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

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 five-dimensional electron beam brightness. We present detailed studies of the beam dynamics associated with such a system, by examining an S-band photoinjector operated at $250\mathrm{MV}/\mathrm{m}$ 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.

Figure 1

Cryogenic, very high field S-band photoinjector, with 1.45 cell Cu gun structure (center) externally coupled to waveguide through a mode-launcher scheme (far right). Also shown: cryostat envelope and liquid neon-based cryocooler (far left), mounting equipment, emittance compensation solenoid (surrounding rf structure).

Figure 2

Breakdown probability in per pulse meter of structure length as a function of peak surface electric field in single cell X-band accelerating structure tests. The introduction of a harder alloy (CuAg, two different samples, indicated as #1 and #2) improves the breakdown as predicted; the effect of operation at 45 deg K is more dramatic, permitting surface electric fields to a threshold at $500\mathrm{MV}/\mathrm{m}$ (from Ref. [31]).

Figure 3

Approximation of ellipsoidal distribution formed after $250\mathrm{MV}/\mathrm{m}$ peak field rf photoinjector (downstream of photocathode $z=1.5\mathrm{m}$) in 125 pC blowout regime case.

Figure 4

(left) Longitudinal phase space for 125 pC beam distribution shown in Fig. 3, after $250\mathrm{MV}/\mathrm{m}$ rf photoinjector. (right) Associated transverse rms beam envelope and normalized emittance evolution.

Figure 5

Beam emittance evolution in low-charge, cigar-beam case, showing an emittance compensated down to ${ϵ}_n=0.005\mathrm{mm}\text{−}\mathrm{mrad}$ level with 1.3 A peak current.

Figure 6

(left) Spatial beam distribution at emittance compensation minimum, in low-charge cigar-beam case, at $z=2.2\mathrm{m}$, where the uniform distribution launched at photocathode is nearly recreated near the emittance compensation waist. (right) Longitudinal emittance for this distribution.

Figure 7

Evolution of transverse beam size and normalized emittance in S-band photoinjector, with 1.45 cell rf gun operated at $250\mathrm{MV}/\mathrm{m}$ followed by two 3-m traveling wave linac sections, using cigarlike beam with 200 pC charge and 10 ps FWHM bunch length.

Figure 8

(a) Horizontal phase space for at end of injector, 100 pC start-to-end simulations; (b) longitudinal phase space for same beam before laser heater.

Figure 9

(a) Current profile after microbunching section in ESASE scheme for LCLS parameters. (b) FEL energy for full beam having microbunch current as in (a), with central wavelength $1.41\AA$ (8.8 keV photons).

Figure 10

Simulation of FEL energy evolution with beam current profile and transverse phase space as in Fig. 9, and ${\lambda }_u=9\mathrm{mm}$, undulator, and 14.1 GeV beam energy, lasing with central wavelength $0.155\AA$ (80 keV photons).

Figure 11

Photoemission temperature $k_B{T}_c$ (left) and quantum efficiency (QE) (right) as a function of photon energy, for atomically clean Cu [55] according to the relations given in [57, 58], using an applied field of $250\mathrm{MV}/\mathrm{m}$. The scale of the quantum efficiency curve is such that at zero field, 270 nm photons produce a quantum efficiency of ${10}^{-5}$, near what was attained in [90]. The energy at 300 K is shown by the dotted red line.

Figure 12

Comparison of the anomalous skin effect surface resistance in $\mathrm{RRR}=400$ Cu at 2.856, 5.712, and 11.424 GHz.

Figure 13

(left) Cutaway model for section of S-band Cu pillbox test cavity. The faceplate is brazed to the bottom, while on the left a feature is included to break dipole mode degeneracies. Through holes host two antennas for $S_{11}$ and $S_{21}$ tests. (right) Calculation of temperature dependence in the ${\mathrm{TM}}_{010}$ accelerating mode using the thermal coefficient of expansion for Cu.

Figure 14

(left) Radio-frequency surface resistance of both accelerating cavities, SLAC (blue) and UCLA (green). This is compared to a theoretical rf surface resistance of copper with IACS 95% and $\mathrm{RRR}=400$. (right) Quality factor in SLAC test cavity.

Figure 15

Peak photocathode electric field as a function of $\beta$ in 1.45 cell rf gun, for four different rf pulse lengths.

Figure 16

(left) Three-dimensional outside view rendering of rf photoinjector and external power coupling, showing on right symmetrized waveguide feeding a mode-launcher style coupler. Cylindrical waveguide then axially couples power into 1.45 cell gun structure. (right) Surface magnetic fields in coupler and gun, as simulated in HFSS, with color map extending from 0 (blue) to $5\times {10}^5\mathrm{A}/\mathrm{m}$ (red).

Figure 17

(left) Current (normalized to uniform launch current $I_i$) as a function of final time $t_f$ within the pulse of initial length $\tau$, in the linear limit of emission, ${\alpha }_{sc}=0.1$; (right) current profile strong bunch lengthening limit, showing near suppression of emission with ${\alpha }_{sc}=0.95$.