1 EX/11-2Rb Density Fluctuations as an Intrinsic Mechanism to Keep - PDF document

1 EX/11-2Rb Density Fluctuations as an Intrinsic Mechanism to Keep Self-consistent Pressure Profile V.A. Vershkov 1 , D.A. Shelukhin 1 , G.F. Subbotin 1 , Yu.N. Dnestrovskii 1 , A.V. Danilov 1 , S.G. Maltsev 1 , E.P. Gorbunov 1 , D.S. Sergeev 1 , S.V. Krylov 1 , T.B. Myalton 1 , D.V. Ryzhakov 1 , V.M. Trukhin 1 , V.V. Chistiakov 1 , S.V. Cherkasov 1 . 1 Institute of Tokamak Physics , National Research Center “Kurchatov Institute”, 123182 Kurchatov Sq. 1, Moscow, Russian Federation E-mail contact of main author: V.Vershkov@fc.iterru.ru Abstract . The paper presents new insight into previous and new experimental data of the turbulent density fluctuations behavior in T-10 OH and ECRH discharges. The experiments confirmed the existence of the same marginal peaked pressure profile in both OH and ECRH tokamak plasmas as well as strong deterioration of particle confinement in the cases when plasma pressure profile meets this profile (fast density decay in OH, “density pump out” in ECRH). Pressure profile peaking could be achieved either with flat density and peaked temperature profile or vice versa. Minimal turbulence level did not depend on heating power and was observed when pressure profile was slightly wider than the marginal one. The density fluctuations did not significantly contribute to the heat transport but determined particle fluxes to maintain the pressure profile. 1. Introduction A variety of techniques for small-scale density fluctuations investigation provides a powerful tool for keen insight into anomalous turbulent transport in tokamaks. The density fluctuations amplitude was considered as the measure of turbulence level correlating with energy confinement. Several experiments seemed to confirm such correlation [1, 2, 3], but detailed DIII-D experiments demonstrated no straight relation between density fluctuations and energy confinement time [4]. Moreover DIII-D proclaimed that density fluctuations level had never risen under electron cyclotron resonance heating (ECRH) and had risen with neutral beam injection (NBI) applied, while the electron temperature fluctuations had risen with both additional heating techniques. The second fact is that in L -mode plasmas the energy confinement degrades for all additional heating methods whereas the particle confinement depends on heating method and discharge conditions. An example is the central density degradation in ECRH plasmas (density pump out) [5]. ASDEX experiments with NBI [6] and T-10 with ECRH [7] demonstrated that this effect could be postponed if heating was applied during density rump up (so called “delayed confinement deterioration”) [ 6]. It was also demonstrated by ASDEX that total replacement of optimal gas puff required in Ohmic plasmas to get optimal confinement and plasma feed by pellet injection leaded to confinement degradation and fast density decay [8]. The paper presented for previous Conference was mostly devoted to electron component dynamics and its relation to density fluctuations [9]. In present paper the authors tried to provide new insight into previous and new measurements of density fluctuations in T-10 tokamak. 2. Experimental setup Experiments were carried out in T-10 tokamak with circular cross section (major radius R = 1.5 m, minor radius a = 0.3 m) in both Ohmic and 2 nd harmonic ECRH plasmas. Electron density profile was measured using 8-channel microwave interferometer and 7-channel HCN- laser interferometer. Time resolved measurements of electron temperature were made with 21-channel radiometer. Each channel was calibrated on electron temperature profile in Ohmic

2 EX/11-2Rb phase of discharge [10]. Absolute temperature values were normalized on X-ray spectrometer measurements data. Turbulence measurements were made with O-mode microwave heterodyne reflectometer [11]. The amplitude of local density perturbation was estimated in 1D geometry optics approach [12]. Reflectometer frequency was varied in a series of reproducible discharges to obtain the turbulence radial profile. 3. Steady-state discharges: plasma parameters and turbulence profiles Comparison of plasma parameters and turbulence profiles were made in steady-state conditions in discharges with toroidal magnetic field on axis B T = 2.3 T and line averaged density about 3·10 19 m -3 for three values of plasma current I p - 140, 200 and 280 kA. On-axis ECRH with total power P ECRH ≈ 1.1 MW, significantly exciding the Ohmic one ( P OH ~ 0.2 ÷ 0.3 MW) was applied in steady-state discharge phase. Figure 1 presents the comparison of density profiles in Ohmic and ECRH discharges and corresponding turbulence profiles. One could see that ECRH profiles tends to be wider than the Ohmic ones with most visible effect for I p = 140 kA. It is also seen that turbulence level decreased in ECRH with respect to Ohmic level with most pronounced variation in 140 kA case. In all cases the turbulence level in plasma core is higher in the case of peaked density profile. Thus turbulence level is related to density profile shape. Solid lines on bottom plots are the approximation of turbulence amplitudes in the form of 0.2 q a · r / L n , where r is the minor radius, L n = (∂ ln n e /∂ ln r ) -1 is density profile length, q a is the safety factor at the boundary; the multiplier 0.2 was chosen to meet the experimental data. This expression provides good approximation of the experiment confirming the correlation between fluctuation amplitude and density profile . It should be underlined that turbulence decreases in ECRH despite the confinement degradation with L-mode scaling [13]. Strong turbulence rise towards the plasma periphery could be in connection with intense particle source at the edge due to ionization. However experiments with deuterium and helium show close turbulence profiles despite the different ionization sources in D 2 and He. Observed link between turbulence and density profile is supported by early tokamak research. Since first experiments it was well known that density profiles have bell-like shape despite the periphery particle source. Coppi and Sharky proposed turbulent anomalous transport to resolve this paradox [14]. It was demonstrated later that density profile remains bell-like even under huge gas puff due to anomalous transport [14,15]. “Ion mixing mode” instability was proposed to explain this anomalous transport, leading to inward turbulent flux and density profile formation [16]. Since pinch fluxes were measured in previous T-10 experiments using -1 ] 0.6 Ohmic Ohmic Ohmic -2 s 6 0.4 -3 ] ECRH ECRH ECRH D e [m 0.2 19 m 0.0 4 8 V p [m] 6 n e [10 4 2 2 0  n /n e [%] 5 I p = 200 kA I p = 280 kA I p = 140 kA 4 3 0 2 1 0  n /n e [percent] Turbulent flux 8 V p n e 1 -2 ] 6 19 m 4  [10 Ohmic ECRH Ohmic ECRH Ohmic ECRH 2 0.1 experiment experiment experiment appoximation appoximation appoximation 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0     Figure 1. Comparison of electron density profiles (top) and Figure 2. From top to bottom: profiles of diffusion coefficient, small-scale density fluctuations amplitude (bottom) for different plasma currents. Ohmic data are plotted in red, pinch velocity, density ECRH ones - in black. fluctuations level, particles fluxes