Design studies for the LCLS 120 Hz RF gun
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Abstract:
A preliminary design studies were carried out at Brookhaven National Laboratory for a photocathode RF gun injection system for LCLS 120 Hz operation. The starting point for the design is 50 Hz BNL Gun IV developed by a BNL/KEK/SHI collaboration. The basic parameters of the 120 Hz gun is discussed in this report. The complete photocathode RF gun injection system is described for a 120 Hz operation. The injector system includes photocathode RF gun, emittance compensation solenoid magnet, laser system and laser beam delivery system, and electron beam diagnostics. The basic design parameters, mechanical modification and the performance will be presented in this report.Keywords:
Photocathode
Electron gun
Solenoid
Photocathode
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A recently developed model of emittance & brightness of photocathode based on "moments" of the electron emission distribution is extended here to non-zero temperatures and fields. Temperature impacts scattering and affects quantum efficiency. Fields affect emission probability and are important in the presence of low-work function coatings characteristic of cesiated dispenser photocathodes under development. Extensions of theoretical models are given, followed by analysis of their comparison with numerical simulations of the intrinsic emittance and brightness of a photocathode. The methodology is designed to facilitate development of photoemission models into comprehensive particle-in-cell (PIC) to address variation in surface coverage and topology.
Photocathode
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Photocathode RF gun has been widely used for generation of high-brightness electron beams for many different applications. We found that the drive laser distributions in such RF guns play important roles in minimizing the electron beam emittance. Characterizing the laser distributions with measurable parameters and optimizing beam emittance versus the laser distribution parameters in both spatial and temporal directions are highly desired for high-brightness electron beam operation. In this paper, we report systematic measurements and simulations of emittance dependence on the measurable parameters represented for spatial and temporal laser distributions at the photocathode RF gun systems of Linac Coherent Light Source. The tolerable parameter ranges for photocathode drive laser distributions in both directions are presented for ultra-low emittance beam operations.
Photocathode
Electron gun
Beam emittance
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A 1.3 GHz photocathode RF gun is under study in peking University. The photocathode RF gun, composed of gun cavity, photocathode and Laser system, is expected to deliver a electron beam with a normalized transverse rms emittance of 1π mm·mrad at a charge of 1nC. Its performance has been studied by the space-charge tracking code PARMELA. The influences of the initialized electron beam were also studied. The emittance compensation technique is employed. After optimization of the parameters was achieved, the normalized transverse emittance is 1.39 πmm·mrad at the gun exit.
Photocathode
Electron gun
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A stable and high brightness electron source using a photocathode RF gun was installed in our laboratory to generate ultra-short X-ray pulses by Compton scattering. Commissioning tests were performed and high quality 12MeV electron bunches were obtained.
Photocathode
Electron gun
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A theoretical expression for the intrinsic emittance of a photocathode is developed based on a method of evaluating moments of the emission distribution function. The method is first used to reevaluate the well-known rms emittance of a thermionic source, and then, using analogous approximations but with an updated theoretical model of photoemission, an equation for the intrinsic emittance and brightness of a photocathode of comparable simplicity to the thermionic case is obtained in the limit of weak field and low temperature.
Photocathode
Thermionic emission
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Photocathode
Electron gun
Beam emittance
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The introduction of the surface plasmon polarizations makes the emittance of the photocathode complicated. In this paper, the emittance of plasmon-enhanced photocathode is analyzed. It is first demonstrated that the plasmonic near field can increase the emittance of the plasmon-enhanced photocathode. A simulation method has been used to estimate the emittance caused by plasmonic near field, and the suppression method also has been discussed, both of which are significant for the design of high performance plasmon-enhanced photocathode.
Photocathode
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We measure the tradeoff between the quantum efficiency and intrinsic emittance from a NaKSb photocathode at three increasing wavelengths (635, 650, and 690 nm) at or below the energy of the bandgap plus the electron affinity, hν≤Eg+Ea. These measurements were performed using a high voltage dc gun for varied photocathode surface fields of 1.4−4.4 MV/m. Measurements of intrinsic emittance are performed using two different methods and were found to agree. At the longest wavelength available, 690 nm, the intrinsic emittance was 0.26 μm/mm-rms with a quantum efficiency of ∼10−4. The suitability of NaKSb emitting at threshold for various low emittance applications is discussed.
Photocathode
Quantum Efficiency
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Transparent-grid scheme for generating cathode-emittance-dominated beams in a gridded thermionic gun
Abstract A transparent-grid scheme is proposed for generating cathode-emittance-dominated beams in a gridded thermionic gun. In the proposed scheme, the grid emittance arising from the potential difference between the grid and grid-mesh center is suppressed by tuning the grid voltage to the gridless gun potential. We developed a 50 kV gridded thermionic gun that is followed by a 238 MHz cavity for 500 kV acceleration. The measured projected emittance as a function of grid voltage shows a clear minimum approaching the cathode emittance and agrees well with a numerical simulation and a theoretical grid emittance taking into account the longitudinal space-charge effect. The measured minimum emittance for a core part which represents 60% of a 1-nC electron bunch is 1.5 μ m · rad making the transparent-gridded thermionic gun a promising electron source for high brightness beam application such as a soft X-ray free-electron laser.
Electron gun
Thermionic emission
Beam emittance
Hot cathode
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