Laser-Photofield Emission from Needle Cathodes for Low-Emittance Electron Beams

Illumination of a ZrC needle with short laser pulses (16 ps, 266 nm) while high voltage pulses ($-60\mathrm{kV}$, 2 ns, 30 Hz) are applied, produces photo-field emitted electron bunches. The electric field is high and varies rapidly over the needle surface so that quantum efficiency (QE) near the apex can be much higher than for a flat photocathode due to the Schottky effect. Up to 150 pC (2.9 A peak current) have been extracted by photo-field emission from a ZrC needle. The effective emitting area has an estimated radius below $50\mu \mathrm{m}$ leading to a theoretical intrinsic emittance below 0.05 mm mrad.

Figure 1

Experimental Setup used for photo-field emission tests; Inset: Micrograph of a ZrC needle.

Figure 2

Measured current pulses when high voltage pulses are applied to a ZrC needle with and without laser illumination (266 nm; ${\sigma }_{t,\mathrm{laser}}=16\mathrm{ps}$; ${\sigma }_{r,\mathrm{laser}}=85\mu \mathrm{m}$; $E_{266\mathrm{nm}}=6\mu \mathrm{J}$; 30 Hz, $V_{\mathrm{Tip}}=-45\mathrm{kV}$).

Figure 3

Current waveforms collected for different $V_{\mathrm{Tip}}$, with and without laser illumination; Laser: 266 nm, $E_{266\mathrm{nm}}=6\mu \mathrm{J}$, ${\sigma }_{z,\mathrm{laser}}=16\mathrm{ps}$, ${\sigma }_{r,\mathrm{laser}}=85\mu \mathrm{m}$.

Figure 4

Emitted charge (average over 20 acquisitions) versus laser spot radial position ${\rho }_{\mathrm{laser}}$ (the zero corresponds to the needle axis), $E_{266\mathrm{nm}}=3\mu \mathrm{J}$, ${\sigma }_{z,\mathrm{laser}}=16\mathrm{ps}$ (rms), ${\sigma }_{r,\mathrm{laser}}=50\mu \mathrm{m}$ (rms), $V_{\mathrm{Tip}}=-32\mathrm{kV}$. Theoretical fit corresponds to Eq. (1) with $\mathrm{QE}(\rho )={\mathrm{QE}}_0\exp (\rho /{\rho }_0)$.

Figure 5

Collected current pulses (single acquisitions) for various delay $\Delta t$ between the instant of laser illumination and the voltage pulse maximum ($V_{\mathrm{Tip}}=-39\mathrm{kV}$, $E_{266\mathrm{nm}}=6\mu \mathrm{J}$, ${\sigma }_{r,\mathrm{laser}}=85\mu \mathrm{m}$, 30 Hz).

Figure 6

Photo-field emitted beam spot on a phosphor screen (43 keV; 50 pC; 30 Hz).

Figure 7

Horizontal projection of the pinhole image transverse profile (left), beamlet divergence centroids versus position centroids (right) (27 keV; 20 pC; 266 nm; $E_{266\mathrm{nm}}=2.3\mu \mathrm{J}$; ${\sigma }_{t,\mathrm{laser}}=16\mathrm{ps}$; 30 Hz).