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An electron cyclotron resonance heating (ECRH) system capable of delivering 2.4 MW cw has been designed to be built at Commissariat a l'Energie Atomique, Cadarache, for the Tore Supra (TS) experiment, to provide plasma heating and current drive by electron cyclotron resonance interaction. The planned system was composed of a generator using six gyrotrons 500 kW for 5 s or 400 kW cw working at 118 GHz. Six transmission lines made of corrugated waveguide, 63.5-mm diameter, carry the HE11 mode to one antenna composed of six fixed mirrors and three independently movable mirrors for the adjustment of the injection angles of the rf beams. The antenna was built and installed in TS, and all transmission line components ordered and installed between the gyrotron locations and the antenna. In the same way, the required six oil tanks, the six cryomagnets, and the six modulating anode devices were designed and manufactured. In parallel, after demonstration in the factory of proper operation of the prototype gyrotron, the manufacture of a first so-called series gyrotron was made. But this gyrotron experienced hard limitations (overheating inducing prohibited outgassing, parasitic oscillations) during the long-pulse tests in Cadarache, and the achieved performance was 300 kW for 110 s. A new study was then carried out in collaboration with Thales Electron Devices, the EURATOM-CEA Association, and the EURATOM-Confederation Suisse Association to understand and overcome the limitations, which led to the construction of a new modified gyrotron. During the tests in factory of this new gyrotron, the output beam showed two peaks, a pattern never predicted by simulations. The gyrotron was nevertheless transferred to Cadarache for long-pulse testing, but an arc on the windows definitely stopped the tests. To understand the cause of the observed two peaks, various low-level tests were then performed on a model of the mode converter with different shapes for the launcher, but without real improvement. Besides measurements, the use of a new software, Surf3D, based on integral equations and providing a complete three-dimensional modeling, showed that the problem mainly comes from the third mirror, whose curvature is too high and consequently not well taken into account by the calculation. These technological problems have seriously delayed the development of the gyrotrons; as a consequence, only two tubes (intermediate developments) are presently available on TS to inject 700 kW in 5-s pulses. In spite of this relatively low power, the localized absorption property of electron cyclotron waves has been used on TS in a wide variety of experiments, such as stabilization and control of the sawtooth period, perturbative transport studies by ECRH modulations, and ECRH-assisted plasma start-up.

Closed loop control ofthe period offast ion stabilized sawtooth has been demonstrated for the first time on Tore Supra by varying the electron cyclotron current drive (ECCD) injection angles in real time. Fast ions generated by up to 4 MW of central ion cyclotron resonance heating (ICRH) increased the sawtooth period from the ohmic value of 25 ms to 60 to 100 ms. This sawtooth period was reduced to 30 ms by the addition of only 300 kW of ECCD. In ICRH heated shots where the normalized minor radius of the ECCD absorption location was swept from 0.4 to 0.05 in 4 s, the sawtooth period showed an abrupt change from 70 to 30 ms when the ECCD deposition normalized minor radius reached ∼0.2. This short period was then maintained until the absorption location moved well inside the sawtooth inversion radius at which point it abruptly returned to 70 ms. A closed loop controller was implemented that allowed the sawtooth period to be switched in real time between short and long sawteeth with a response time of the order of I s.

The long pulse capability of Tore Supra and its ECRH system makes it an ideal machine to prove steady state feedback control as required in ITER. Although Neoclassical Tearing Modes (NTMs) have not yet been observed on Tore Supra, the control of other MHD modes represents a very similar task from a control point of view and the stabilisation of such modes for long periods using ECRH will provide essential experience for the implementation of such control schemes on ITER. For this work to progress on Tore Supra, it must be possible to vary the injection angles in real time under feedback control from measured plasma parameters. At Tore Supra the front mirror position - and hence the injection angles - is adjusted using stepper motors controlled through a serial link. The use of a serial link limiting the sampling time for the control system to 50-100 ms and the dynamic response of the stepper motors results in a system frequency response

