Analysis of experiments with electron cyclotron resonance heating (ECRH) requires a good knowledge of the ECRH power profile. This profile is reconstructed by analysis of the transient process after on-axis ECRH switching on in special experiments with suppressed sawtooth oscillations in the T-10 tokamak. The calculations show that the absorbed ECRH power, P T EC , determined by the change in time derivative of the electron temperature at the region of ECRH power input, and the absorbed ECRH power, P β EC , determined by the magnetic measurements, are several times different. Depending on the plasma density and plasma current, their relation, γ EC = P T EC /P β EC , changes from 0.2 to 0.4. Analysis of different explanations for this effect shows that adequate description of the transient process demands introduction of a ballistic jump in the total heat flux just after on-axis ECRH switching on. The effective heat diffusivity increases up to values of 10-15 m 2 s −1 in the first 100-200 µs and decreases down to values of 1.5-2.0 m 2 s −1 during the following 1-2 ms. Note that such a non-monotone dependence of the effective heat diffusivity cannot be described by the modern critical gradient models. It seems that plasma reacts directly to the deposited power but not to the corresponding consequences (the increase in temperature or gradients). Different physical mechanisms could be proposed for this process (partial destruction of magnetic surfaces, fast transition of information through the turbulent cell connections), but each of them needs further confirmation.
New experimental observations of the plasma potential using the heavy ion beam probe diagnostic are presented together with a theoretical description of the formation of the electric field E r in the T-10 circular tokamak (B 0 = 1.5-2.5 T, R = 1.5 m, a = 0.3 m). Ohmically heated (OH) deuterium plasmas with main plasma parameters ne = (0.6-4.7) × 10 19 m −3 , T e (0) < 1.3 keV, T i (0) < 0.6 keV are characterized by a negative potential ϕ(ρ) with maximum negative values of ϕ(6 cm) = −1400 V with respect to the wall. The potential profile monotonically increases towards the plasma edge. A density rise due to gas puff is accompanied by a plasma potential that becomes increasingly negative. When the density approaches values in the range ne = (2.5-3.5) × 10 19 m −3 , the value of the plasma potential saturates, while the energy confinement time still increases up to a saturation value that is obtained at a slightly higher density. With auxiliary heating by electron cyclotron resonance heating (ECRH) up to 1.2 MW, T e (0) increases (up to 3 keV) and the absolute value of the plasma potential decreases. In some cases the plasma potential changes its sign and becomes positive at the edge. The radial profile of E r and its dependence on n e and T i are qualitatively explained by a neoclassical model in the core, and a turbulent dynamic model (Braginskij magnetohydrodynamic equations) in the edge.
Tokamak plasmas have a tendency to self-organization: the plasma pressure profiles obtained in different operational regimes and even in various tokamaks may be represented by a single typical curve, called the self-consistent pressure profile. About a decade ago local zones with enhanced confinement were discovered in tokamak plasmas. These zones are referred to as internal transport barriers (ITBs) and they can act on the electron and/or ion fluid.Here the pressure gradients can largely exceed the gradients dictated by profile consistency. So the existence of ITBs seems to be in contradiction with the selfconsistent pressure profiles (this is also often referred to as profile resilience or profile stiffness). In this paper we will discuss the interplay between profile consistency and ITBs. A summary of the cumulative information obtained from T-10, RTP and TEXTOR is given, and a coherent explanation of the main features of the observed phenomena is suggested. Both phenomena, the selfconsistent profile and ITB, are connected with the density of rational magnetic surfaces, where the turbulent cells are situated. The distance between these cells determines the level of their interaction, and therefore the level of the turbulent transport. This process regulates the plasma pressure profile. If the distance is wide, the turbulent flux may be diminished and the ITB may be formed. In regions with rarefied surfaces the steeper pressure gradients are possible without instantaneously inducing pressure driven instabilities, which force the profiles back to their self-consistent shapes. Also it can be expected that the ITB region is wider for lower dq/dρ (more rarefied surfaces).
It has been observed in the T-10 tokamak that immediately after off-axis electron cyclotron resonance heating (ECRH) switch-off, the core electron temperature stays constant for some time, which can be as long as several tens of milliseconds, i.e. several energy confinement times (τ E), before it starts to decrease. Whether or not the effect is observed depends critically on the local magnetic shear in the vicinity of the q = 1 rational surface, which should be close to zero. It is hypothesized that a small shear can induce the formation of an internal transport barrier. Measurements of density fluctuations in the transport barrier with a correlation reflectometer show immediately after the ECRH switch-off a clear reduction in the fluctuation level, corroborating the above results. The delayed temperature decrease has also been observed in similar discharges in the TEXTOR tokamak; however, the delay is restricted to ∼1×τ E .
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