L. Porte

First experimental results from the European Union 2 MW coaxial cavity ITER gyrotron prototype

Abstract The European Union is working toward providing 2-MW, coaxial-cavity, continuous-wave (cw) 170-GHz gyrotrons for ITER. Their design is based on results from an experimental preprototype tube having a pulse length of several milliseconds, in operation at Forschungszentrum Karlsruhe (FZK) for several years now. The first industrial prototype tube was designed for cw operation but, in a first phase, aimed at a pulse length of 1 s at the European Gyrotron Test Facility in Lausanne, Switzerland, as part of a phased testing/development program (1 s, 60 s, cw). The first experimental results of the operation of this prototype gyrotron are reported here. The microwave generation was characterized at very short pulse length (<0.01 s) using a load on loan from FZK, and the highest measured output power was 1.4 MW, at a beam energy significantly lower than the design value (83 kV instead of 90 kV), limited by arcing in the tube. The radio-frequency (rf) beam profile was measured to allow reconstruction of the phase and amplitude profile at the window and to provide the necessary information permitting proper alignment of the compact rf loads prior to pulse extension. Arcs in the tube limited the pulse length extension to a few tens of milliseconds. According to present planning, the tube is going to be opened, inspected, and refurbished, depending on the results of the inspection, to allow testing of an improved version of the mode launcher and replacement of some subassemblies.

Complete multi-field characterization of the geodesic acoustic mode in the TCV tokamak

The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and toroidal structure of the magnetic component of the turbulence-driven GAM has been mapped in the TCV tokamak. Radially localized measurements of the fluctuating density, ECE radiative temperature and poloidal flow show that the GAM is a fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.

Development of a 2-MW, CW Coaxial Gyrotron at 70 GHz and Test Facility for ITER

In ITER, EC heating and current drive (H&CD) is foreseen not only as a principal auxiliary system for plasma heating and as assist for plasma start-up, but is considered essential in meeting the key requirement of neoclassical tearing mode (NTM) stabilisation, by localized current drive. In the reference ECH design, ITER requires a total of 20 MW/CW power at 170 GHz using gyrotrons with a unit power of 1 MW. A higher power per unit (2 MW/gyrotron) would result in a strong reduction of the cost of the whole ECRH system, and would also relax the room constraints on the launcher antenna design. In view of the capability of coaxial cavity gyrotrons demonstrated with short pulse experiments at FZK, the European Fusion Development Agreement (EFDA) has started in 2003 the development of an industrial 170 GHz 2 MW/CW coaxial cavity gyrotron, in a collaborative effort between European research associations CRPP/EPFL, FZK, TEKES and Thals Electron Devices (TED). The development plan includes three steps to reach successively 2 MW/1s, 2 MW/60s and finally 2 MW/CW operation. The procurement of the first prototype is in progress and it scheduled to be delivered during the first quarter of 2006. The experimental tests of the prototypes will be carried out at CRPP/EPFL, where an ITER relevant test facility is presently under construction and will be achieved during the second half of 2005. The test facility is designed to be flexible enough, allowing the possible commissioning of tubes with different characteristics, as well the tests of the launcher antenna at full performances.

Plasma dynamics with second and third-harmonic ECRH and access to quasi-stationary ELM-free H-mode on TCV

Intense electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) are employed on the Tokamak a Configuration Variable (TCV) both in second- and third-harmonic X-mode (X2 and X3). The plasma behaviour under such conditions is driven largely by the electron dynamics, motivating extensive studies of the heating and relaxation phenomena governing both the thermal and suprathermal electron populations. In particular, the dynamics of suprathermal electrons are intimately tied to the physics of X2 ECCD. ECRH is also a useful tool for manipulating the electron distribution function in both physical and velocity space. Fundamental studies of the energetic electron dynamics have been performed using periodic, low-duty-cycle bursts of ECRH, with negligible average power injection, and with electron cyclotron emission (ECE). The characteristic times of the dynamical evolution are clearly revealed. Suprathermal electrons have also been shown to affect the absorption of X3 radiation. Thermal electrons play a crucial role in high density plasmas where indirect ion heating can be achieved through ion-electron collisions. In recent experiments approximate to 1.35 MW of vertically launched X3 ECRH was coupled to a diverted ELMy H-mode plasma. In cases where >= 1.1 MW of ECRH power was coupled, the discharge was able to transition into a quasi-stationary ELM-free H-mode regime. These H-modes operated at beta(N) approximate to 2, (n) over bar (e)/n(G) approximate to 0.25 and had high energy confinement, H-IPB98(y,H-2) up to approximate to 1.6. Despite being purely electron heated and having no net particle source these discharges maintained a density peaking factor (n(e,o)/ approximate to 1.6). They also exhibited spontaneous toroidal momentum production in the co-current direction. The momentum production is due to a transport process as there is no external momentum input. This process supports little or no radial gradient of the toroidal velocity.

