P. Lamalle

Preparing ITER ICRF: development and analysis of the load resilient matching systems based on antenna mock-up measurements

The reference design for the ICRF antenna of ITER is constituted by a tight array of 24 straps grouped in eight triplets. The matching network must be load resilient for operation in ELMy discharges and must have antenna spectrum control for heating or current drive operation. The load resilience is based on the use of either hybrid couplers or conjugate-T circuits. However, the mutual coupling between the triplets at the low expected loading strongly counteracts the load resilience and the spectrum control. Using a mock-up of the ITER antenna array with adjustable water load matching solutions are designed. These solutions are derived from transmission line modelling based on the measured scattering matrix and are finally tested. We show that the array current spectrum can be controlled by the anti-node voltage distribution and that suitable decoupler circuits can not only neutralize the adverse mutual coupling effects but also monitor this anti-node voltage distribution. A matching solution using four 3 dB hybrids and the antenna current spectrum feedback control by the decouplers provides outstanding performance if each pair of poloidal triplets undergoes a same load variation. Finally, it is verified by modelling that this matching scenario has the same antenna spectrum and load resilience performances as the antenna array loaded by plasma as described by the TOPICA simulation. This is true for any phasing and frequency in the ITER frequency band. The conjugate-T solution is presently considered as a back-up option.

New Techniques For The Improvement Of The ICRH System ELM Tolerance On JET

Two complementary improvements to the ELM tolerance of the existing A2 antennas on JET are being assessed. The use of external conjugate‐T matching of straps of adjacent antenna arrays could reduce the VSWR levels at RF amplifier output during fast load perturbations. The scheme under consideration uses coaxial line‐stretchers (trombones) for tuning the conjugate‐T to low resistive impedance (3–6 Ohm) with subsequent stub/trombone circuit impedance transformation to 30 Ohms. Another technique is to modify the RF plant protection system logic to reduce the high VSWR trip duration to an absolute minimum corresponding to a typical ELM response (∼1–2ms) without compromising the plant safety. Both projects are presently being tested and could increase the average power delivered by RF plant into ELMy plasmas at JET.

Status of the ITER IC H&CD System

The ITER Ion Cyclotron Heating and Current Drive system will deliver 20 MW of radio frequency power to the plasma in quasi continuous operation during the different phases of the experimental programme. The system also has to perform conditioning of the tokamak first wall at low power between main plasma discharges. This broad range of requirements imposes a high flexibility and a high availability. The paper highlights the physics and design requirements on the IC system, the main features of its subsystems, the predicted performance, and the current procurement and installation schedule.

R&D activities on RF contacts for the ITER ion cyclotron resonance heating launcher

Abstract Embedded RF contacts are integrated within the ITER ICRH launcher to allow assembling, sliding and to lower the thermo-mechanical stress. They have to withstand a peak RF current up to 2.5xa0kA at 55xa0MHz in steady-state conditions, in the vacuum environment of the machine. The contacts have to sustain a temperature up to 250xa0°C during several days in baking operations and have to be reliable during the whole life of the launcher without degradation. The RF contacts are critical components for the launcher performance and intensive R&D is therefore required, since no RF contacts have so far been qualified at these specifications. In order to test and validate the anticipated RF contacts in operational conditions, CEA has prepared a test platform consisting of a steady-state vacuum pumped RF resonator. In collaboration with ITER Organization and the CYCLE consortium (CYclotron CLuster for Europe), an R&D program has been conducted to develop RF contacts that meet the ITER ICRH launcher specifications. A design proposed by CYCLE consortium, using brazed lamellas supported by a spring to improve thermal exchange efficiency while guaranteeing high contact force, was tested successfully in the T-resonator up to 1.7xa0kA during 1200xa0s, but failed for larger current values due to a degradation of the contacts. Details concerning the manufacturing of the brazed contacts on its titanium holder, the RF tests results performed on the resonator and the non-destructive tests analysis of the contacts are given in this paper.

