Demonstration of sub-$\mathrm{GV}/\mathrm{m}$ accelerating field in a photoemission electron gun powered by nanosecond $X$-band radio-frequency pulses

Radio-frequency (rf) photoemission electron guns operating at higher accelerating gradients offer a pathway to produce brighter electron bunches. Brighter beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an rf photoemission electron source supporting an accelerating field close to $400\mathrm{MV}/\mathrm{m}$ at the photocathode surface without major rf breakdown or significant dark current over a 3-week experimental run. This unprecedented gradient was achieved by operating the gun in a new regime with short rf pulses, $\sim 9\mathrm{ns}$ $X$-band (11.7 GHz). The demonstrated paradigm provides a viable path to form relativistic electron beams with unprecedented brightness.

Figure 1

Schematic of the XRF gun (a) and simulated peak field on photocathode (b) for a short (${\tau }_p=3$-ns, solid trace) and long (${\tau }_p=100$-ns, dash trace) flattop 300-MW peak power rf pulse (the inset details the forward power $P_{\mathrm{FWD}}$ rf-pulse profile with 3-ns rising and falling edges). Overview of the experimental setup (c): the upper (magenta) path corresponds to the relevant section of the AWA main accelerator and the lower (green) path is the XRF gun beamline. The pink path shows the UV laser path. The labels $\mathrm{X}i$, $\mathrm{M}i$, and $\mathrm{L}i$, respectively, refer to $\mathrm{Ce}:\mathrm{YAG}$ scintillating screens with CCD cameras, optical mirrors, and lenses.

Figure 2

rf conditioning history of the XRF gun (a) where the photocathode peak field $E_0$ was gradually increased (blue circle) and the ratio of the peak value of the measured to simulated reflected rf signals, $R$, recorded (orange square). The upper plots (b)–(d) compared the measured (dash red trace) and simulated (green solid trace) rectified rf-reflection signals at different stages of the conditioning history indicated with the dash vertical lines.

Figure 3

Measured bunch charge as a function of laser-launch phase (circles with shaded area giving the uncertainty) compared with astra simulations with (solid line, Schottky-strength parameter set to $\xi =2\times {10}^{-3}$) and without (dash line, $\xi =0$) including the Schottky effect. The inset (b) gives the transverse beam distribution on a $10\times 10{\mathrm{mm}}^2$ area at X1 for ${\phi }_0\simeq 95°$ with associated projections (solid lines).

Figure 4

Simulated kinetic-energy isoclines as a function of rf-gun operating conditions $K(E_0,{\phi }_0)$ and retrieved operating points (“$+$” symbols) for four measurements. The labeled isoclines give the kinetic energy in MeV units. The shaded areas represent the uncertainty on the measured ${\phi }_0$ and inferred $E_0$ values (reported in the legend in $\mathrm{MV}/\mathrm{m}$ units).

Figure 5

Waterfall plot of measured energy spectra density $\lambda$ over 853 shots (a) with associated drive-beam charge transmitted through the PETS (b), and corresponding bunch charge photoemitted from the XRF gun (c). Plots (d)–(f) are the associated histograms. The dashed trace in (a) is a spline interpolation of the slow drift which is superimposed as a dashed curve in (b) after applying an affine transformation.