Plasma rotation and momentum transport studies in the TCV tokamak based on charge exchange spectroscopy measurements
Thermonuclear controlled fusion research is a highly active branch of plasma physics. The main goal is the production of energy from the fusion reaction of hydrogen isotope nuclei, the same reaction that powers stars. The most promising present approach are Tokamaks, toroidal devices where high temperature plasmas are confined by means of magnetic fields. This thesis is devoted to a detailed and systematic study of plasma rotation nthe Tokamak à Configuration Variable (TCV), at the Centre de Recherches en Physique des Plasmas (CRPP) in Lausanne Switzerland. In a tokamak, confinement is limited by particle and energy transport from the hot core to the cold edge and by macroscopic perturbations of magnetic equilibrium. Recently, plasma rotation has been demonstrated to beneficially affect both confinement and stability, explaining the great recent interest in plasma rotation studies Relatively little is understood about plasma rotation physics and, in particular, the so called "intrinsic" rotation that will constitute the main component of plasma rotation in next generation machines. That is why a great theoretical and experimental effort is being deployed in studying intrinsic rotation and this work is part of this context. In TCV, plasma rotation is measured by the Charge eXchange Recombination Spectroscopy diagnostic (CXRS). The spectroscopic signal comes from the perpendicular observation of a low power Diagnostic Neutral Beam Injector (DNBI), which applies a negligible torque to the plasma. Hence, the DNBI/CXRS pair is an effective tool for the experimental study of intrinsic tokamak plasma rotation. During this work, the pre existing toroidal observation view was complemented with two new systems, permitting the measurement of toroidal rotation, on inboard plasma radius, and poloidal rotation in the plasma periphery. The implementation of an automated wavelength calibration procedure, based on reference Neon spectra, permitted the first viable (toroidal and poloidal) rotation measurements of TCV, with uncertainties down to 1km/s. Using upgraded light collection optics and fiber optic transmission lines, simultaneous measurement of core and edge plasma was achieved, with a doubling of of the radial resolution of the toroidal rotation measurements. The measurable range of plasma parameters was also extended to higher densities by the installation of back illuminated CCD detectors. In the present configuration (CXRS09), the diagnostic is capable of routinely measuring toroidal and poloidal plasma rotations with a radial resolution of ≲ 1 cm and a sample frequency of 10 ÷ 20 Hz, for plasma densities of 0.8 ≲ ne,av ≲ 8 × 1019 m-3. The basic scenario of Ohmically heated discharges in limiter L-mode configuration was initially addressed. A large core toroidal rotation up to uφ ≈ 50 km/s in the counter current direction is measured, reversing nearly exactly upon reversal of plasma current Ip. The toroidal rotation profile may be schematically divided into a core, a peripheral and an intermediate region. For qe ≈ 3 (magnetic safety factor at the plasma edge) the core region velocity is relatively flat or slightly "bulged" in the co current direction inside the sawtooth inversion radius. In the peripheral region, the toroidal rotation is small with a monotonic intermediate region. The central rotation appears to be limited to approximately its value at the sawtooth inversion radius. Poloidal rotation uθ ≲ 3 km/s, measured in Ohmic discharges, is only weakly dependent on plasma parameters but reverses with reversed magnetic field, with values and direction coherent with neoclassical predictions. Combining uφ and uθ measurements, the profile of the radial electric field Er was estimated through the radial force balance equation. Er down to 8 kVm (inward directed) is found in the plasma bulk, and close to zero at the plasma edge. A spontaneous reversal of the toroidal rotation profile is observed when the average density exceeds ne,av ≈ 4 × 1019 m-3 at low qe ≈ 3, with the plasma now rotating in the co current direction. The transition between the co and counter rotation regimes is studied dynamically using ne and Ip ramps and with the application of Electro Cyclotron Heating (ECH). The rotation reversal is weakly sensitive to impurity concentration and positive plasma triangularity appears to be a key ingredient. Whilst a physical explanation has not been identified yet, dynamic uφ reversal observations indicate that it results from a changed balance of radial non-diffusive fluxes of toroidal momentum. The study was extended to the divertor magnetic configuration in which the plasma column rotates in the co current direction at low ne, and uφ reverses at high ne opposite from the limited configuration. The rotation profile may again be divided into three regions, although the rotation at the plasma periphery does not always remain close to zero but evolves with the plasma parameters. In particular, independently on the core rotation regime, peripheral uφ decreases with ne and Ip and is strongly sensitive to the ion B→ × ∇B direction, suggesting a link with parallel fluxes in the Scrape-Off Layer (SOL). Combined measurements of of CXRS and Mach probe indicate that in the plasma edge toroidal rotation matches the toroidal component of SOL flows. From the analysis of toroidal rotation in stationary and transient phases, a characterization of the momentum transport is presented. The resulting radial momentum diffusivity, of the order of Χφ ∼ 0.1 – 0.3 m2/s, exceeds by 2 orders of magnitude the neoclassical estimation. A remarkable result is the existence of a "residual stress" component, which sustains a substantial stationary rotation gradient for null background rotation. Conversely, the analysis suggests a minor role for the convective (pinch) component, that is preliminarily confirmed by gyro-kinetic simulations including turbulent Coriolis convective pinch. Neoclassical predictions are in quantitative and qualitative disagreement with the experimental observations. The effect of the sawtooth instability on the core rotation was addressed in a specific experimental scenario where the inter crash evolution of the core uφ could be measured. The measurement required the development of a fast CXRS acquisition scheme based on a trigger constructed in real time from a Soft X-ray measurement. At the sawtooth crash the plasma core undergoes a strong acceleration in the co current direction (∆uφ ≈ 9 km/s in the experimental scenario), possibly related to a strong transient toroidal electric field. The systematic and varied observations reported in this work extend the experimental knowledge of bulk plasma rotation in low confinement regimes. In particular, the rotation reversal phenomena constitute an important test for momentum transport models of tokamak plasmas.
Keywords: plasma physics ; nuclear fusion ; tokamak ; TCV plasma rotation ; intrinsic rotation ; toroidal rotation ; poloidal rotation ; rotation reversal ; momentum transport ; plasma diagnostic ; charge exchange ; CXRSThèse École polytechnique fédérale de Lausanne EPFL, n° 4569 (2009)
Programme doctoral Physique
Faculté des sciences de base
CRPP Association Euratom
Record created on 2009-11-09, modified on 2016-08-08