G. Bosia

RF current distribution and topology of RF sheath potentials in front of ICRF antennae

The 2D (radial/poloidal) spatial topology of RF-induced convective cells developing radially in front of ion cyclotron range of frequency (ICRF) antennae is investigated, in relation to the spatial distribution of RF currents over the metallic structure of the antenna. This is done via a Greens function, determined from the ICRF wave coupling equations, and well-suited to open field lines extending toroidally far away on both sides of the antenna. Using such formalism, combined with a full-wave calculation using the 3D antenna code ICANT (Pecoul S. et al 2000 Comput. Phys. Commun. 146 166–87), two classes of convective cells are analysed. The first one appears in front of phased arrays of straps, and depending on the strap phasing, its topology is interpreted using the poloidal profiles of either the RF current or the RF voltage of the strip line theory. The other class of convective cells is specific to antenna box corners and is evidenced for the first time. Based on such analysis, general design rules are worked out in order to reduce the RF-sheath potentials, which generalize those proposed in the earlier literature, and concrete antenna design options are tested numerically. The merits of aligning all strap centres on the same (tilted) flux tube, and of reducing the antenna box toroidal conductivity in its lower and upper parts, are discussed.

High-Power Density Ion Cyclotron Antennas for Next Step Applications

It is generally recognized that the ion cyclotron (IC) heating and current drive technology is applicable to the next step devices, but it is still debated whether current IC launching structures are adequate for high-power density operation in discharges where the edge density profile is modified by the presence of edge-localized mode (ELM) activity. Important issues are the antenna tolerance to load variations (such as those due to ELMs) and minimum parasitic power losses. The proposed ITER IC antenna design addresses these issues through an advanced antenna design. The design principles and the projected performances of the ITER IC antenna are discussed.

Key results of long pulse ICRH operation in Tore Supra

Long pulse operation on the Tore Supra tokamak has entered a new phase, characterized by the use of heating power level in excess of 10?MW, during pulses lasting several tens of resistive times. This has been made possible by the use of ion cyclotron range of frequency (ICRF) heating (9?MW coupled to the plasma at 57?MHz), combined with lower hybrid current drive (LHCD: 3?MW at 3.7?GHz) and efficient fuelling techniques (supersonic gas injection, pellets). This paper addresses key technological, operational and physics issues related to the long pulse operation of the Tore Supra ICRF system and required for a reactor: R&D on the ICRF plant, real-time control and safety procedures, integration with other tokamak subsystems, experimental investigation and theoretical modelling of the edge ICRF physics (wave coupling, heat loads on antenna front faces). As far as possible lessons are drawn from the experience gained on Tore Supra for the design and operation of a next-step device.

Validation of the load-resilient ion cyclotron resonance frequency antenna concept on Tore Supra plasmas

In the framework of the ion cyclotron resonance frequency heating development at CEA Cadarache, a prototype antenna based on the load-resilient electrical layout foreseen for ITER has been built. This prototype was recently tested in Tore Supra. The ITER-like electrical scheme has been validated during fast perturbations at the edge plasma. Clear load resilience properties are reported. The main conclusions and consequences learned from the development of the ITER antenna are discussed.

Development of the JET ICRH plant

Abstract The JET 32 MW ICRF system is modular in construction and consists of eight independent generator-antenna units operating at frequencies in the range 23–57 MHz. The system is controlled remotely, and all real time functions, such as the matching to the plasma load, are performed automatically during the tokamak pulse. The flexibility and versatility of the plant have successfully permitted a continuing development programme since the system commenced operation in 1985. The total power of the eight generators has been upgraded from 24 to 32 MW, of which over 22 MW has been coupled into the plasma centre. The plant and power availability has been considerably improved by enhancing the automatic matching and protection circuitry. The operating regimes have been extended beyond the original conceptual design, making it possible to heat during L to H mode transitions and perform preliminary Fast Wave Current Drive experiments. New facilities will enable phased conductor FWCD experiments with the four-element (A2) antenna arrays during the pumped divertor phase of JET. This paper reviews the enhancements to the plant since its inception, especially to the control circuitry, which have resulted from analysis and understanding of the fast varying antenna-plasma load and from operating under increasingly more stringent conditions. References to the associated ICRH physics are given. The developments of the JET antennae are described in the companion paper “Present and Future JET ICRH Antenna” by A.S. Kaye.

Dynamics of rotating tearing modes under phase locked feedback control

Low-order single-helicity rotating resistive modes in a tokamak can, in principle, be stabilized in amplitude by means of externally applied fields, provided that control of the phase difference between mode and field is achieved. Possible feedback loop schemes are presented and the non-linear dynamics is studied analytically and numerically. Other physical effects currently observed in modern tokamaks are discussed and interpreted in the general framework of phase locking phenomena.

Design Status of EC System in ITER - Design of Reliable RF Beam Launching System

The design of two types of EC-launchers which fulfil many functions requested to EC system, is described. One with toroidal RF beam steering is mainly used for main plasma heating and current drive. The other with poloidal RF beam steering is mainly used for neoclassical tearing mode stabilization. The RF beam is deflected by a steerable mirror. In this design, one mirror, steerable in a single direction, is assigned to one row of waveguides. A reliable RF beam launching system can be constructed by using a spiral pipe for cooling water and a simple push/pull shaft for mirror rotation without a cam and link. The design includes a mirror composed of a high melting temperature material plate and a SS pipe.

Tore supra ICRH antenna prototype for next step devices

ITER-like IC antennas, fulfilling or possibly exceeding ITER requirements and designed for long pulse operation (1000 s) are currently being developed at Cadarache. As part of these developments, a project of an IC antenna prototype obtained by modifying a tore supra (TS) resonant double loop (RDL) antenna is proposed, to assess in real working conditions the performance of this scheme. After a brief comparison between the new RF circuit version and the conventional RDL circuit, which has been tested in TS for 10 years, the paper focuses on the design and manufacturing aspects of the prototype.

ICRH-produced H-modes in the JET tokamak

H-modes produced by ion-cyclotron resonance heating (ICRH) in a double-null X-point open divertor configuration in the JET Tokamak are studied, where the ICRH antennas, operated in the dipole (0, pi ) phasing, are located on the low-field side of the Tokamak. Beryllium evaporation on the nickel antenna screen and the first wall of the Tokamak played a crucial role in achieving H-modes with ICRH alone. In these discharges, all the characteristics typical of neutral-beam injection heating H-modes are found, and the global energy confinement time approaches two times the Goldston L-mode prediction. For most of their duration, ICRH H-modes are free from ELMs (edge-localized modes). In addition, the monster sawtooth (long sawtooth-free period) feature of ICRH is maintained during H-modes, leading to a central electron temperature (Tc0) of 10 keV, nearly twice the value of the central ion temperature (Ti0). ICRH H-modes often occur as a two-step transition and the antenna plasma coupling resistance (Rc) also decreases in two steps. Theoretical values of Rc agree well with experimental values during the H-phase.

Preliminary ICRF results from JET

As a first step in the JET ICRF programme, two antenna generator units have been installed at JET and operated up to the design specification of 3 MW coupled in the Torus for 1 second. After a brief description of the system, the experimental results of wave coupling to the plasma and matching the plasma loaded antenna are discussed. The first heating results are presented concentrating on the analysis of a case where the total power to the plasma increases by a ratio of 2.5.