Investigation of ion trapping in high-power electron beams for electron coolers
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Abstract
Electron coolers utilize high-current electron beams to counteract emittance blow
up of hadron beams circling in storage rings with equal velocity, by enveloping the
hadrons and transferring their momentum to the electrons in a dedicated cooling
section of the accelerator. To achieve effective cooling, electron beams with currents
in the Ampere regime are required. Diagnostics of such high-current electron beams
is crucial for effective operation. In this thesis, the possibility of using beam induced
fluorescence (BIF) of residual gas as a means of beam diagnostics was investigated.
At the electron cooler test bench located at Helmholtz-Institute Mainz the BIF signal,
which is caused by ionized residual gas particles trapped in the electromagnetic
potential of the electron beam, indicated the center of charge and the boundary of
said electron beam. A simple model, describing the intensity distribution of the
BIF, could be established. Brief beam interruptions were used to significantly suppress
the BIF, while preserving a large duty cycle. It also became evident, that
the residual gas in the ultra-high vacuum vessel of the test bench is infused with
barium, evaporating from the dispenser cathode, which in turn influences the BIF.
This type of cathode is commonly used in most applications that rely on high DC
beam currents. In addition, the electron source was improved to be more robust
against Penning discharges for certain electromagnetic field configurations, allowing
for an electron beam current of 1A.