and the difficulty of synchronizing the Thomson scattering laser pulses with the sawtooth crash. A rough estimate of the hollowness of the electron temperature profile can be obtained from the soft X ray measurements, assuming ∆T e /T e = γ∆S X /S X , where γ = d(ln S X )/d(ln T e0 ) is the factor describing the temperature dependence of the soft X ray emissivity of the dominant impurity, carbon, in TCV [19]. For the case shown in Fig.…”

Tokamak elongation – how much is too much? Part 2. Numerical results

“…Confinement may improve at high triangularity due to reduced MHD turbulence associated with the n = 1 ballooning-kink mode (Eriksson & Wahlberg 2001). On the other hand, it was found in experiments on the TCV tokamak that increasing triangularity led to reduced plasma confinement (Weisen et al 1997), which was explained by the increase of drift-wave turbulent transport for high triangularity (Camenen et al 2007). Unfortunately, the present empirical scaling relations for τ E do not explicitly take triangularity into account.…”