Electron-cyclotron-current-drive efficiency in DEMO plasmas

Abstract

The achievable efficiency for external current drive through electron-cyclotron waves in a demonstration tokamak reactor is investigated. Two possible reactor designs, one for steady state and one for pulsed operation, are considered. Beam propagation, absorption and current drive are modelled employing the beam tracing technique and including momentum conservation in electron-electron collisions. It is found that for midplane injection the achievable current drive efficiency is limited by second-harmonic absorption at levels consistent with previous studies. Higher efficiencies can be achieved by injecting the beams from the top of the machine, exploiting wave absorption by more energetic (less collisional) electrons. Current drive efficiencies competitive with those usually obtained by neutral beam current drive are reported. These optimum efficiencies are found for frequencies around 230 GHz and 290 GHz for the steady-state and the pulsed DEMO, supposed to operate at a magnetic field B = 5.84 T and B = 7.45 T, respectively.

DEMO: Heating and current drive system integration with blanket system

“…The opening dimensions are 300 x 300 mm. The toroidal injection angle has been chosen due to a beam tracing analysis [2], which aims to establish for a reference DEMO plasma scenario, the maximum CD efficiency with ECH. The poloidal injection angle has been decided so that the three beams could deposit power all on the same plasma volume, located at R = 9 m, vacuum vessel center.…”

“…The study has been carried out assuming each OB-MMS to be kept as single unit in order to allow their extraction from the vertical ports [1], as foreseen by the current maintenance approach for the blanket, defining the beam injection angles in order to maximize CD efficiency [2], and it is carried on for a pulsed DEMO machine (see Tab. 1 for the parameters considered).…”

Preliminary conceptual design of DEMO EC system

“…The principle of frequency step-tunability has been demonstrated for the KIT 1MW D-band multi-frequency short-pulse prototype gyrotron for example [8]. Operating frequencies above 230GHz will be required for optimum ECCD efficiencies if assuming a high magnetic field at the tokamak as defined for DEMO baseline 2012 [9]. Therefore, the principle feasibilities of a 236GHz 1 MW conventional-cavity and, alternatively, a 238GHz 2MW coaxial-cavity gyrotron are currently under investigation at EUROfusion [10].…”

Interaction circuit design and RF behavior of a 236 GHz gyrotron for DEMO

“…DEMO is the prototype of a fusion power plant which will follow ITER. The detailed goals and design parameters of the DEMO gyrotron according to the 2012 baseline of DEMO (aspect ratio of 4.0) are listed in Table 1 [7,8]. Along with these specifications, it is also preferred to use a quasi-optical mode converter for the axial output and a single disk CVD-diamond RF window [9].…”