Tamar Thesis CEBAF
The continuous electron beam accelerator facility (CEBAF) at Thomas Jefferson National Lab contains two anti-parallel 0.5 GeV linacs connected by five sets of recirculated arcs and a high quality 45 MeV electron beam injector with transverse emittence of less than mm-mrad and longitudinal emittence of less than keV-degrees. In oder to minimize the total accelerator circumference the accelerator structure was arranged in two separate linac segments. Each linac segment contains 25 crymodules, which itself are made of 8 superconducting radio-frequency cavities. Each crymodule is seperated by a vacuum equipment, beam diagnostics and quadrupole and dipole magnets. Quadrupole and dipole magnets are used for steering the beam. Electrons are accelerated by microwave fields generated in klystrons and propagated through wave guides to radio-frequency accelerating cavities.
Klystron is a powerful type of microwave amplifier in which a low energy microwave signal breaks a continuous electron beam into so called bunches. After that the beam passes through the resonant waveguide, where it induces a high energy microwave signal. The transmitted microwave radiation is transported to a superconducting radio-frequency(RF) cavity where it accelerates electrons.
The electromotive force(EMF) induced in the RF cavity is roughly parallel to the beam axis and decaying to zero radially at the walls. The EMF induces charge on the interior surfaces of the cavities such that the electrons moving through the cavity see a positive charge in front of them and accelerate towards that charge.
After the beam reaches the desired energy the three beams are separated at the switch yard and send to the A, B and C Halls with the separation between the bunches of 2.04 ns.
The CEBAF accelerator parameters
| Quantity | Value |
|---|---|
| Electron beam energy E (GeV) | |
| Electron beam current () | 100 |
| Energy spread () | |
| Electron beam polarization | 40-80% |
| Simultaneous beams | 3 |
| Number of linac segments | 2 |
| Number of passes | 4 |
| Maximum energy gain per pass (GeV) | 1.5 |
The polarized electron beams were achieved since 1998 at JLAB using polarized gun. The polarized electrons are produced by creating bandgap photoemission from a strained GaAs cathode. when the cathode is exposed by a circularly polarized laser light, the polarized electrons move from the valence region to the conduction band. In order to free the electrons from the conduction band the surface of the material (GaAs) is coated with a single layer of cesium and fluorine. Electrons in the conduction band are bound to the surface of the material by 4 eV. The sign of the electron polarization is flipped with a frequency of 1 Hz by reversing the laser polarization. The electron beam polarization for each hall can be changed with a Wien Filter, it can rotate electron spin without changing its momentum.
The beam polarization is monitored using a 5 MeV Mott polarimeter and a Moller polarimeter at the injector and in Hall B respectively. The average electron beam polarization achieved during the eg1b experiment was around 70%. The integrated electron beam current is measured using a Farady cup, which is a lead block and placed downstream of the CLAS target. The position of the beam is monitored with a three harp beam position monitors. The profile of the beam is monitored with a harp scan, a device with thin iron wires moved around the beam.
Notes
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