Electron cooling of 8 GeV antiprotons at Fermilab’s Recycler: Results and operational implications * L.R. Prost # , D. Broemmelsiek, A. Burov, K. Carlson, C. Gattuso, M. Hu, T. Kroc, J. Leibfritz, S. Nagaitsev, S. Pruss, G. Saewert, C.W. Schmidt, A. Shemyakin, M. Sutherland, V. Tupikov, A. Warner FNAL, Batavia, IL 60510, USA Abstract Electron cooling of 8 GeV antiprotons at Fermilab’s Recycler storage ring is now routinely used in the collider operation. It requires a 0.1-0.5 A, 4.3 MeV dc electron beam and is designed to increase the longitudinal phase- space density of the circulating antiproton beam. This paper briefly describes the characteristics of the electron beam that were achieved to successfully cool antiprotons. Then, results from various cooling force measurements along with comparison to a non- magnetized model are presented. Finally, operational aspects of the implementation of electron cooling at the Recycler are discussed, such as adjustments to the cooling rate and the influence of the electron beam on the antiproton beam lifetime. INTRODUCTION In the Recycler, the goal of the electron cooler is to reduce the longitudinal phase-space area of the stored antiprotons either to allow for additional incoming transfers or to prepare the bunch for extraction to the Tevatron collider. Soon after the first cooling demonstration, the electron cooler became part of normal operation of the accelerator complex and allowed for the latest advances in the Tevatron luminosity. Meanwhile improvements to the electron beam quality were pursued and the cooling force characterized. ELECTRON BEAM CHARACTERISTICS Electron cooling of 8.9-GeV/c antiprotons requires a dc electron beam with kinetic energy of 4.3 MeV and a beam current of 0.1-0.5 A. The main parameters of the cooler are listed in Table 1. Table 1: Electron cooler main parameters Parameter Symbol Value Unit Electron energy E b 4.34 MeV Beam current (for cooling) I b 0.1 A Terminal voltage ripple, rms δU 250 V Cooling section (CS) length L 20 m Solenoid field in CS BB cs 105 G Beam radius in CS r b 3.3 mm The dc electron beam is generated by a thermionic- cathode gun, located in the high-voltage (HV) terminal of the electrostatic (Van de Graff-type) accelerator operated in a so-called ‘energy recovery’ mode [1] and is referred to as ‘recirculation’ in this paper. To provide cooling, the electron beam should recirculate for hours at the nominal energy and a current of hundreds of mAmps. Although this accelerator is in principle capable of sustaining dc beam currents to ground of up to 300 μA, stable operation can only be achieved for very low current losses, typically 2 × 10 -5 at 0.5 A (Figure 1). The losses to the acceleration/deceleration tubes are monitored by measuring the resistive divider currents at the top and at the bottom of the column. The beam stability improved greatly when the tube current changes were maintained to less than 1-2 μA, at the detriment of the beam loss to ground. This was achieved through optimization of the optics in the Pelletron, notably in the deceleration tube. ACTUBE DCTUBE ATUBEI DTUBEI BIASI 35.0 35.5 36.0 36.5 37.0 37.5 38.0 38.5 39.0 0 0.1 0.2 0.3 0.4 0.5 0.6 Beam current [A] Tubes current [ μ A] 0 2 4 6 8 10 12 14 16 Anode current change [ μ A] Figure 1: Current losses vs beam current. Blue diamonds are the changes to the anode current, representing the beam current loss. Brown circles and green crosses are the acceleration and deceleration tube resistive divider currents at the bottom. Pink squares and yellow triangles are those at the top. The main figures of merit to assess the quality of the beam are its rms transverse and longitudinal velocity spreads, which, along with the total beam current, ultimately determine the cooling force. In our case, the longitudinal velocity spread, expressed as the total electron energy spread in the laboratory frame, is estimated to be approximately 250 eV (rms), dominated by the power supply ripple, δU. Multiple-coulomb scattering and electron beam density fluctuations [2] are estimated to contribute to the order of ~100 eV, added in quadrature. The transverse velocity spread can be expressed as an angle, θ e , in the laboratory frame and has several origins ____________________________________________ * Operated by URA Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of Energy # lprost@fnal.gov FERMILAB-CONF-06-098-AD