Publications by Roland Behn | EPFL Graph Search

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A new far-infrared polarimeter diagnostic for the tokamak a configuration variable (TCV) is under construction at CRPP. It uses two FIR lasers at 432.5 mu m, optically pumped by a 120 W continuous wave CO2 laser. The two FIR cavities will be detuned such t ...

For future divertor studies on TCV measurements of electron temperature and density and their spatial gradients along open flux surfaces will be of major interest. A Thomson scattering system is proposed for this purpose, which takes advantage of the good access via horizontal ports on three vertical levels. The modular design of the existing Thomson scattering diagnostic on TCV offers the possibility to modify it for simultaneous measurements in the main plasma and in the divertor region using the same las

A novel method for inverting time-resolved line-integrated interferometric plasma density measurements is described. The method uses singular value decomposition of local density profiles from Thomson scattering measurements obtained at low sampling rates ...

This paper presents experimental results on the accessibility and the properties of plasmas with improved confinement in TCV. First, the H-mode threshold power is measured in Ohmic plasmas. Above an Ohmic threshold density, the threshold power increases with the density. A lower threshold density is found when additional electron cyclotron heating (ECH) is applied. At these low densities, the threshold power increases dramatically with decreasing density. Only a small fraction of the wide operational domain

On the Tokamak a Configuration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs ra ...

Experiments with high-power electron cyclotron heating (ECH) and current drive (ECCD) in the TCV tokamak are discussed. Power up to 2.7 MW from six gyrotrons is delivered to the tokamak at the second-harmonic frequency (82.7 GHz) in X-mode. The power is transmitted to the plasma by six independent launchers, each equipped with steerable mirrors that allow a wide variety of injection angles in both the poloidal and toroidal directions. Fully non-inductive operation of the tokamak has been achieved in steady

Electron cyclotron resonance heating (ECRH) experiments at 39 GHz with power levels similar to those of Ohmic heating have been conducted in the TCA tokamak, in either the X-mode or the O-mode. ECRH was observed to be more efficiently absorbed in low density and low q(a) discharges. At low density the central electron temperature increased by a factor of two over Ohmic values and the stored energy increased by 40% when up to 80% of the plasma energy was supplied by ECRH. During low density ECRH operation, t

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, sug

Improvement of the spatial resolution of the TCV Thomson scattering system has permitted measurements of the pedestal height and gradient of electron density and temperature profiles near; the plasma boundary during ELMy H-mode. Measured profiles were fitt ...

The physics of tokamak plasmas, in which electrons are heated by electron cyclotron heating (ECH) and whose current is driven by electron cyclotron current drive (ECCD), is investigated in this paper together with applications on tokamak A configuration variable (TCV) using modifications of the pressure and current profiles to improve the operational regimes. In order to explain the experimentally determined current drive efficiency and hard x-ray and electron cyclotron emission measurements, it is shown th

Turbulence and Transport Reduction with Negative Triangularity : Correlation ECE Measurements in TCV Due to turbulence, core energy transport in fusion devices such as tokamaks generally exceeds collisional transport by at least an order of magnitude. It is therefore crucial to understand the instabilities driving the turbulent state and to find ways to control them. Plasma shape is one of these fundamental tools. In low collisionality plasmas, such as in a reactor, changing the plasma shape from Dee-shape

Although ohmic H-modes have long been produced on the TCV tokamak and the effects of ELMs at the divertor targets studied in some detail [1], no attempt has yet been made to model the scrape-off layer (SOL) in these plasmas. This contribution describes det ...

