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Register a new userRapid thermal emittance and quantum efficiency mapping of a cesium telluride cathode in an rf photoinjector using multiple laser beamlets
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Thermal emittance and quantum efficiency (QE) are key figures of merit of photocathodes, and their uniformity is critical to high-performance photoinjectors. Several QE mapping technologies have been successfully developed; however, there is still a dearth of information on thermal emittance maps. This is because of the extremely time-consuming procedure to gather measurements by scanning a small beam across the cathode with fine steps. To simplify the mapping procedure, and to reduce the time required to take measurements, we propose a new method that requires only a single scan of the solenoid current to simultaneously obtain thermal emittance and QE distribution by using a pattern beam with multiple beamlets. In this paper, its feasibility has been confirmed by both beam dynamics simulation and theoretical analysis. The method has been successfully demonstrated in a proof-of-principle experiment using an L-band radiofrequency photoinjector with a cesium telluride cathode. In the experiment, seven beamlets were generated from a microlens array system and their corresponding thermal emittance and QE varied from 0.93 to 1.14 $mu$m/mm and from 4.6 to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.
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Proposed fourth generation light sources using SASE FELs to generate short pulse, coherent, X-rays require demonstration of high brightness electron sources. The Gun Test Facility (GTF) at SLAC was built to test high brightness sources for the proposed Linac Coherent Light Source at SLAC. The transverse emittance measurements are made at nearly 30 MeV by measuring the spot size on a YAG screen using the quadrupole scan technique. The emittance was measured to vary from 1 to 3.5 mm-mrad as the charge is increased from 50 to 350 pC using a laser pulse width of 2 ps FWHM. The measurements are in good agreement with simulation results using the LANL version of PARMELA.
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.
Recent work at Fermilab in collaboration with the Advanced Photon Source and members of other national labs, designed an experiment to study the relationship between the RF repetition rate and the average current per RF pulse. While existing models anticipate a direct relationship between these two parameters we observed an inverse relationship. We believe this is a result of damage to the barium coating on the cathode surface caused by a change in back-bombardment power that is unaccounted for in the existing theories. These observations shed new light on the challenges and fundamental limitations associated with scaling an un-gated thermionic cathode RF gun to high average current machines.
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.
Ionization injection triggered by short wavelength laser pulses inside a nonlinear wakefield driven by a longer wavelength laser is examined via multi-dimensional particle-in-cell simulations. We find that very bright electron beams can be generated through this two-color scheme in either collinear propagating or transverse colliding geometry. For a fixed laser intensity $I$, lasers with longer/shorter wavelength $lambda$ have larger/smaller ponderomotive potential ($propto I lambda^2$). The two color scheme utilizes this property to separate the injection process from the wakefield excitation process. Very strong wakes can be generated at relatively low laser intensities by using a longer wavelength laser driver (e.g. a $10 micrometer$ CO$_2$ laser) due to its very large ponderomotive potential. On the other hand, short wavelength laser can produce electrons with very small residual momenta ($p_perpsim a_0sim sqrt{I}lambda$) inside the wake, leading to electron beams with very small normalized emittances (tens of $ anometer$). Using particle-in-cell simulations we show that a $sim10 femtosecond$ electron beam with $sim4 picocoulomb$ of charge and a normalized emittance of $sim 50 anometer$ can be generated by combining a 10 $micrometer $ driving laser with a 400 $ anometer$ injection laser, which is an improvement of more than one order of magnitude compared to the typical results obtained when a single wavelength laser used for both the wake formation and ionization injection.
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