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.

Preliminary results of top launch third harmonic X-mode electron cyclotron heating in the TCV tokamak

The electron cyclotron heating (ECH) system in TCV has recently been upgraded to a total of 4.5 MW of installed power. In order to extend the density range of ECH-heated plasmas up to electron densities of 1.2 x 10(20) m(-3), three 0.5 MW gyrotrons at a frequency of 118 GHz have been added to the existing 3 MW second harmonic system operating at a frequency of 82.7 GHz. The launcher consists of a mirror placed at the top of the vessel that collects the power of the three microwave sources and can be steered radially and poloidally to ensure maximum flexibility. The choice of the top launch scheme results as a compromise between heating of high-density plasmas and maximization of single-pass absorption in Ohmically heated target plasmas. In this launching configuration, the high sensitivity of single-pass absorption to launching geometry has been experimentally demonstrated. A comparison between the experimental results and the linear ray-tracing/absorption code TORAY-GA shows a good agreement for densities up to 6.0 x 10(19) m(-3).

First experimental results from the EU 2 MW coaxial cavity ITER gyrotron prototype

The EU is working towards providing 2 MW, coaxial-cavity, CW, 170 GHz gyrotrons for ITER. Their design is based on results from an experimental pre-prototype tube in operation at FZK for several years, having a pulse length of several milliseconds. The first industrial prototype tube is designed for CW operation, but, in a first phase, will be tested out to Is at the European Gyrotron Test Facility in Lausanne, Switzerland as part of a phased testing/development program (1s, 60 s, CW). It is known that RF beam profile shaping, stray radiation handling, and collector cooling at these high power levels are three issues for the gyrotron. The gyrotron, magnet and body power supply have been delivered and successfully installed at the test stand, hosted by the CRPP. The main high voltage power supply delivery is delayed, so one of the power supplies dedicated to 3 of 9 gyrotrons in the TCV EC system is being used as a backup power source (all 3 TCV power sources can be interfaced with the test stand). Cathode conditioning began in November 2007 followed by collector conditioning in December. Parasitic low frequency oscillations have not hindered operation, and the tests have progressed to conditioning out to 0.14 s pulses by March 2008. During this period, the performance concerning microwave generation has been characterised and the RF beam profile has been measured at several planes to allow reconstruction of the phase and amplitude profile at the gyrotron window and to provide the necessary information permitting proper alignment of the compact RF loads prior to pulse extension. The power will be measured, according to the pulse length, using either a very-short pulse (<0.01 s) load on loan from FZK, or short-pulse (<0.2 s) or long-pulse (CW), spherical, calorimetric loads developped as part of this program by CNR. This paper presents the preliminary results of these operations.

Recent electron cyclotron emission results on TCV

Electron cyclotron emission (ECE) diagnostics on Tokamak à Configuration Variable (TCV) allow study of the electron temperature evolution in time with good spatial and temporal resolution at the high field side and low field side at various lines of sight. That is why ECE is being widely used to obtain both qualitative and quantitative information on heat transport, magnetohydrodynamics (MHD) phenomena, and fast electron dynamics. In this paper, a new regime on TCV with regular oscillations of the electron temperature in electron cyclotron current drive (ECCD) driven fully noninductive discharges and in discharges with a combination of ohmic/ECCD driven current is discussed. These oscillations are reminiscent of the oscillations of the central electron temperature (O-regime) seen on Tore Supra in fully noninductive lower hybrid current drive plasmas. A link between evolutions of the electron temperature, the MHD modes, and the current density profile on TCV is considered. In order to yield information on the properties of microturbulence of electrostatic and magnetic origin on TCV, a correlation ECE radiometer is currently under development. A technical description of the diagnostic is presented in this paper.