Studies on the Radio Frequency power supply system for the ITER NB Injector Ion Source

The most recent studies on the development of a Radio Frequency (RF) driven Negative Ion Source for the ITER NB Injector, performed at IPP Garching laboratory, besides being aimed at demonstrating the fulfilment of the main ITER requirements, were also addressed at defining the technical specifications for the mechanical and electrical components [W. Kraus et al. '' RF-source development for ITER: Large area H- beam extraction, modifications for long pulse operation and design of a half size ITER source '', Fusion Eng. And Design, Vol. 74, Part A pp 337-341.]. As far as the RF Power Supply (PS) system is concerned, the most relevant input are the necessary RF power, the matching configuration, the number of the RF drivers to supply. On the basis of these data, the design of a 1 MHz, 1 MW RF power supply system has been worked out in cooperation between IPP and RFX in the framework of the European activities on the ITER NB Injector. The first analyses to outline the RF power supply scheme were oriented toward the definition of the best modularity level to be adopted, which is related to the requirements in terms of number of RF drivers independently controlled to achieve the desired spatial uniformity of the negative ion production. The solution which will be presented in the paper was worked out with the aim of achieving the best compromise between the required performance and flexibility level, size and cost. A key issue in a RF circuit is the achievement of the matching between the source and load impedances that allows the maximum power transfer to the load and avoids the presence of reflected RF waves. In the ITER NBI RF source, load impedance variations are expected, therefore provisions to assure the capability of adjusting the matching also during the pulse is required. In the existing IPP test-beds, this function is performed by a matching network close to the source, made by a transformer, and a couple of capacitors, one of which variable so as to provide for the adjustment required. In ITER, the presence of remote controlled mechanical actuators inside the ion source should be avoided due to the radioactive environment and the difficult accessibility. Analyses were addressed to optimize the matching circuit and to identify possible alternative strategies to perform the matching adjustment; they led to an improved and simplified matching network without both matching transformers and remotely controlled variable capacitors in the source, the adjustment being made by varying the RF generator frequency. The paper will report on the analyses performed, and the resulting present design of the RF power supply system. The results of the experimental tests made at IPP to verify the proposed solutions will be also described. (author)