Electron beam collector with a low back flow

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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 ... continued below

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86 Kilobytes pages

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Shemyakin, A. March 17, 2004.

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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{sup -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 {center_dot} 10{sup -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 {center_dot} 10{sup -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 {center_dot} 10{sup -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{sub c}=4 kV, the maximum beam current of 0.9 A and {delta}I/I= (5-20) {center_dot} 10{sup -6} were demonstrated [3].

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86 Kilobytes pages

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  • 3rd IEEE International Vacuum Electronics Conference (IVEC 2002), Monterey, CA (US), 04/23/2002--04/25/2002

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  • Report No.: FERMILAB-Conf-02/416-AD
  • Grant Number: AC02-76CH03000
  • Office of Scientific & Technical Information Report Number: 822094
  • Archival Resource Key: ark:/67531/metadc786238

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  • March 17, 2004

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  • Dec. 3, 2015, 9:30 a.m.

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  • Aug. 5, 2016, 2:58 p.m.

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Shemyakin, A. Electron beam collector with a low back flow, article, March 17, 2004; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc786238/: accessed September 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.