Omegatron ion mass spectrometer for the Alcator C-mod tokamak

Abstract

A new ion mass spectrometry probe that operates at high magnetic field (∼8 T) has been recently commissioned on the Alcator C-Mod tokamak. The probe combines an omegatron E(t)×B ion mass spectrometer and a retarding field energy analyzer. The probe samples the plasma in the far scrape-off layer, on flux surfaces between 25 and 50 millimeters from the separatrix. Radio-frequency (rf) power is used to collect ions with resonant cyclotron frequency on the side walls of an rf cavity. Scanning the frequency results in a spectrum ordered by the ratio of ion mass to charge, M/Z. Resonances are presently resolved for 1⩽M/Z<12 down to signal levels as low as 5×10−4 times that of the bulk plasma species. Well-resolved resonances have widths within a factor of two of theoretical values obtained from particle orbit theory. Absolute impurity fluxes and individual impurity charge state temperatures are quantified by varying the applied rf power and recording the change of the amplitude of the resonant ion current. The design of the hardware and electronics design is described, the principles of operation are discussed, and initial experimental results are presented. Sources of noise which presently limit the sensitivity of the device are also discussed along with techniques to improve the signal–to–noise ratio.