Fusion Engineering and Design 82 (2007) 744–750 Transverse field collector sweep system for high power CW gyrotrons M. Schmid a,∗ , S. Illy a , G. Dammertz a , V. Erckmann c , M. Thumm a,b a Forschungszentrum Karlsruhe, Institut f¨ ur Hochleistungsimpuls und Mikrowellentechnik, D-76021 Karlsruhe, Germany b Universit¨ at Karlsruhe, Institut f¨ ur H ¨ ochstfrequenztechnik und Elektronik, D-76131 Karlsruhe, Germany c Max Planck-Institut f¨ ur Plasmaphysik, EURATOM-Association, D-17491 Greifswald, Germany Received 31 July 2006; received in revised form 29 May 2007; accepted 5 June 2007 Available online 25 July 2007 Abstract The spent electron beam of megawatt-class CW gyrotrons requires magnetic sweeping to keep the specific heat dissipation on the collector surface within technically acceptable limits of about 500 W per cm 2 . Conventionally this is realised with a solenoid coil placed over the collector, which sweeps the strike point of the electron beam up and down the collector-wall at low frequency (<10Hz). The low frequency is necessitated by the strong screening effects due to eddy-currents in the copper-collector-wall. This paper describes the realisation and test results obtained with a six-coil 50 Hz transverse field sweep system (TFSS) on the 140 GHz 1 MW CW prototype gyrotron for W7-X. The advantages of this system compared to the conventional scheme are discussed and an outlook on further enhancements is given. © 2007 Elsevier B.V. All rights reserved. Keywords: ECRH; Gyrotron; Collector; Electron-beam; Dissipation; Sweeping 1. Introduction During the development of the W7-X gyrotrons the wall material of the lowest section of the collector was changed from copper to stainless steel (SS). The original purpose was to reduce the level of stray radi- ation inside the gyrotron by increasing the microwave absorbing surface. The conductivity of stainless steel (SS) is at least 40 times lower than that of copper and the RF absorption is therefore 6–7 times higher. Soon ∗ Corresponding author. the idea came up, to make use of this low conductiv- ity section and to introduce a rotating transverse field sweep system (TFSS) operating at 50 Hz, thus reduc- ing the considerable cost of the conventional sweep system which consists of a large solenoid coil and a rather sophisticated low frequency IGBT power sup- ply. The SS-section can be seen in Fig. 1, which shows the W7-X prototype gyrotron without cooling jacket. Numerical simulations [1] demonstrated the feasi- bility of such a scheme, although the simulation-codes could not calculate the field-attenuation through the 0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.06.008