F. Louche

Re-evaluation of ITER ion cyclotron operating scenarios

Various ICRH scenarios for ITER-FEAT are evaluated. A wave propagation and damping study confirms the potential of the second harmonic tritium heating scenario for both heating and current drive purposes. The fundamental deuterium heating scheme is dominated by beryllium and alpha particle absorption. Owing to the reduced ITER size, the low frequency current drive window is lost in practice. A 3He minority greatly enhances the performance. In the early stage of the discharge, the power absorbed by the 3He is transferred to the background ions but, later on, the power of the fast particles (from both the 3He and the tritium tails) is roughly equally distributed between electrons and ions. Using the experimentally established expression for the L to H mode threshold, it is found that the H mode regime can always be reached using RF heating. To achieve Q = 10, high density operation is required. Minority current drive competes with heating and with electron current drive. The scenarios foreseen for the non-activated ITER-FEAT phase are also discussed.

Study of the ITER ICRH system with external matching by means of a mock-up loaded by a variable water load

A mock-up of the complete antenna array (24 straps grouped in 8 triplets) of the ICRH system with external matching for ITER has been constructed with a length reduction factor of 5. At a frequency increased by the same factor the electrical properties of the full-scale system can be measured in the presence of non-dispersive medium. A movable water tank in front of the array simulates variable plasma loading. Measurements of the matching performances of various external circuit configurations and of the scattering matrix of the system show (i) the non-negligible effect of mutual coupling on load resilient matching by Conjugate T (CT) or hybrid leading to coupling between the matching actuators and the generators and asymmetry in power distribution, (ii) good load resilience of a single CT for the right choice of configuration and number of matching parameters, (iii) the large number of matching solutions for coupled CTs and (iv) the benefit of passive power distribution to the straps. This has been successfully tested in the case of the complete array. The power is passively distributed among the upper half and the bottom half of the 24 radiating straps of the antenna plug. The 4 top and 4 bottom triplets are, respectively, set in parallel outside the antenna plug near a voltage anti-node by means of T junctions. The load resilient matching (VSWR <1.3 for an antenna loading variation of about 1–8 Ω m−1) is then obtained by a 4-parameters single CT configuration or a hybrid. The maximum voltage along the line remains equal to the one in the antenna plug and there is a fair power share between the straps. A straightforward robust matching procedure of the complete array is described.The effective radiation resistance of different toroidal and poloidal phasing conditions is measured and compared. The paper also underlines the significant influence of the presence of the electrostatic screen and the resulting increase in the recess of the straps on the reduction of the coupling to the load and of the mutual coupling between the straps.

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.

Eigenmode analysis of the ITER ICRF antenna plug and electrical solution to the grounding of the antenna

The excitation of non-TEM modes around the ITER ICRF (ion cyclotron range of frequency) antenna plug can considerably increase the level of RF currents and voltages on the ITER plug. We report on a study of these modes and a solution to avoid them in the ITER ion cyclotron range of frequencies. From a transmission line approach we show that resonances can be avoided if the distance between the mouth of the line and an added short-circuit is sufficiently small. We therefore propose to position short-circuits at about 1 m from the antenna mouth, constituted by rows of closely spaced non-capacitive contacts all around the plug. These conclusions are further validated with Microwave Studio® simulations. A simplified model of the ITER ICRF array is used for comparing various grounding solutions and it is proved that the solution already inferred from transmission line approximation suppresses the resonances from the frequency domain relevant for ICRH in ITER. The MWS calculations predict that the proposed solution also avoids large RF currents in the blanket modules and in their connectors. Their amplitude barely exceeds 5% of the strap RF currents. This solution avoids perturbation of the antenna array impedance matrix by the coaxial gap. We also discuss resonances appearing at harmonic frequencies which could be excited by various non-linear effects due to the plasma or the generator. This kind of excitation should not lead to large voltage build-up.

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 8 MW/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...

