Production Of High Brightness Electron Beams Research Articles

Nanomechanical and microtribological properties of yttrium thin films for photocathode engineering

The authors study the nanomechanical and microtribological properties of yttrium (Y) thin films deposited by pulsed laser deposition on Cu polycrystalline substrates. Nanoindentation tests reveal that such films have a high hardness of H = 2.3 GPa and a reduced elastic modulus of 71.7 GPa with respect to the Cu substrates. The friction coefficient between a diamond tip and the Y film reaches a steady state value of μ ∼ 0.34, lower than that for the Cu (μ ∼ 0.38). Moreover, nanoscratch experiments show that Y films are more scratch-resistant than the Cu substrates, probably due to their greater hardness, higher elastic recovery, and lower friction coefficient. Their results confirm that the mechanical and tribological properties of the Y films are suitable for designing and fabricating scratch-resistant hybrid photocathodes and can reduce instabilities and unwanted discharges in the cavity of the radio-frequency gun. Furthermore, the low surface roughness and the low work function of the material are important characteristics for a photocathode based on the Y thin film for the production of high-brightness electron beams.

Effects of laser pulse heating of copper photocathodes on high-brightness electron beam production at blowout regime**Supported by National Natural Science Foundation of China (11375097)

Producing high-brightness and high-charge (>100 pC) electron bunches at blowout regime requires ultrashort laser pulses with high fluence. The effects of laser pulse heating of the copper photocathode are analyzed in this paper. The electron and lattice temperature is calculated using an improved two-temperature model, and an extended Dowell-Schmerge model is employed to calculate the thermal emittance and quantum efficiency. A time-dependent growth of the thermal emittance and the quantum efficiency is observed. For a fixed amount of charge, the projected thermal emittance increases with decreasing laser radius, and this effect should be taken into account in laser optimization at blowout regime. Moreover, laser damage threshold fluence is simulated, showing that the maximum local fluence should be less than 40 mJ/cm2 to prevent damage to the cathode.

UV pulse shaping for the photocathode RF gun

Recently, manipulation with the drive laser plays a significant role in high brightness electron beam production by the photocathode RF gun. The article takes efforts on the temporal shaping of the driving laser for the photocathode RF gun. Method based on pulse stacking by birefringent crystal of α-BBO serials was tried to directly shape ultraviolet laser pulse. Using four pieces of α-BBO crystals to separate an input UV pulse with appropriate duration into 16 sub-pulses can form a ps-spaced pulse train suitable for coherent THz production. The group delay dispersion induced by the crystals was also carefully considered. To avoid beam deterioration by long path propagation, imaging relay of the shaped pulse was applied.

TW Laser system for Thomson scattering X-ray light source at Tsinghua University

A TW (Tera Watt) laser system based on Ti:sapphire mainly for the Tsinghua Thomson scattering X-ray light source (TTX) is being built. Both UV (ultraviolet) laser pulse for driving the photocathode radio-frequency (RF) gun and the IR (infrared) laser pulse as the electron-beam-scattered-light are provided by the system. Efforts have also been made in laser pulse shaping and laser beam transport to optimize the high-brightness electron beam production by the photocathode RF gun.

Slice emittance measurements at the SLAC gun test facility

A goal of the Gun Test Facility (GTF) at SLAC is to investigate the production of high-brightness electron beams for the Linac Coherent Light Source (LCLS) X-ray FEL. High brightness in the RF photocathode gun occurs when the time-sliced emittance is nearly the same as the cathode thermal emittance and when the slices are all lined up, i.e., their Twiss parameters are nearly identical. In collaboration with the BNL Source Development Lab (SDL), we have begun a systematic study of the slice emittance at GTF. The technique involves giving the bunch a near linear energy chirp using the booster linac and dispersing it with a magnetic spectrometer. Combined with knowledge of the longitudinal phase space, this establishes the energy–time correlation on the spectrometer screen. The slice emittances are determined by varying the strengths of the quadrupoles in front of the spectrometer. Spectrometer images for a range of quadrupole settings are then binned into small energy/time windows and analysed for the slice emittance and Twiss parameters. Results for various gun parameters are presented.

Computer simulations of cathodeless, high-brightness electron-beam production by multiple laser beams in plasmas

The use of two crossed laser pulses in a plasma for the cathodeless production of high-current low-emittance electron beams [D. Umstadter, J. K. Kim, and E. Dodd, Phys. Rev. Lett. 76, 2073 (1996)] is examined with fully relativistic, two-and-a-half-dimensional particle-in-cell simulations. Estimates for the number of injected particles, their energy spread, and their emittance are given as functions of the amplitude and timing of the injection pulse relative to the drive pulse of the laser wake field accelerator. The physical mechanism of the trapping of particles is examined based on single particle phase space trajectories in the simulations and numerical calculations.

Production of high-brightness electron beams for the ELSA-FEL

The photo-injector built at Bruyères-le-Châtel for the production of high-brightness electron beams is running routinely. Electron bunches containing up to 20 nC charge have been extracted from a CsK 2Sb photocathode illuminated by a Nd:YAG laser and accelerated to about 1 MeV. Measurements of electron and laser temporal profiles are in progress in addition to measurements of macropulse averaged quantities.