Abstract
The internal structure of electron bunches generated in an injector line with a dc photoelectron gun is investigated. Experiments were conducted on the ALICE (accelerators and lasers in combined experiments) energy recovery linac at Daresbury Laboratory. At a relatively low dc gun voltage of 230 kV, the bunch normally consisted of two beamlets with different electron energies, as well as transverse and longitudinal characteristics. The beamlets are formed at the head and the tail of the bunch. At a higher gun voltage of 325 kV, the beam substructure is much less pronounced and could be observed only at nonoptimal injector settings. Experiments and computer simulations demonstrated that the bunch structure develops during the initial beam acceleration in the superconducting rf booster cavity and can be alleviated either by increasing the gun voltage to the highest possible level or by controlling the beam acceleration from the gun voltage in the first accelerating structure.
Highlights
Short, picosecond-range, electron bunches generated by photoinjector-based accelerators are not always of the highest quality, as might be expected
This should be especially relevant to injectors with dc photoelectron guns that operate at voltages below 500 kV, and this is the topic of this investigation
Experimental studies of the structure of electron bunches were conducted on the ALICE energy recovery linac [4,5,6], where the beam is provided by a dc high-voltage (HV) photoelectron gun
Summary
Picosecond-range, electron bunches generated by photoinjector-based accelerators are not always of the highest quality, as might be expected. Physical mechanisms governing the beam formation in the injector are among various factors that may cause electron beams to become non-Gaussian This should be especially relevant to injectors with dc photoelectron guns that operate at voltages below 500 kV, and this is the topic of this investigation. Experimental studies of the structure of electron bunches were conducted on the ALICE (accelerators and lasers in combined experiments) energy recovery linac [4,5,6], where the beam is provided by a dc high-voltage (HV) photoelectron gun. Some transverse beam structure was evident at the exit from the booster consisting of a tightly focused central part of the beam, the “core,” and a larger “halo” [Fig. 2(a)]. We present the results of investigations into the causes of this beam structure, at the gun voltage of both 230 and 325 kV, concentrating on the observation of two distinct beamlets in the injector (see Fig. 2)
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