Simulation of ITER full-field ICWC scenario in JET: RF physics aspects

ITER as a superconducting fusion machine needs efficient wall conditioning techniques for application in the presence of the permanent high toroidal magnetic field for (i) reducing the in-vessel impurity content, (ii) controlling the surface hydrogen isotopic ratio and (iii) mitigating the in-vessel long-term tritium inventory build-up. Encouraging results recently obtained with ion-cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have raised ICWC to the status of one of the most promising techniques available to ITER for routine inter-pulse and overnight conditioning with the ITER main ICRF heating system in the presence of the permanent high toroidal magnetic field. This paper is dedicated to a milestone experiment in ICWC research: the first simulation of ICWC operation in an equivalent ITER full-field scenario and the assessment of the wall conditioning effect on the carbon wall in the largest present-day tokamak JET. In addition, we address in this paper the following topics: (i) an analysis of the radio frequency (RF) physics of ICWC discharges, (ii) the optimization of the operation of ICRF antennas for plasma startup and (iii) an outlook for the performance of ICWC in ITER using the ICRF heating system. Important operational aspects of the conventional ICRF heating system in JET (the so-called A2 antenna system) for use in the ICWC mode are highlighted: (i) the ability of the antenna to ignite the cleaning discharge safely and reliably in different gases, (ii) the capacity of the antennas to couple a large fraction of the RF generator power (>50%) to low-density (≈1016–1018 m−3) plasmas and (iii) the ICRF absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. Successful optimization of the JET-ICWC discharge parameters (BT = 3.3 T, f = 25 MHz) resulted in a reliable operation of the JET A2 antennas and a high conditioning efficiency in a scenario imitating closely ITER full-field operation (BT = 5.3 T, f = 40 MHz) with the fundamental ion-cyclotron resonance for deuterium located on-axis. Numerical modelling with the 3D electromagnetic code Micro Wave Studio, a 1D RF full wave code and a 0D plasma code allows extrapolating the results obtained on JET and other present-day tokamaks to ITER and provides good prospects for the use of the ITER ICRF antennas for ICWC purposes.

ITER ICRF Antenna Optimization and Broad‐Banding Validation by use of a Reduced‐Scale Mock‐Up

The ITER ICRF antenna has been optimized by making use of numerical tools. Flexible reduced‐scale mock‐ups have been constructed in order to validate the results of the optimization. The different mock‐ups and their purpose are briefly described. First measurements are given, including the confirmation of the broad‐banding effect of the service stub. The comparison of the measurements with results of numerical simulations is the subject of a companion paper. The measurements and the response of the system in function of frequency are interpreted in the light of transmission line models.

Validation of the Electrical Properties of the ITER ICRF Antenna using Reduced‐Scale Mock‐Ups

Experimental measurements on reduced‐scale mock‐ups allow validating the electrical properties and RF numerical optimization of the ITER ICRF antenna. Frequency response in the different regions of the antenna is described and key parameters for performance improvement are given. Coupling is improved by acting on the front‐face geometry (strap width, antenna box depth and vertical septa recess). The 4‐port junction acts as a frequency filter and together with the service stub performs pre‐matching in the whole frequency band. Influence of the Faraday screen on coupling is limited. The effect of voltage limitation on the maximum total radiated power is given. The importance of a good decoupling network and of grounding is emphasized. Finally the control of the antenna wave spectrum is performed by implementing feedback controlled load‐resilient matching and decoupling options and control algorithms are tested.

Performance of the ITER ICRF Antenna plug as expected from TOPICA matrices

The performances of the present ICRF antenna plug design is evaluated by means of the TOPICA 24×24 matrix for plasma loading supplemented by 4‐port junction (4PJ) matrices. For their interpretation these results are compared with a cruder modeling by the semi‐analytical code ANTITER II. From this analysis we conclude: (1) The broadbanding effect of the service stub on the response of the 4PJ made for one triplet is maintained for the complete array for all the heating and current drive phasings. (2) For a given maximum voltage of the 8 feeding lines the radiated power capability is roughly constant in the entire frequency band. (3) The power capability of the array is significantly dependent on the distance of the antenna to the Last Closed Flux Surface, the density profile in the scrape‐off layer (SOL) and on the toroidal and poloidal phasings. The dependence on phasing is stronger for wider SOL (4) For a not too optimistic plasma density profile (Sc2 shortl7) a power capability exceeding 20 MW is only o...

ICRF/ECR plasma production for wall conditioning in TEXTOR-94

To develop an alternative scenario for efficient wall conditioning in superconducting fusion devices, comparative studies of ICRF and ECR Discharge Conditioning (ICRF-DC/ECR-DC) have been undertaken on TEXTOR-94 using the present ICRH and ECRH systems without changes in the hardware. The first results clearly indicate essential differences in performance and cleaning between the two types of RF discharges for the same machine parameters {BT=2.0 T,pHe=(3÷7)×10−2 Pa,PICRF≈PECRH≈0.1 MW÷0.2 MW,τICRF=τECRH=0.2 s}: (i) The ICRF plasma produced by double-strap unshielded antenna (ω=4ωcHe+=2ωcHe++=ωcH) has a low density (ne0≈0.4×1018 m−3) and homogeneous distribution in the torus during all phases of the discharge; (ii) The ECR plasma produced by quasi-optical beam launching antenna (ω=2ωce) has a high density (ne0=2.4×1018 m−3) and is strongly localized along the equatorial trajectory of the focussed microwave beam; (iii) Hydrogen desorption from the wall looks more pronounced and efficient in the ICRF-DC than i...