Enhancement and simplification of EBIS operation by evaporative ion cooling through gas mixing

Abstract

The stepwise ionization in EBIS sources can be extended beyond th time at which neutralization of the electron beam space charge is reached by trapping ions from the residual gas: We have added minor amounts of argon to the residual gas in the main trap. From this gas mixture ions are formed during the whole containment time and extracted to a TOF analyser. At short containment times, the amount and the charge states of the residual gas ions increase with time − no argon seems to be present. At long containment times, argon ions dominate the spectrum more and more and increase their charge state according to time, while the total amount of charges does not increase further. The transition occurs, when the containment time equals the time, where a maximum of ions is collected. Then evaporative cooling of argon ions by residual gas ions is starting, ending at long containment times in a stationary state, where most of the residual gas ions are lost radially.This simple method to run an EBIS, first investigated with CEBIS I and besides extraction similar to EBIT operation, shows enhancement in the yield of highly charged ions by gas mixing, as it is also known from ECR sources. The gas mixing in EBIS and EBIT provides during the whole containment time colder and lower charged ions, which take up the heat from the heavier highly charged ones and by this get evaporated out of the electron beam.