J. Jacquinot

Present and future JET ICRF antennae

Abstract Since the initial operation of the JET ICRF system in 1985, up to 22 MW has been coupled to the plasma, many heating scenarios have been demonstrated and the main technological problem of RF-specific impurity production overcome. Many developments of the antennae have taken place over this period, notably the replacement of the water-cooled nickel screens with indirectly cooled beryllium screens, and the forthcoming installation of eight new A2 antennae for operation during the pumped divertor phase of JET. The A2 antennae include enhanced provision for fast wave current drive experiments on JET. This paper describes the beryllium screens, the technological results from operation and subsequent inspection of these screens, the design of the A2 antennae and the results from high power RF testing of a model of the A2 antenna.

Impurity release from the ICRF antenna screens in JET

During ICRF heating in JET highly-localized impurity influxes from the Faraday screens (FS) of RF antennas are observed. The influxes result from the acceleration of deuterium and impurity ions in the RF-enhanced Bohm-Debye sheaths. The RF-enhanced sheaths form when the equilibrium magnetic field line, linking a large RF magnetic flux, connects two surfaces and closes a loop where the RF voltage is induced. The FS influxes depend on two intimately-linked parameters, the antenna voltage and the density at the FS, as well as on the magnetic field angle, antenna phasing, FS geometry and material.

The modification of the plasma edge and impurity production by antenna phasing during ICRF heating on JET

During r.f. experiments on JET a strong modification of the plasma edge is generally observed. Density and temperature profiles in the scrape-off layer usually flatten and enhanced impurity and neutral influx is observed. Metallic impurity influx originating from antennae screens is also observed. This is especially true when the antennae are phased so that the excited spectrum contains a large fraction of power in the waves with k/sub ///=0-3 m-1. A radially localized enhancement of the particle diffusivity Dperpendicular to (a) across the plasma edge results. When the antennae are phased in the quadrupole configuration with the maximum power emitted at k/sub ///=7 m-1 a strong modification of the edge is not observed. The scrape-off layer profiles scale as in the ohmic discharges. Then both the light and metal impurity influxes become substantially reduced.

JET recent results on wave heating and current drive consequences for future devices

The latest developments of the JET ICRH and LHCD systems are reviewed. Feedback controls have alleviated the traditional difficulties with plasma-antenna coupling. Experimental results on high plasma performance, basic confinement aspects and issues relevant to the next generation of tokamaks are described. ICRH allows a narrow power deposition profile to be localised at will independently of the plasma density and size. The authors demonstrate that in large tokamaks, the constraint to use a minority species can be removed and heating a balanced D/T mixture at omega = omega CD in ITER is proposed. LHCD allows the highest current efficiency to be obtained. The conditions for wave accessibility are discussed in light of the JET results. LHCD is well adapted to save volt seconds in the current rise phase. The observed synergistic LHCD/ICRH acceleration of fast electrons is a promising route towards higher current drive efficiency but raises unresolved physics aspects which are analysed. They finally consider the prospects of a steady state fusion reactor.

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.

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.

Effect of beryllium evaporation on the performance of ICRH on JET

Abstract Beryllium films (150 to 300 A) have been evaporated on all JET in-vessel components (carbon limiters, nickel screen, inconel walls). Immediate and lasting improvements of the plasma have been observed in all plasma regimes. We concentrate on the results obtained with ICRH and NBI on the density limit and on H-mode plasmas which are particularly sensitive to impurity release. Specific effects produced by RF waves are reduced to a negligible level when the following 3 aspects are combined - beryllium evaporation, dipole phasing, correct screen angle. The density limit obtained with ICRF is now identical to NBI heated cases and substantially higher than the values obtained in the pre-beryllium phase. Long H-modes (1.3 s) have been obtained with RF alone. The H mode confinement is similar to that of NBI cases. In limiter plasmas Zeff decreased to 1.2–1.4 at high density and for 10 MW input power. The results are related to oxygen gettering, suppression of nickel sputtering from the antenna screens and to the use of dipole phasing.

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.

Invited paper: ICRF heating/plasma edge interaction in jet with beryllium gettering

During Ion Cyclotron Range of Frequencies (ICRF) heating in JET (Joint European Torus) with beryllium gettered walls and RF antenna screens both the global and local impurity influxes are significantly reduced as compared to the previous operation. In particular oxygen is reduced by an order of magnitude and the deuterium is efficiently pumped. The deuterium recycling RD < 1 except at the onset of RF pulse (1 < RD < 1.1) for t = 0.75 s, indicating modest desorption. This provides the density control. Except for the lowest densities, Zeff = 2 at PRF = 10 MW is routinely achieved. At At comparable densities, PRAD/PTOT is 30–50% lower. If we assume that the long term evolution of the beryllium signal at the antenna screen is due to erosion, rather than coverage by carbon, we estimate the corresponding influx in monopole configuration (k11 = 0 m−1) to be ΦBeSCREEN = 1.16 × 1019 atoms MW−1 s−1 from an effective area AEFF = 0.5 m2. At the same coupling resistance the beryllium influx scales with the RF power of the antenna and with the plasma density. With dipoles (k11 = 7 m−1), the influx is reduced by more than a factor 3. The contribution of antenna influxes to Zeff is small, ΔZeffBe = 0.08 and ΔZeffNi = 0.05 at PRF = 5 MW.

Invited paper: Impurity release by ICRF antennas in jet

Abstract The release of Faraday screen material into the plasma by a powered ICRF antenna is investigated in this report. JET spectroscopic data has been analyzed to give scalings of the metal impurity influx from a chromium-plated screen with antenna voltage and edge density. Modeling of the metal influx data by an RF sheath mechanism and scrape off layer model has been carried out including the sputtering contributions of deuterium and carbon ions and the self-sputtering of the Faraday screen material. The predictions of the model for JET parameters are compared with the Cr data and are also employed to investigate the sputtering influx of Be from Be-coated Faraday screens.