An ECRH (Electron Cyclotron Resonance Heating) system capable of delivering 2.4 MW CW is presently under construction at CEA (Commissariat a l'Energie Atomique) Cadarache, for the Tore Supra experiment to provide plasma heating and current drive by Electron Cyclotron Resonance interaction. Due to some limitations observed on the first series tube which achieved 300 kW output power for 110 s, a new study carried out in a collaboration between TED (Thales Electron Devices), the Association Euratom-CEA and the Association Euratom-Confederation Suisse has led to the construction of a new modified gyrotron. The new gyrotron, with a new launcher profile and a better cooling system is now installed in this test bed. A clear improvement in the time required to condition the tube has been observed. On the other hand poor mode purity in the output beam has resulted in the need to implement a cooling system for the waveguides transmitting the power to the dummy load. The gyrotron tests have been temporarily suspended while a new system for the automatic filling of the cryostats with liquid nitrogen and helium is being installed. The experience gained from tests operations including some of the problems related both to auxiliary equipment and to the control of the gyrotrons will be presented with a special focus on long pulse related issues.

The 118 GHz electron cyclotron heating and current drive (ECRH/ECCD) system under development in Cadarache, France, for use on the Tore Supra tokamak (Pain M. et al 1994 Proc. 18th SOFT (Karlsruhe) pp 481–4: Darbos C. et al 2000 Proc. 21st SOFT (Madrid) pp 605–9), is designed to launch 2.4 MW of power for up to 10 min into the plasma. At present two out of six gyrotrons are installed and available for injection of up to 800 kW. This paper concentrates on the generation and transmission of the ECRH/ECCD power for very long pulse operation. The power is injected into the plasma as Gaussian beams by an antenna which, using actively cooled mirrors inside the Tore Supra vacuum vessel, allows extensive control of both the poloidal and toroidal injection angles. The toroidal field on Tore Supra is normally in the range of 3.8–4 T, which for 118 GHz gives almost central deposition at the fundamental electron cyclotron resonance. A pair of actively cooled corrugated mirrors is installed in each matching optics unit at the output of each gyrotron allowing complete control of the polarization of the wave transmitted to the antenna, with the result that pure O-mode—or pure X-mode—power injection can be achieved for all injection angles. In tokamak experiments, a world record energy of 17.8 MJ has been injected into the plasma. New upgraded gyrotrons specified to produce 400 kW for up to 10 min will be introduced over the next 3–4 years.

This year TORE-SUPRA celebrated its 25 years of operation. During this long time a number of technologies have been developed [1]. First of all it was mandatory to develop reliable superconducting magnets at ∼ - 4 K, with superfluid helium as efficient coolant. For the production of steady state discharge, 3 types of Radio Frequency (RF) additional heating systems have been developed: Lower Hybrid Current Drive (LHCD), Ion Cyclotron Resonance Heating (ICRH) and Electron Cyclotron Resonance Heating (ECRH) [2]. To cope with long lasting discharges (up to 380 s × 2.8 MW) and large RF additional heating power (12.3 MW × 3 s), Actively Cooled (AC) Plasma Facing Components (PFC) were deployed in TORE-SUPRA for the first time in a Tokamak environment. TORE-SUPRA is now being modified into an axisymmetric tokamak with actively cooled tungsten main chamber walls and a divertor, the WEST project (W - for tungsten - Environment in Steady-state Tokamak) [3]. This new facility has the objective to offer ITER a test bed for validating the relevant actively cooled metallic technologies in D-shape H-mode plasmas. In contrast to other metallic devices such as JET and ASDEX, WEST will rely only on RF additional power systems. A set of plasma scenarios have been identified, ranging from a high total RF power scenario up to 15 MW during 30 seconds, to a high fluence scenario of 1000 seconds with up to 10 MW of injected RF power. These scenarios are able to reproduce ITER relevant conditions of steady state heat loads of 10 to 20 MW/m, to test tungsten actively cooled divertor technologies with relevant power heat fluxes and particle fluence. The paper presents the main issues regarding WEST project and especially the additional RF power injection systems (2 LHCD antennas, 3 + 4 = 7 MW continuous wave and 3 ICRH antennas, 3 × 3 = 9 MW-30 s or 3 MW-1000 s) for WEST. The front face of the LHCD antennas will be modified to account for the different plasma position and smaller toroidal field ripple, due to the more inward antenna position in the vessel. No other modifications are needed on the Passive-Active Multijunction (PAM) or the Fully-Active Multijunction (FAM) LHCD antennas, or the associated generator (2 × 8 klystrons, 600 kW each CW). Concerning the ICRH system, the main challenges are its ELM-resilience, its compatibility with continuous operation, and the interaction of the RF near fields with neighbouring plasma facing components. 3 new actively cooled antennas are being designed to be matched with an ELMs resilient electric circuit. The proposed solution is based on the JET-EP antenna and CEA prototype tested in 2007, both having identical internal conjugate-T electrical layout and a demonstrated load resilience capacity to plasma edge transients during ELMs. © 2013 IEEE.

