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Register a new userPhotoinjector design for the LCLS
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The design of the Linac Coherent Light Source assumes that a low-emittance, 1-nC, 10-ps beam will be available for injection into the 15-GeV linac. The proposed rf photocathode injector that will provide a 150-MeV beam with rms normalized emittances of 1 mm in both the transverse and longitudinal dimensions is based on a 1.6-cell S-band rf gun that is equipped with an emittance compensating solenoid. The booster accelerator is positioned at the beam waist coinciding with the first emittance maximum and is provided with an accelerating gradient of ~25 MeV/m, i.e., the new working point. The uv pulses required for cathode excitation will be generated by tripling the output of a Ti:sapphire laser system consisting of a highly stable cw mode-locked oscillator and two bow-tie amplifiers pumped by a pair of Q-switched Nd:YAG lasers. The large bandwidth of the Ti:sapphire system accommodates the desired temporal pulse shaping. Details of the design and the supporting simulations are presented.
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We report the results of a recent beam dynamics study, motivated by the need to redesign the LCLS photoinjector, that lead to the discovery of a new effective working point for a split RF photoinjector. We consider the emittance compensation regime of a space charge beam: by increasing the solenoid strength, the emittance evolution shows a double minimum behavior in the drifting region. If the booster is located where the relative emittance maximum and the envelope waist occur, the second emittance minimum can be shifted to the booster exit and frozen at a very low level (0.3 mm-mrad for a 1 nC flat top bunch), to the extent that the invariant envelope matching conditions are satisfied. Standing Wave Structures or alternatively Traveling Wave Structures embedded in a Long Solenoid are both candidates as booster linac. A careful measurement of the emittance evolution as a function of position in the drifting region is necessary to verify the computation and to determine experimentally the proper position of the booster cavities. The new design study and supporting experimental program under way at the SLAC Gun Test Facility are discussed.
Polarized electron beams are now in routine use in particle accelerators for nuclear and high energy physics experiments. These beams are presently produced by dc-biased photoelectron sources combined with rf chopping and bunching systems with inherently high transverse emittances. Low emittances can be produced with an rf gun, but the vacuum environment has until now been considered too harsh to support a negative electron affinity GaAs photocathode. We propose to significantly improve the vacuum conditions by adapting a PWT rf photoinjector to achieve reasonable cathode emission rates and lifetimes. This adaptation can also be combined with special optics that will result in a flat beam with a normalized rms emittance in the narrow dimension that may be as low as 10-8 m.
The Linac Coherent Light Source (LCLS) project at SLAC uses a dense 15 GeV electron beam passing through a long undulator to generate extremely bright x-rays at 1.5 angstroms. The project requires electron bunches with a nominal peak current of 3.5kA and bunch lengths of 0.020mm (70fs). The bunch compression techniques used to achieve the high brightness impose challenging tolerances on the accelerator RF phase and amplitude. The results of measurements on the existing SLAC linac RF phase and amplitude stability are summarised and improvements needed to meet the LCLS tolerances are discussed.
LAL is now equiped with its own platform for photoinjectors tests and Research and Developement, named PHIL (PHotoInjectors at LAL). This facility has two main purposes: push the limits of the photoinjectors performances working on both the design and the associated technology and provide a low energy (MeV) short pulses (ps) electron beam for the interested users. Another very important goal of this machine will be to provide an opportunity to form accelerator physics students, working in a high technology environment. To achieve this goal a test line was realised equipped with an RF source, magnets and beam diagnostics. In this article we will desrcibe the PHIL beamline and its characteristics together with the description of the first two photoinjector realised in LAL and tested: the ALPHAX and the PHIN RF Guns.
Photoinjectors are widely used for linear accelerators as electron sources to generate high-brightness electron beam. Drive laser, which determines the timing structure and quality of the electron beam, is a crucial device of photoinjector. A new drive laser system has been designed and constructed for the upgraded 3.5-cell DC-SRF photoinjector at Peking University. The drive laser system consists of a 1064 nm laser oscillator, a four- stage amplifier, the second and fourth harmonic generators, the optical system to transfer the UV pulses to the photocathode, and the synchronization system. The drive laser system has been successfully applied in the stable operation of DC-SRF photoinjector and its performance meets the requirements. 266 nm laser with an average power close to 1W can be delivered to illuminate the Cs2Te photocathode and the instability is less than 5% for long time operation. The design consideration for improving the UV laser quality, a detailed description of laser system, and its performance are presented in this paper.
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