Collective Thomson scattering, using a high-power pulsed D2O laser at 385-mu-m and a heterodyne receiver system, has provided local ion temperature (T(i)) measurements of the plasma in the tokamak chauffage Alfven (TCA) tokamak. Recent improvements in the noise-equivalent power of the Schottky barrier diode mixers permitted us to achieve a typical precision of +/- 12% for a single shot measurement at densities above 10(20) m-3. Even at densities of standard TCA discharges (5 x 10(19) m-3) the uncertainty is

Advanced tokamak regimes obtained in ASDEX Upgrade, DIII-D, FF-U, JET, JT-60U, TCV and Tore Supra experiments are assessed both in terms of their fusion performance and capability for ultimately reaching steady-state using data from the international internal transport barrier database. These advanced modes of tokamak operation are characterized by an improved core confinement and a modified current profile compared to the relaxed Ohmically driven one. The present results obtained in these experiments are s

We present applications of a recently developed method, the singular value decomposition inversion (SVD-I), for inverting line integrated multichannel interferometer measurements in fusion devices. The method expands the local electron density profiles into a limited series of orthogonal basis functions formed from the singular value decomposition of Thomson scattering measurements, which are obtained at much lower frequencies than the interferometer data. Because these basis functions are formed from a loc

Different types of central relaxation oscillations are observed in the presence of ECH depending on the location of the deposited power. In the TCV tokamak, normal sawteeth, i.e. triangular sawteeth similar to ohmic sawteeth, and saturated sawteeth are observed with central ECH power deposition, while giant sawteeth and 'humpback' oscillations occur when heating close to the sawtooth inversion surface of the local soft X ray emissivity. New measurements with high temporal resolution show that the crash phas

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. Apa ...

Confinement in TCV electron cyclotron heated discharges was studied as a function of plasma shape, i.e. as a function of elongation, 1.1 < kappa < 2.15, and triangularity, -0.65 less than or equal to delta less than or equal to 0.55. The electron energy confinement time was found to increase with elongation, owing in part to the increase of plasma current with elongation. The beneficial effect of negative triangularities was most effective at low power and tended to decrease at the higher powers used. The l

The TCV tokamak has obtained ohmic H-modes in virtually all diverted plasmas with the ion del B drift directed towards an X-point and in several elongated limiter plasmas. Troyon factors (beta(tor)aB/l(p)) up to 2 and line average densities up to 2.2 x 10(20) m(-3), corresponding to the Greenwald limit, have been obtained in diverted ELM-free H-modes. Quasi-stationary H-modes lasting for the entire current flat top (1.5 s) have been obtained in the presence of regular ELMs. The occurrence and magnitude of E

A comparative study of X- and O-mode high field side launch for electron cyclotron heating (ECH) breakdown and startup of tokamak plasmas has been performed. It is found that X-mode power is not absorbed at the cyclotron resonance but uniquely at the upper hybrid resonance, displaced to the low field side of the cyclotron resonance. O-mode power, however, is absorbed at the cyclotron resonance as well. The displacement of the upper hybrid resonance to the low field side with O-mode launch is significantly s

Roland Behn

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10-channel far-infrared polarimeter for the tokamak configuration variable

A Thomson scattering diagnostic for measurements in the divertor region of TCV

A new method for the inversion of interferometry data using basis functions derived from singular value decomposition of local measurements in tokamak plasmas

Accessibility and properties of ELMy H-mode and ITB plasmas in TCV

Safety factor profile requirements for electron ITB formation in TCV

High-power ECH and fully non-inductive operation with ECCD in the TCV tokamak

Electron-Cyclotron Resonance Heating on the Tca Tokamak

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

Edge profiles of electron temperature and density during ELMy H-mode in ohmically heated TCV plasmas

ECH physics and new operational regimes on TCV

Transport and turbulence reduction with negative triangularity : Correlation ECE measurements in TCV

SOLPS5 modelling of ELMing H-mode on TCV

Ion Temperature-Measurements in a Tokamak by Collective Thomson Scattering

Status of and prospects for advanced tokamak regimes from multi-machine comparisons using the 'International Tokamak Physics Activity' database

Application of the singular value decomposition method for inversion of interferometer measurements in fusion plasmas

Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

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

Energy confinement and MHD activity in shaped TCV plasmas with localized electron cyclotron heating

Ohmic H-modes in the TCV tokamak

X-Mode and O-Mode Electron-Cyclotron Heating Breakdown and Startup in Tca