Injection, Positron and Electron Beam Generation
In order to produce the desired luminosity it is required to generate a large number of positrons, namely N{sub +} = 2 {divided_by} 5 x 10{sup 10} positrons per bunch at a repetition rate of 3 + 12 KHz. These specifications are very demanding and well above the present state of the art. e{sup +}/e{sup -} conversion in a high Z target is performed with an overall efficiency, {eta}, of at most 7%/GeV, including e{sup +} capture and acceleration efficiencies. The value refers to a situation where all positrons in an energy bite, {Delta}{Epsilon}, of about 15 MeV are captured. About 15 MeV is therefore the positron beam absolute energy spread. Higher efficiencies can be obtained at the expense of increasing {Delta}{Epsilon}, but the resulting beam absolute energy spread would not be accepted by the following stages of the machine. In order to limit the charge in the primary electron bunch so as to avoid too strong an interaction with the accelerating structure, and taking into account the fact that the conversion efficiency is proportional to the energy of the electrons, it is advisable to convert electrons into positrons at the highest possible energy. Another important design parameter that determines the minimum energy to which positrons have to be accelerated before being injected into the machine is the maximum relative energy spread tolerated by the first recirculation arc; it has been assumed throughout that it is at most 4%. Given the absolute energy spread of the positrons at production and folding-in the spread induced by the interaction of the bunch with the accelerating structure, the minimum energy of the positron beam entering the first recirculation arc cmes out to be about 450 MeV. Since before entering the arc the beam passes once through one side of the machine accelerating structure, thereby gaining 350 Me, it has to be accelerated by the injection linac following the converter to at least 100 MeV. A one-sided race track acceleration scheme could be considered. Another fundamental design specification that has to be dealt with at injection level concerns the possibility of producing electron beams with an emittance to match that specified for the beams in collision (namely a normalized emittance {epsilon}{sub n} {<=} 2 x 10{sup -6} mrad) in order to save the complication and expense of passing through a damping ring. For positrons this is not possible since they are of course created with a very large emittance and a damping ring is mandatory. Values of the normalized emittance as small as the specified can only be obtained by careful design and require an improvement over what is commonly achieved by a factor {>=} 3.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE Office of Energy Research (ER) (US)
- DOE Contract Number:
- AC05-84ER40150
- OSTI ID:
- 829653
- Report Number(s):
- CEBAF-PR-87-38; DOE/ER/40150-2880; TRN: US200430%%47
- Resource Relation:
- Conference: Workshop On Heavy Quarks Factory And Nuclear Physics Facility With Superconducting Linacs, Courmayeur (IT), 12/14/1987--12/18/1987; Other Information: PBD: 1 Dec 1987
- Country of Publication:
- United States
- Language:
- English
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