R. Behn

Impact of plasma triangularity and collisionality on electron heat transport in TCV L-mode plasmas

The impact of plasma shaping on electron heat transport is investigated in TCV L-mode plasmas. The study is motivated by the observation of an increase in the energy confinement time with decreasing plasma triangularity which may not be explained by a change in the temperature gradient induced by changes in the geometry of the flux surfaces. The plasma triangularity is varied over a wide range, from positive to negative values, and various plasmas conditions are explored by changing the total electron cyclotron (EC) heating power and the plasma density. The mid-radius electron heat diffusivity is shown to significantly decrease with decreasing triangularity and, for similar plasma conditions, only half of the EC power is required at a triangularity of −0.4 compared with +0.4 to obtain the same temperature profile. Besides, the observed dependence of the electron heat diffusivity on the electron temperature, electron density and effective charge can be grouped in a unique dependence on the plasma effective collisionality. In summary, the electron heat transport level exhibits a continuous decrease with decreasing triangularity and increasing collisionality. Local gyro-fluid and global gyro-kinetic simulations predict that trapped electron modes are the most unstable modes in these EC heated plasmas with an effective collisionality ranging from 0.2 to 1. The modes stability dependence on the plasma triangularity is investigated.

Behaviour of central plasma relaxation oscillations during localized electron cyclotron heating on the TCV tokamak

During initial studies of ECRH in the TCV tokamak, non-standard central MHD activities, such as humpbacks and saturated and inverted sawteeth, have been observed while changing the heating location, the ECRH power, the plasma shape and the safety factor. For edge safety factors q(alpha) > 4.5, safety factors on-axis q(0) < 1 and small plasmas, complete sawtooth stabilization was achieved with the present 1 MW gyrotron power, and it is likely that sawtooth stabilization can be achieved for all conditions at. higher ECRH power. The conditions under which the various relaxation activities are produced or suppressed are reported, and the origins for such tron-standard behaviour are discussed.

Recent results from the electron cyclotron heated plasmas in Tokamak à Configuration Variable (TCV)

In noninductively driven discharges, 0.9 MW second harmonic (X2) off-axis co-electron cyclotron current drive deposition is combined with 0.45 MW X2 central heating to create an electron internal transport barrier (eITB) in steady plasma conditions resulting in a 1.6-fold increase of the confinement time (τEe) over ITER-98L-mode scaling. The eITB is associated with a reversed shear current profile enhanced by a large bootstrap current fraction (up to 80%) and is sustained for up to 10 current redistribution times. A linear dependence of the confinement improvement on the product of the global shear reversal factor (q0/qmin) and the reversed shear volume (ρq-min2) is shown. In other discharges heated with X2 the sawteeth are destabilized (respectively stabilized) when heating just inside (respectively outside) the q=1 surface. Control of the sawteeth may allow the avoidance of neoclassical tearing modes that can be seeded by the sawtooth instability. Results on H-mode and highly elongated plasmas using the...

An overview of results from the TCV tokamak

The Tokamak Configuration Variable (TCV) tokamak (R = 0.88 m, a < 0.25 m, B < 1.54 T) programme is based on flexible plasma shaping and heating for studies of confinement, transport, control and power exhaust. Recent advances in fully sustained off-axis electron cyclotron current drive (ECCD) scenarios have allowed the creation of plasmas with high bootstrap fraction, steady-state reversed central shear and an electron internal transport barrier. High elongation plasmas, kappa = 2.5, are produced at low normalized current using far off-axis electron cyclotron heating and ECCD to broaden the current profile. Third harmonic heating is used to heat the plasma centre where the second harmonic is in cut-off. Both second and third harmonic heating are used to heat H-mode plasmas, at the edge and centre, respectively. The ELM frequency is decreased by the additional power. In separate experiments, the ELM frequency can be affected by locking to an external perturbation current in the internal coils of TCV. Spatially resolved current profiles are measured at the inner and outer divertor targets by Langmuir probe arrays during ELMs. The strong, reasonably balanced currents are thought to be thermoelectric in origin.

Electron heat transport in shaped TCV L-mode plasmas

Electron heat transport experiments are performed in L-mode discharges at various plasma triangularities, using radially localized electron cyclotron heating to vary independently both the electron temperature T-e and the normalized electron temperature gradient R/L-Te over a large range. Local gyrofluid (GLF23) and global collisionless gyro-kinetic (LORB5) linear simulations show that, in the present experiments, trapped electron mode (TEM) is the most unstable mode. Experimentally, the electron heat diffusivity chi(e) is shown to decrease with increasing collisionality, and no dependence of chi(e) on R/L-Te is observed at high R/L-Te values. These two observations are consistent with the predictions of TEM simulations, which supports the fact that TEM plays a crucial role in electron heat transport. In addition, over the broad range of positive and negative triangularities investigated, the electron heat diffusivity is observed to decrease with decreasing plasma triangularity, leading to a strong increase of plasma confinement at negative triangularity.

