Conceptual study of an ICRH traveling-wave antenna system for low-coupling conditions as expected in DEMO

Abstract

For the central heating of a fusion reactor ion cyclotron radio frequency heating (ICRH) is the first choice method as it is able to couple RF power to the ions without density limit. The drawback of this heating method is the problem of excitation of the magneto-sonic wave through the plasma boundary layer from the antenna located along the wall, without exceeding its voltage standoff. The amount of coupling depends on the antenna excitation and the surface admittance at the antenna output due to the plasma profile. The paper deals with the optimization of the antenna excitation by the use of sections of traveling-wave antennas (TWAs) distributed all along the reactor wall between the blanket modules. They are mounted and fed in resonant ring system(s). First, the physics of the coupling of a strap array is studied by simple models and the coupling code ANTITER II. Then, after the study of the basic properties of a TWA section, its feeding problem is solved by hybrids driving them in resonant ring circuit(s). The complete modeling is obtained from the matrices of the TWA sections connected to one of the feeding hybrid(s). The solution is iterated with the coupling code to determine the loading for a reference low-coupling ITER plasma profile. The resulting wave pattern up to the plasma bulk is derived. The proposed system is totally load resilient and allows us to obtain a very selective exciting wave spectrum. A discussion of some practical implementation problems is added.

Study of a distributed ICRF antenna system in DEMO

“…The details of the modelling procedure are explained in [4]. It is worth to remind that, in its pass band, the combline acts as transmission line with characteristic impedance Z 0 = Z it = (Z 11 2 -Z 12 2 ) 0.5 [1] where: Z it is the iterative impedance of the structure [6] and Z ii (i=1,2) are the terms of the symmetric Z matrix that characterise the two-port network. In other words, Z it is the impedance seen at the input of a two-port network when the output is terminated on the same input impedance.…”

“…6). A TWA section fed by a resonant ring system was analysed in [1] and will be here compared with the first case where the resonant ring is connected to two arbitrary but consecutive elements, e.g. #28 and #1.…”

“…While an independent feeding for each element allows the best control on the radiated spectrum, it poses some technical difficulties like the large number of intrusions in the reactor vessel and the intrinsic coupling between adjacent lines that complicates the external matching circuitry. A travelling wave array [1] is then a possible solution allowing a considerably smaller number of feeding lines and a simplified external circuitry. Integration in the DEMO design requires a detailed analysis of the possible system solutions for the launcher and the corresponding feeding mechanism.…”

“…The aim of this contribution is to explore some of those possibilities. Three different feeding cases are here analysed: (i) an array with all elements fed [2], (ii) an array with periodic conventional feeding [3] and (iii) an array with resonant ring recirculation feeding system [1]. We first analyse case (i), the case with the best possible performances [4] although with the drawback of requiring a large number of mutually coupled feeding lines.…”

“…In the second case the dependency of the response of the structure with different relative phasing of the generators is analysed and the power capabilities are studied. In the last section an array fed with one resonant ring is compared to the traveling wave array (TWA) of [1] and a general case of two consecutive resonant rings is studied. Conclusions are then presented.…”