[en] Generation of a DC electron beam in the future Fermilab electron cooler [1] employs an electrostatic acceleration and a beam energy recovery, so that electrons are decelerated from the nominal energy of 4.3 MeV they have in the cooling section to few keV in the collector. Stable performance of this scheme requires a current loss (delta)I below 10 (micro)A at the beam current up to the nominal value of I = 0.5 A. One of sources of the loss is a back flow of secondary electrons from the beam collector. The paper discusses principles and performance of a collector with the low current loss. Electric and magnetic fields in the collectors used in existing electron coolers are axially symmetrical. For practically interesting parameters, such collectors can not provide (delta)I/I<10^-4 because of the reversibility of trajectories in the collectors: a secondary electron with the kinetic energy equal to the energy of the primary one can come out of the collector following the trajectory of the ''parent'' electron. The back flow can be dramatically decreased if the reversibility is broken by a transverse magnetic field in the collector cavity. In our case, the field was formed by a system of permanent magnets. Several versions of the system were tested at a low-energy test bench. The first of them is optimized for operation without a longitudinal magnetic field. The transverse field is formed by two groups of 6 Nd-Fe-B square permanent magnets, mounted on both sides of the collector and magnetized along the X-axis. Because the directions of magnetization in groups are opposite, the magnetic field in the vicinity of the Z-axis has a quadrupole configuration with the gradient of 10-15 G/cm. The field focuses electrons in X direction and defocuses in Y, so that the beam is absorbed on collector walls mainly along a narrow band near the plane X=0. The transverse field in this region, with the magnitude of 50-70 G, effectively confines secondary electrons. The only exception is electrons entering the collector with small Y offsets, which fly through the collector and hit its bottom. Because the transverse field strength near the bottom is low, the produced secondary electrons have a high probability of escaping from the collector. Measurements of the collector efficiency at various beam positions at the collector entrance, made with a low-current, small size beam, show a narrow band of the beam positions with high relative current losses (up to 1 · 10^-3) near Y axis. At higher currents, when the beam size is comparable with the entrance opening, the beam cannot be shifted from the high-loss region, and the total efficiency is determined by the beam part overlapped with the band. The best relative current loss in the symmetric configuration is 1.5 · 10^-5 at the beam current up to 0.5 A. To eliminate the effect, the magnets on one side of the collector were shifted along Z with respect to the second group. Arising asymmetry of the magnetic field results in a displacement of the high-loss band from the center and in decreasing of the relative losses down to (delta)I/I= 3 · 10^-6 at the beam current of up to I=0.6A [2]. When the collector was used in a 1.2 MeV beam recirculation experiment at the collector voltage of U_c=4 kV, the maximum beam current of 0.9 A and (delta)I/I= (5-20) · 10^-6 were demonstrated [3]

[en] Fermilab's 4.3 MeV electron cooler is based on an electrostatic accelerator, which generates a DC electron beam in an energy recovery mode. Effective cooling of the antiprotons in the Recycler requires that the beam remains stable for hours. While short beam interruptions do not deteriorate the performance of the Recycler ring, the beam may provoke full discharges in the accelerator, which significantly affect the duty factor of the machine as well as the reliability of various components. Although cooling of 8 GeV antiprotons has been successfully achieved, full discharges still occur in the current setup. The paper describes factors leading to full discharges and ways to prevent them

[en] The newly installed Recycler Electron Cooling system (REC) at Fermilab [1] will work at an electron energy of 4.34 MeV and a DC beam current of 0.5 A in an energy recovery scheme. As a part of the Electron cooling project, the efficiency of the collector for the REC was optimized at a dedicated test bench to the level of relative current losses of 5 · 10^-6. The paper discusses the test bench measurements for several distributions of a transverse magnetic field in the collector cavity

[en] Fermilab's Recycler ring was used as a storage ring for accumulation and subsequent manipulations of 8 GeV antiprotons destined for the Tevatron collider. To satisfy these missions, a unique electron cooling system was designed, developed and successfully implemented. The most important features that distinguish the Recycler cooler from other existing electron coolers are its relativistic energy, 4.3 MV combined with 0.1-0.5 A DC beam current, a weak continuous longitudinal magnetic field in the cooling section, 100 G, and lumped focusing elsewhere. With the termination of the Tevatron collider operation, so did the cooler. In this article, we summarize the experience of running this unique machine.

[en] The Fermilab Electron Cooling Project requires the operation of a 4.34 MeV electron beam in the same enclosure that houses the 120, 150 GeV Main Injector. Effective shielding of the magnetic fields from the ramped electrical buses and local static fields is necessary to maintain the high beam quality and recirculation efficiency required by the electron cooling system. This paper discusses the operational tolerances and the design of the beamline shielding, bus design, and bus shielding as well as experimental results from the prototype and final installation

[en] Antiprotons in Fermilab's Recycler ring are cooled by a 4.3 MeV, 0.1-0.5 A DC electron beam (as well as by a stochastic cooling system). The unique combination of the relativistic energy (γ = 9.49), an Ampere-range DC beam, and a relatively weak focusing makes the cooling efficiency particularly sensitive to ion neutralization. A capability to clear ions was recently implemented by way of interrupting the electron beam for 1-30 (micro)s with a repetition rate of up to 40 Hz. The cooling properties of the electron beam were analyzed with drag rate measurements and showed that accumulated ions significantly affect the beam optics. For a beam current of 0.3 A, the longitudinal cooling rate was increased by factor of ∼2 when ions were removed.

[en] A 0.1 A, 4.3 MeV DC electron beam is routinely used to cool 8 GeV antiprotons in Fermilab's Recycler storage ring [1]. The primary function of the electron cooler is to increase the longitudinal phase-space density of the antiprotons for storing and preparing high-density bunches for injection into the Tevatron. The longitudinal cooling rate is found to significantly depend on the transverse emittance of the antiproton beam. The paper presents the measured rates and compares them with calculations based on drag force data

[en] A powerful electron beam (4.3 MeV, 0.1 A DC) generated by an electrostatic accelerator has been used at Fermilab for three years to cool antiprotons in the Recycler ring. For electron cooling to be effective, the electron energy should not deviate from its optimum value by more than 500V. The main tool for studying the energy stability is the electron beam position in a high-dispersion area. The energy ripple (frequencies above 0.2 Hz) was found to be less than 150 eV rms; the main cause of the ripple is the fluctuations of the chain current. In addition, the energy can drift to up to several keV that is traced to two main sources. One of them is a drift of the charging current, and another is a temperature dependence of generating voltmeter readings. The paper describes the efforts to reach the required level of stability as well as the setup, diagnostics, results of measurements, and operational experience

[en] A 0.1-0.5 A, 4.3 MeV DC electron beam provides cooling of 8 GeV antiprotons in Fermilab's Recycler storage ring. The most detailed information about the cooling properties of the electron beam comes from drag rate measurements. We find that the measured drag rate can significantly differ from the cooling force experienced by a single antiproton because the area of effective cooling is significantly smaller than the physical size of the electron beam and is comparable with the size of the antiproton beam used as a probe. Modeling by the BETACOOL code supports the conclusion about a large radial gradient of transverse velocities in the presently used electron beam

[en] Electron cooling in the Fermilab Recycler ring is found to create correlation between longitudinal and transverse tails of the antiproton distribution. By separating the core of the beam from the tail and cooling the tail using 'gated' stochastic cooling while applying electron cooling on the entire beam, one may be able to significantly increase the overall cooling rate. In this paper, we describe the procedure and first experimental results