A. Walden

Implementation of load resilient ion cyclotron resonant frequency (ICRF) systems to couple high levels of ICRF power to ELMy H-mode plasmas in JET

The paper summarizes the continuous developments made to the ion cyclotron resonant frequency (ICRF) system at JET in order to improve the reliability of the power coupled to plasma. It details the changes and improvements made to the system so that more power is coupled during ELMy plasmas as well as increasing the power density to demonstrate reliable operation in the range of the requirements for ITER. Results obtained using the conventional matching (stubs and trombones) system, 3 dB couplers and the conjugate-T scheme with variable matching elements outside the wave launching structure (external conjugate-T) and inside the wave launching structure (ITER-like antenna) are described. The presence of the three different approaches to load resilient ICRF systems at JET creates a unique opportunity to compare these methods under very similar plasma conditions and to assess the results of ICRF power delivery to ELMy plasmas, an important issue for ITER. The impact of the availability of increased levels of reliable ICRF power on plasma physics studies in JET is illustrated.

Fundamental ion cyclotron resonance heating of JET deuterium plasmas

Radio frequency heating of majority ions is of prime importance for understanding the basic role of auxiliary heating in the activated D-T phase of ITER. Majority deuterium ion cyclotron resonance heating (ICRH) experiments at the fundamental cyclotron frequency were performed in JET. In spite of the poor antenna coupling at 25 MHz, this heating scheme proved promising when adopted in combination with D neutral beam injection (NBI). The effect of fundamental ICRH of a D population was clearly demonstrated in these experiments: by adding similar to 25% of heating power the fusion power was increased up to 30-50%, depending on the type of NBI adopted. At this power level, the ion and electron temperatures increased from T-i similar to 4.0 keV and T-e similar to 4.5 keV (NBI-only phase) to T-i similar to 5.5 keV and T-e similar to 5.2 keV (ICRH + NBI phase), respectively. The increase in the neutron yield was stronger when 80 keV rather than 130 keV deuterons were injected in the plasma. It is shown that the neutron rate, the diamagnetic energy and the electron as well as the ion temperature scale roughly linearly with the applied RF power. A synergistic effect of the combined use of ICRF and NBI heating was observed: (i) the number of neutron counts measured by the neutron camera during the combined ICRF + NBI phases of the discharges exceeded the sum of the individual counts of the NBI-only and ICRF-only phases; (ii) a substantial increase in the number of slowing-down beam ions was detected by the time of flight neutron spectrometer when ICRF power was switched on; (iii) a small D subpopulation with energies slightly above the NBI launch energy was detected by the neutral particle analyzer and gamma-ray spectroscopy.

Design and operations of a load-tolerant external conjugate-T matching system for the A2 ICRH antennas at JET

A load-tolerant External Conjugate-T (ECT) impedance matching system for two A2 Ion Cyclotron Resonance Heating (ICRH) antennas has been successfully put into operation at JET. The system allows continuous injection of the RF power into plasma in the presence of strong antenna loading perturbations caused by Edge Localized Modes (ELMs). Reliable ECT performance has been demonstrated under a variety of antenna loading conditions including H-mode plasmas with Radial Outer Gaps (ROG) in the range of 4-14 cm. The high resilience to ELMs predicted during the circuit simulations has been fully confirmed experimentally. Dedicated arc detection techniques and real-time matching algorithms have been developed as a part of the ECT project. The new Advanced Wave Amplitude Comparison System (AWACS) has proven highly efficient in detection of arcs both between and during ELMs. The ECT system has allowed the delivery of up to 4 MW of RF power without trips into plasmas with Type-I ELMs. Together with the 3dB system and the ITER-Like Antenna (ILA), the ECT has brought the total RF power coupled to ELMy plasma to over 8 MW, considerably enhancing JET research capabilities. This paper provides an overview of the key design features of the ECT system and summarizes the main experimental results achieved so far.

Coupling Of The JET ICRF Antennas In ELMy H‐mode Plasmas With ITER Relevant Plasma—Straps Distance

In ITER, the requirement for the ICRF antenna is to deliver 20 MW in ELMy H‐mode plasmas with an averaged antenna—plasma separatrix distance of 14 cm [1]. Two major problems will have to be solved: the very fast change in antenna loading during ELMs and the decrease of the loading when the plasma is pushed far away from the antenna. JET has the capability to combine these conditions and for the first time, experiments were performed in ELMy H‐mode at antenna—separatrix distance, referred as ROG, varied from 10 to 14 cm. When ROG was increased, the perturbation caused by ELMs was found to decrease significantly and the loading between ELMs was found to deteriorate to very low values. In order to compensate the latter unwanted effect, different levels of deuterium gas were injected in the edge either from the divertor, the midplane or the top of the tokamak. Using this technique, the loading was increased by up to a factor 6 and up to 8 MW of ICRF power were coupled.

Commissioning of the wideband matching system for ICRH of ELMy JET plasmas

The conventional JET ICRH antenna matching system cannot respond to large and rapid load variations induced by the ELMs. Unacceptably large bursts of reflected power occur at the generators, severely limiting the ability to deliver RF power to ELMy plasmas. A wideband matching system based on prematching sliding impedances and fast frequency control is under commissioning on one of the antenna arrays. When suitably configured, the new system, which operates in 4 frequency bands inside the JET plant range, should strongly reduce the ELM-induced power reflection by rapid adjustments of generator frequency. The paper reports recent progress achieved in its commissioning. Successful operation of the fast frequency control loop has been demonstrated during ELMs, and work is underway to bring the system to full operation.

Fast Wave Current Drive in JET ITB-Plasma

Fast wave current drive has been performed in JET plasmas with internal transport barriers, ITBs, and strongly reversed magnetic shear. Although the current drive efficiency of the power absorbed on the electrons is fairly high, only small effects are seen in the central current density. The main reasons are the parasitic absorption of RF power, the strongly inductive nature of the plasma and the interplay between the fast wave driven current and bootstrap current. The direct electron heating in the FWCD experiments is found to be strongly degraded compared to that with the dipole phasing.

Recent Developments in the External Conjugate‐T Matching Project at JET

The External Conjugate‐T (ECT) matching system is planned for installation on two A2 ICRH antenna arrays at JET in 2007. This will enhance the operational capabilities of the RF plant during ELMy plasma scenarios and create new opportunities for ITER‐relevant matching studies. The main features of the project are discussed in the paper focusing on the specific challenges of the ECT automatic matching and arc detection in optimized ELM‐tolerant configurations. A ‘co/counter‐clockwise’ automatic control mode selection and an Advanced Wave Amplitude Comparison System (AWACS) complementing the existing VSWR monitoring are proposed as simple and viable solutions to the identified problems.

The internal vacuum transmission lines of the ITER-like ICRH antenna project for JET

Abstract A new ITER-like (International Thermonuclear Experimental Reactor) ICRH antenna is being developed to deliver over 7 MW at high power density (8 MW/m2) to the JET plasmas. The paper discusses the RF, mechanical and hydraulic design aspects of the option for Internal Vacuum Transmission Lines based upon in-vessel motor actuators.