Optics of Electron Beam in the Recycler

Abstract

Abstract. Electron cooling of 8.9 GeV/c antiprotons in the Recycler ring (Fermilab) requires high current and good quality of the DC electron beam. Electron trajectories of ~0.2 A or higher DC electron beam have to be parallel in the cooling section, within ~ 0.2 mrad, making the beam envelope cylindrical. These requirements yielded a specific scheme of the electron transport from a gun to the cooling section, with electrostatic acceleration and deceleration in the Pelletron. Recuperation of the DC beam limits beam losses at as tiny level as ~0.001%, setting strict requirements on the return electron line to the Pelletron and a collector. To smooth the beam envelope in the cooling section, it has to be linear and known at the transport start. Also, strength of the relevant optic elements has to be measured with good accuracy. Beam-based optic measurements are being carried out and analysed to get this information. They include beam simulations in the Pelletron, differential optic (beam response) measurements and simulation, beam profile measurements with optical transition radiation, envelope measurements and analysis with orifice scrapers. Current results for the first half-year of commissioning are presented. Although electron cooling is already routinely used for pbar stacking, its efficiency is expected to be improved.

Electron Beam Size Measurements in the Fermilab Electron Cooling System

“…The error of the tilt angle ν is about 16% and the A x and A y are determined to within about 10%. These results indicate that the envelope oscillations are ~220 µrads [6]. The beam shape appears to be fairly uniform through the cooling section although it is somewhat elliptical.…”

The Recycler Electron Cooler

“…The beam envelope was found to be close to being round and was adjusted with two lenses right upstream of the cooling section. The residual 2D angle at the beam periphery was reported to be 0.22 mrad [28]. This was sufficiently good to obtain electron cooling rates adequate to demonstrate cooling and early operation.…”

“…The beam boundary was determined by scraping ~10 -5 of the beam intensity. Therefore, this procedure was sensitive only to the beam halo, which properties may be very different from the core's [28]. At the very beginning, the measured beam size exceeded the prediction from Eq.…”

“…Repeating the procedure in all scrapers one by one gave the beating of the beam ellipse along the cooling section. While by design this diagnostics allows only to align the beam halo, which angles can differ significantly from the core [28], it was sufficiently good to demonstrate cooling and put the cooler into operation. All further tuning was made using cooling itself as an indicator of the electron beam angles.…”

Attainment of a High-Quality Electron Beam for Fermilab’s 4.3 MeV Cooler

“…[3], shows that presently the angles across the beam are determined by envelope oscillations. The original design assumed operation of the gun in the Pierce mode, with a nearly uniform current density distribution.…”