Studies of JET ICRH Antenna Coupling During ELMs

Details are provided of a new fast data acquisition system that records RF data for a complete four‐strap JET ICRH antenna array with sampling rates up to 250 kHz, triggered by the rapid increase in plasma Dα emission intensity during an ELM. The coupling properties are deduced from the transmission line voltage amplitude and phase measured using 80dB directional couplers installed close to the antenna. Resistive and reactive components of antenna loading perturbation are reported during different types of ELMs and discharge magnetic configurations.

Overview on Experiments On ITER‐like Antenna On JET And ICRF Antenna Design For ITER

Following an overview of the ITER Ion Cyclotron Resonance Frequency (ICRF) system, the JET ITER‐like antenna (ILA) will be described. The ILA was designed to test the following ITER issues: (a) reliable operation at power densities of order 8u2009MW/m2 at voltages up to 45 kV using a close‐packed array of straps; (b) powering through ELMs using an internal (in‐vacuum) conjugate‐T junction; (c) protection from arcing in a conjugate‐T configuration, using both existing and novel systems; and (d) resilience to disruption forces. ITER‐relevant results have been achieved: operation at high coupled power density; control of the antenna matching elements in the presence of high inter‐strap coupling, use of four conjugate‐T systems (as would be used in ITER, should a conjugate‐T approach be used); operation with RF voltages on the antenna structures up to 42 kV; achievement of ELM tolerance with a conjugate‐T configuration by operating at 3Ω real impedance at the conjugate‐T point; and validation of arc detection sys...

ITER ICRF system: R&D progress and technical choices

This paper describes the ITER ICRF system main requirements and the latest developments for its different parts which are to be procured by the ITER Parties. The built in margins will allow reaching the requirements on a large parameter range, or delivering more power in a restricted range.

ICRF Wall Conditioning: Present Status and Developments for Future Superconducting Fusion Machines

ITER and future superconducting fusion machines need efficient wall conditioning techniques for routine operation in between shots in the presence of permanent high magnetic field for wall cleaning, surface isotope exchange and to control the in-vessel long term tritium retention. Ion Cyclotron Wall Conditioning (ICWC) based on the ICRF discharge is fully compatible and needs the presence of the magnetic field. The present paper focuses on the principal aspects of the ICWC discharge performance in large-size fusion machines: (i) neutral gas RF breakdown with conventional ICRF heating antennas, (ii) antenna coupling with low density (similar to 10(17) m(-3)) RF plasmas and (iii) ICWC scenarios with improved RF plasma homogeneity in the radial and poloidal directions. All these factors were identified as crucial to achieve an enhanced conditioning effect (e.g. removal rates of selected marker masses). All the observed effects are analyzed in terms of RF plasma wave excitation/absorption and compared with the predictions from I-D RF full wave and 0-D RF plasma codes. Numerical modeling and empirical extrapolation from the existing machines give good evidence for the feasibility of using ICWC in ITER with the main ICRF antenna.

Optimizing ion-cyclotron resonance frequency heating for ITER: dedicated JET experiments (vol 53, 124019, 2011)

Reference EPFL-ARTICLE-178250doi:10.1088/0741-3335/54/6/069601View record in Web of Science Record created on 2012-06-15, modified on 2017-05-12

Development of RF Tools and Scenarios for ITER on JET

The improvement of LH coupling with local puffing of D2 gas, which made operation at ITER relevant distances (10 cm) and with ELMs a reality, has been extended to ITER‐ like plasma shapes with higher triangularity. With ICRF, we developed tools such as (1) localized direct electron heating using the 3He mode conversion scenario for electron heat transport studies, (2) the production of 4He ions with energies in the MeV range by 3 ωc acceleration of beam injected ions at 120 keV to investigate Alfven instabilities and test α diagnostics, (3) the stabilisation and destabilisation of sawteeth and (4) ICRF as as a wall conditioning. Several ITER relevant scenarios were tested. The (3He)H minority heating scenario, considered for the non‐activated start‐up phase of ITER, produces at very low concentration energetic 3He which heat the electrons indirectly. For n3He/ne > 2%, the scenario transforms to a mode conversion scenario where the electrons are heated directly. The (D)H minority heating is not accessible ...