In a tokamak plasma, sawtooth oscillations in the central temperature, caused by a magnetohydrodynamic instability, can be partially stabilized by fast ions. The resulting less frequent sawtooth crashes can trigger unwanted magnetohydrodynamic activity. This Letter reports on experiments showing that modest electron-cyclotron current drive power, with the deposition positioned by feedback control of the injection angle, can reliably shorten the sawtooth period in the presence of ions with energies >or=0.5 MeV. Certain surprising elements of the results are evaluated qualitatively in terms of existing theory.

The first TH1506B prototype showing problems of overheating and spurious oscillations, a new modified gyrotron was built. During the tests, the extwo peaks, which was never predicted by simulations. Various low evel tests were performed on the mode converter with different shapes for the launcher but without real improvement. Besides measurements, the use of a new software Surf3D[l] showed that the problem mainly comes from the 3rd mirror whose curvature is too high and not well taken nto account by the calculation. This analysis software is based on ntegral equations and the complete 3D modelling alowed to determine a new profile for the 3rd mirror. An aluminium model of a new mirror was manufactured and thorough low level tests made at FZK showed that there was no double peak.The next step would consist in building a gyrotron based on this new design, to confirm the simulation and to valdate it for long pulses.

Sawtooth control is a potentially important method for avoiding MHD instabilities triggered by sawtooth crashes, such as neoclassical tearing modes in a reactor‐size tokamak. The aim of the present study is to investigate the effect of localised current drive on Sawtooth activity. In recent experiments in the Tore Supra tokamak, Electron Cyclotron Current Drive has been observed to strongly affect the Sawtooth period, even at modest ECCD power levels. For instance, with 8 MW of ICRH long, fast ion stabilised, sawteeth were shortened from 0.6 s to 0.3 s by application of 0.6 MW of co‐ECCD. In these experiments it is estimated that only 2–3% of the total current was driven by ECCD, but this was sufficient to significantly affect the sawtooth period provided the driven current was localized accurately with respect to the q = 1 surface

ECRH pre-ionization and assisted start-up will be necessary in ITER due to the low electric field available (⩽0.3 V m−1). This paper reports on the results recently obtained on assisted start-up at the fundamental EC resonance on the tokamak Tore Supra (BT= 3.85 T,R= 2.4 m,a= 0.72). The Tore Supra ECRH/ECCD system is based on two 118 GHz gyrotrons (EC resonance atR∼ 2.17 m). The power is injected into the plasma as Gaussian beams by an antenna located on the low field side which, using actively cooled mirrors inside the vacuum vessel, allows extensive control of both poloidal and toroidal injection angles. During the last experimental campaign, the ECRH system has been used for the first time to assist the plasma start-up. The usual start-up voltage on Tore Supra is 25 V (1.7 V m−1) with deuterium prefill pressure of around 20 mPa. The minimum stray field value (B⊥) in most of the vacuum vessel is lower than 5 mT, which gives toroidal connection lengths (a0BT/B⊥) longer than 770 m. In these conditions, attempts have been made to lower the start-up voltage. Sustained breakdowns were obtained down to ∼0.4 V m−1without external assistance at 5 mPa. Besides, ∼300 kW of ECRH has been applied during 50 ms (one gyrotron in O-mode). Pre-ionization was successfully achieved at the resonance location and ∼30 kA of current was established in 10 ms. Successful plasma initiation has been obtained down to ∼0.28 V m−1at 5 mPa.