Recent results from the TCV Tokamak

During the first two years of operation, the TCV tokamak has produced a large variety of plasma shapes and magnetic configurations, with 1.0≤Btor≤1.46T,Ip≤800kA,k≤2.05, −0.7≤δ≤1. A new shape control algorithm, based on a finite element reconstruction of the plasma current in real time, has been implemented. Vertical growth rates up to 1000s−1 have been stabilized using the external coil system. Ohmic H-modes with Troyon factors (βtoraB/Ip) up to two and densities up to 2.25×1020m−3, corresponding to the Greenwald limit, have been obtained in diverted discharges. Limiter H-modes with line averaged electron densities up to 1.7×1020m−3 have been obtained in elongated D-shaped plasmas with 360 kA≤IP≤600 kA.

Steady-state fully noninductive operation with electron cyclotron current drive and current profile control in the tokamak à configuration variable (TCV)

Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak a configuration variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi...

Recent Alfven-Wave Heating Results on the Tca Tokamak

In this paper, we present the most recent results obtained during the Alfven wave Heating experiments being carried out on the TCA Tokamak. We have obtained both significant electron heating and some ion heating using rf powers up to 230 kW, greater than the value of the ohmic heating power in the target plasma. Firstly, we shall describe the AWH method as applied to TCA and then briefly cover some of the low power experiments carried out during the last year. The higher power experiments will then be discussed, along with some of the problems we have encountered.

Modelling of the electron cyclotron current drive experiments in the TCV tokamak

With the very high electron cyclotron (EC) wave power density achieved in the TCV tokamak, more than 20 MW m(-3), quasilinear modelling predicts an electron cyclotron current drive (ECCD) efficiency well in excess to the experimental value, by up to a factor of 10. Experimentally, radial transport of suprathermal electrons consistent with a diffusion coefficient larger than 1.5 m(2) s(-1) has been observed. This implies that the radial transport time is of the same order as the electron deflection time, suggesting that the key to resolving the discrepancy is to include radial transport in the kinetic simulations. In this paper we show that with a diffusion coefficient in accordance with the experimental estimation, we can reproduce the observed current drive efficiency in the fully EC current driven plasmas of TCV by solving the Fokker-Planck equation. Experimentally the total wave-driven current is well-known since the current in the Ohmic transformer is set to a constant value. We study the radial profile and the velocity dependence of the radial diffusion coefficient. A specific model is employed for steady-state electron internal transport barriers, produced by off-axis ECCD, with the electrons divided into two groups according to their energy. A small diffusion coefficient is assigned for the low-energy electrons, while at higher energies the diffusion level is chosen such as to obtain the experimental ECCD efficiency. The total current density, which is the sum of the wave-driven part and the bootstrap current, is found to be hollow, supporting the hypothesis that the reversed shear is the cause of the transport barrier.

Influence of non-Maxwellian velocity distributions during ECRH and ECCD on electron temperature measurements by Thomson scattering

Deposition of electron cyclotron waves at high-power densities for heating and current drive in a tokamak plasma generates a population of fast electrons and therefore the velocity distribution of the electron population will deviate from a Maxwellian. Apart from the formation of a high energy tail, modifications to the low-energy part of the distribution function have also been observed. This may have consequences for electron temperature measurements by incoherent Thomson scattering, which usually assumes a Maxwellian distribution function to interpret the scattered light spectrum. In order to investigate and quantify such perturbations, a few typical cases of electron cyclotron heating (ECH) and current drive (ECCD) experiments on the Tokamak a Configuration Variable (TCV) have been analysed in detail. The experimental results from the TCV Thomson scattering system have been compared with simulated data assuming different velocity distribution functions. These were obtained either as results of numerical modelling, using the Fokker-Planck code CQL3D, or in the form of a model bi-Maxwellian distribution function based on information from electron cyclotron emission measurements. In some of the investigated cases the deviations from an ideal Maxwellian were significant and systematic errors in the T-e measurements from Thomson scattering could be identified. In particular, for a case with an injected power of 1.35 MW for ECCD, the discrepancies in T, measurements based on different parts of the scattered light spectrum, i.e. different electron velocity classes, were found to be as large as 25-30%. The use of a bi-Maxwellian model distribution function with three free parameters has permitted us to identify the parameter range in which systematic errors exceed the experimental uncertainties of Thomson scattering measurements on TCV.