End-point energy measurements of field emission current in a continuous-wave normal-conducting rf injector

The LANL/AES normal-conducting radio-frequency injector has been tested at cw cathode gradients up to $10\mathrm{MV}/\mathrm{m}$. Field-emission electrons from a roughened copper cathode are accelerated to beam energy as high as 2.5 MeV and impinge on a stainless steel target. The energies of the resulting bremsstrahlung photons are measured at varying levels of injector cavity rf power corresponding to different accelerating gradients. At low cavity power, the bremsstrahlung spectra exhibit well-defined end-point energies at the positions where the number of single-photon events decreases to one ($\mathrm{S}/\mathrm{N}\mathrm{\text{ratio}}=1$). Increasing the cavity power raises the probability of two-photon events in which two photons simultaneously arrive at the detector and register counts at twice the photon energy. The end-point energies at high cavity power are recorded at positions where the single-photon events transition to two-photon events. The measured end-point energies using this method are in excellent agreement with PARMELA calculations based on the cavity gradients deduced from the cavity rf power measurements.

Figure 1

Diagram of the LANL/AES normal-conducting rf injector.

Figure 2

Plots of calculated normalized rms emittance versus solenoid field at $9\mathrm{MV}/\mathrm{m}$ (blue) and $10\mathrm{MV}/\mathrm{m}$ (red) cathode gradients.

Figure 3

PARMELA-predicted temporal profile of the field-emission current pulse.

Figure 4

Dependence of electron energy on rf phase of the injector cavity.

Figure 5

PARMELA-predicted energy spectrum of field-emitted electrons at cavity pickup loop power of 702 mW.

Figure 6

Number of photons versus photon energy for experiment runs at 250 mW rf power. The mean energy of all data points at $N_{\mathrm{ph}}=1$ is 1449 keV.

Figure 7

Plot of photon counts versus energy at the highest power level showing the transition from single-photon events to two-photon events at 2.5 MeV.

Figure 8

End-point photon energy versus cavity power. The vertical bars are measured data $\pm 2$ standard deviations, the red circles are PARMELA prediction, and the line is analytic calculation.