Next generation high brightness electron beams from ultrahigh field cryogenic rf photocathode sources

Abstract

Recent studies of the performance of radio-frequency (rf) copper cavities operated at cryogenic temperatures have shown a dramatic increase in the maximum achievable surface electric field. We propose to exploit this development to enable a new generation of photoinjectors operated at cryogenic temperatures that may attain, through enhancement of the launch field at the photocathode, a significant increase in fivedimensional electron beam brightness. We present detailed studies of the beam dynamics associated with such a system, by examining an S-band photoinjector operated at 250 MV=m peak electric field that reaches normalized emittances in the 40 nm-rad range at charges (100-200 pC) suitable for use in a hard x-ray free-electron laser (XFEL) scenario based on the LCLS. In this case, we show by start-to-end simulations that the properties of this source may give rise to high efficiency operation of an XFEL, and permit extension of the photon energy reach by an order of magnitude, to over 80 keV. The brightness needed for such XFELs is achieved through low source emittances in tandem with high current after compression. In the XFEL examples analyzed, the emittances during final compression are preserved using microbunching techniques. Extreme low emittance scenarios obtained at pC charge, appropriate for significantly extending temporal resolution limits of ultrafast electron diffraction and microscopy experiments, are also reviewed. While the increase in brightness in a cryogenic photoinjector is mainly due to the augmentation of the emission current density via field enhancement, further possible increases in performance arising from lowering the intrinsic cathode emittance in cryogenic operation are also analyzed. Issues in experimental implementation, including cavity optimization for lowering cryogenic thermal dissipation, external coupling, and cryocooler system, are discussed. We identify future directions in ultrahigh field cryogenic photoinjectors, including scaling to higher frequency, use of novel rf structures, and enabling of an extremely compact hard x-ray FEL.

Towards an ultra-compact x-ray free-electron laser (Conference Presentation)

“…As discussed in Ref. [20], the fivedimensional brightness is expected to scale as B 5D ∝ E n i , where E i is the amplitude of the initial launch field, and the exponent n is between 1.5 and 2, depending on the beam shape. The six-dimensional brightness, including the effect of space-charge on slice energy spread scales similarly [37,38], as B 6D ∝ E 2 i .…”

“…The combination of resulting lower dissipation due to diminished surface resistivity with increased material yield strength and mitigation of thermal expansion are the physical effects underpinning this remarkable advance. Applying increased fields in the photoinjector should have profound implications for beam brightness, which stands to be increased 50-fold over the original LCLS design [19,20,21], and similarly advance the recent state-of-the-art [22]. We discuss the expected performance of such a high field photoinjector, operated at E 0 = 240 MV/m below.…”

“…Small improvements in the injector 6D brightness performance are needed, and these have already been obtained in simulation [20]. A path to the undulator design, utilizing the comb Halbach approach, is also outlined above, but has yet to be realized.…”

Upgrade possibilities for continuous wave rf electron guns based on room-temperature very high frequency technology

“…At cryogenic temperature, both the electric conductivity and thermal conductivity of the cavity copper walls increase significantly, which alleviates the thermal loading and improves the cooling efficiency. Additionally, recent experimental results showed an increase of the copper electric rigidity at low temperature significantly reducing the rf breakdown rate [19].…”

High-gradient low- β accelerating structure using the first negative spatial harmonic of the fundamental mode

“…The direct scaling of BTW cells design from β ¼ 0.38 to β ¼ 0.3 beam velocities leads to intolerable peak fields (>220 MV=m), well above the demonstrated levels of 192 MV=m for a single structure [15]. For multicell rf guns the achieved peak electric field limits are lower: ∼160 MV=m [16]. This happens due to the very close nose arrangement, or to dramatically lower power efficiencies if the noses around the aperture are removed, which lowers the shunt impedance.…”