Electron cyclotron (EC) heating in a magnetised plasma is performed by launching a radio-frequency (RF) wave which is resonant with the plasma electrons having a cyclotron frequency, fcycl, (or integer multiple of fcycl) equal to the RF. The cyclotron frequency is proportional to the local magnetic field amplitude (fcycl = 28 [GHz]/[Tesla]). The resonant nature of the interaction, together with the space dependent magnetic field in a fusion device, creates a situation in which extremely localised power deposition can be achieved. This property can be used either for localised electron heating (ECH) or for generating non-inductive current drive (ECCD). Compared to other RF auxiliary heating systems EC waves have the additional advantage that the radiation propagates in freespace, which significantly simplifies the launching system.

“Tokamak à Configuration Variable” is the main experimental facility on the Lausanne (Switzerland) site of the Association Euratom CRPP (“Centre de Recherches en Physique des Plasmas”).

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The magnetic island produced by a tearing mode perturbs the bootstrap current, which further amplifies the island and degrades confinement or leads to a disruption. This instability is the neo-classical tearing mode.

Device used for generating high power microwaves in the electron cyclotron range of frequencies (≈ 50 – 200 GHz).

Multi-megawatt ECH and ECCD system exists on several of today’s toroidal magnetic fusion devices. In ITER, ECH and ECCD is foreseen not only as a principal auxiliary system for plasma heating and for plasma start-up, but is considered essential in meeting the key requirement of neoclassical tearing mode (NTM) stabilisation by localised current drive. The EC system is therefore distributed between a main horizontal port, through which 20 MW of EC power at 170 GHz can be launched, and three upper ports, also operating at 170 GHz, which are to be used principally for the stabilisation of NTMs.

TCV Paramete rs:
Plasma height:
Plasma width:
Plasma major radius:
Plasma current:
Plasma elongation:
Aspect ratio:
Toroidal magnetic field on the
magnetic axis:
Additional heating (ECRH):
Transformer flux:
Loop voltage:
Plasma duration:
Vessel width:
Vessel height:
Vessel ohmic resistance:
Time constant of the vessel:

The recently completed ECH system installed on the TCV tokamak, with a total of 4.1 MW of injected power, and with a highly flexible launching mirror system allowing real-time control of the toroidal and poloidal mirror angles, is, at present, the most relevant ECH systems for ITER. It is made up of 9 gyrotrons grouped in three “clusters” of three gyrotrons. Two of the clusters operate at 82.7 GHz (0.5 MW/gyrotron, 2s) and are used for heating and/or current-drive, coupling to the X-mode plasma wave from the outboard (low field side) of the machine and being absorbed at the 2nd harmonic resonance (X2). The third cluster operates at 118 GHz (0.5 MW/gyrotron, 2s), couples to the X-mode from the top of the machine and provides the ECH power which is absorbed at the 3rd harmonic resonance (X3).

Each gyrotron RF output is connected to an RF conditioning unit (RFCU) whose optics is designed both for adapting the RF beam to the evacuated waveguide and for controlling the wave polarisation. The required polariser angles are calculated for the target plasma equilibrium and desired heating/current-drive scenario and then remotely set to these values using a motorised system. Each RFCU is connected to ~30 m of a 63.5 mm diameter evacuated waveguide, the reference transmission system for ITER. For the X2 gyrotrons each wave guide is connected to a low-field side launcher, which has two angular degrees of freedom. One degree of freedom has real-time steering capability with a maximum angular mirror sweep rate of 48 °/sec.

The mirror orientation can be such as to sweep either the poloidal or the toroidal injection angles during the plasma shot. The 3 wave guides from the X3 gyrotrons converge to a single top-launch mirror which has radial-position and poloidal-angle degrees of freedom. The X3 mirror angle has real-time steering capability of 20 °/sec. With the very flexible position and shape control capability of the TCV poloidal field coil system, the real-time steering capabilities of the launching angles both for the X2 and X3 have proven to be essential. During a plasma discharge, the real-time steering of the RF beams primarily permits experiments to be performed that would be significantly more difficult to analyse and interpret if performed on a shot to shot basis. It also opens the door to the implementation of a feedback system for real-time control of the power deposition location and/or driven current, which are essential for ITER.

A variety of plasma heating and current drive experiments have been carried out in recent years. With the X2 gyrotrons, fully sustained non-inductive discharges are routinely achieved by replacing the inductively driven current with CO-ECCD. These discharges have been extended to the study of advanced tokamak regimes, specifically to the formation of a steady state electron internal transport barrier (e-ITB) in conjunction with a large bootstrap fraction (up to 80% of the total plasma current). The stabilisation of local MHD instabilities, such as the sawtooth instability by ECCD at the q=1 surface has also been extensively studied. The developed tools may be used to prevent long sawtooth periods, which have been shown to trigger the onset of NTMs in low beta discharges on JET, which are similar to the operating regimes planned for ITER.

More information:

FZK, Karlsruhe (Germany)


TEKES, Helsinki (Finland)


CEA (France)


THALES Electron Device


CNR, Milano (Italy)


ENEA (Italy)


FOM, Utrecht (The Netherlands)
http:// www. rijnh. nl/ n0/ f1234. htm

IPF, Stuttgart (Germany)


IPP, Garching (Germany)


UKAEA, Culham (UK)


The X3 system broadens the operational space of TCV with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system (ne = 4.2 x 1019 m-3). In recent experiments with a target plasma density of ne = 4.5 x 1019 m-3 and 1.35 MW of X3 injected from the top launcher, full single- pass absorption has been achieved, with the total plasma energy increasing by a factor of 2.5.

In parallel to the physics studies with the ECH system installed on TCV, within the frame of EFDA, the Association Euratom-CRPP is strongly involved in the European gyrotron development programme, which is, from its inception, focused on the development of sources having high unit power. Within this programme, the technical development by industry (gyrotrons for TCV, Tore-Supra and W7-X) aims at gaining all the necessary knowledge to build a high power Continous Wave (CW) tube. The European community is now engaged in the joint development by Associations and industry of a 2 MW, CW, coaxial cavity gyrotron at 170 GHz, its test stand (including the power supplies, series switch, transmission line, RF load) and a prototype upper launcher for the ITER EC system. Contributions to the gyrotron development come from the Euratom Associations CRPP, FZK and TEKES, with the participation of CEA, and from the industrial partner THALES Electron Device. The test stand involves a collaboration between the Euratom Associations CNR, CRPP, ENEA, and FOM while CEA, CNR, CRPP, FOM, FZK, IPP, IPF and UKAEA are collaborating on the development of the upper launcher and the modeling. These activities are preparing for high power tests to be carried out at CRPP once a suitable gyrotron is available.