B. Joye

Cold Boronization in Tca

Boronisation of the TCA vacuum vessel was performed at ambient temperature. The paper presents detailed surface analysis of the coating and a description of the tokamak performance after boronisation. The obtained coatings showed a film composition close to the feed gas composition although a high oxygen content was measured. Apart from the dominating carbidic B-C bonds, the existence of at least two non-equivalent carbon bonding sites were found. Boronisation gave an immediate and long-term improvement of many plasma parameters in TCA with a strong reduction of both low and high-Z impurities in the plasma. In particular oxygen was reduced by over 20 despite a high level oxygen contamination in the coating. The gas retention in the film has been shown to be temporarily reduced by glow discharge cleaning. With these techniques we are able to obtain nearly isotopically pure discharges. Boronisation has been shown to be a very efficient wall conditioning procedure in TCA.

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.

Magnetic broadband turbulence measurements and ohmic confinement in the TCA tokamak

Abstract The amplitude of the magnetic broadband fluctuations in an ohmically heated tokamak is measured to have an inverse relationship with both the density and the electron confinement time.

Effect of Rf Heating on Edge Parameters in the Tca Tokamak

Abstract Detailed scrape-off measurements during Alfven wave heating experiments ( P RF ~ 200 kW) in TCA are presented. The Measurements are performed by means of electrical probes and by deposition probes fixed onto the antennae. It is shown that different antenna designs strongly influence the behaviour of the boundary plasma. Even during low power RF heating experiments is it found that the plasma-antenna interaction leads to a non-Maxwellian plasma. Low frequency density fluctuations are associated with changes of the density and temperature profiles during RF. High frequency fluctuations at the driving frequency and their harmonics are observed.

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.

Shape dependence of sawtooth inversion radii and profile peaking factors in TCV L mode plasmas

Sawtooth inversion radii and profile peaking factors of a large variety of ohmic and ECH heated L mode plasmas, including, elongations up to 2.6 and triangularities between -0.5 and 0.75, have been investigated in the TCV tokamak. In ohmic plasmas, normalized inversion radii and electron temperature profile peaking factors (corrected for sawtoothing effects) depend solely on the parameter (j)/q(0)j(0), irrespective of plasma shape. With ECH this parameter remains the main scaling parameter. Density profiles are well described as functions of poloidal flux, in agreement with turbulent equipartition theories. Parameter conversions are also provided that allow the observed scalings to be expressed using the conventional scaling variables q(95), delta(95) and kappa(95).

Effect of plasma shape on confinement and MHD behaviour in TCV

Note: 38th Annual Meeting, APS Division of Plasma Physics, Denver, CO, USA, November 1996, Bull. Amer. Phys. Soc. 41(7), 1513 (1996) Reference CRPP-CONF-1996-037 Record created on 2008-05-13, modified on 2016-08-08