Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun

The femtosecond photoemission yield from a copper cathode and the emittance of the created electron beams has been studied in a $12\mathrm{MeV}/\mathrm{m}$, 100 keV dc electron gun over a wide range of laser fluence, from the linear photoemission regime until the onset of image charge limitations and cathode damaging. The measured photoemission curves can be described well with available theory which includes the Schottky effect, second-order photoemission, and image charge limitation. The second-order photoemission can be explained by thermally assisted one-photon photoemission (1PPE) and by above-threshold two-photon photoemission (2PPE). Measurements with a fresh cathode suggest that the 2PPE process is dominant. The beam emittance has been measured for the entire range of initial surface charge densities as well. The emittance measurements of space-charge dominated beams can be described well by an envelope equation with generalized perveance. The dc gun produces 0.1 pC bunches with 25 nm rms normalized emittance, corresponding to a normalized brightness usually associated with rf photoguns. In this experimental study the limits of femtosecond photoemission from a copper cathode have been explored and analyzed in great detail, resulting in improved understanding of the underlying mechanisms.

Figure 1

One-photon photoemission (1PPE) and two-photon photoemission (2PPE) compared with thermionic emission as a function of absorbed fluence.

Figure 2

Peak surface charge density versus peak laser fluence, simulated using GPT.

Figure 3

Measured peak surface charge density ${\sigma }_0\equiv Q/(2\pi s^2)$ as a function of peak fluence $F_0$ for acceleration voltages of 10 and 100 kV, corresponding to acceleration fields ($G_a$) of 1.23 and $12.3\mathrm{MV}/\mathrm{m}$. The blue arrows indicate the conditions at which waist scans have been performed (at 100 kV).

Figure 4

First-order coefficient $b_1$ as a function of acceleration field $G_a$.

Figure 5

Second-order coefficient $b_2$ as a function of acceleration field $G_a$.

Figure 6

Measured photoemitted yield at an acceleration voltage of 100 kV for the new and used cathodes.

Figure 7

Waist scans for different bunch charges.

Figure 8

Top row: images of electron bunches for $I_{C2}=0.1\mathrm{A}$ for 10 fC, 100 fC, and 1 pC (from left to right); the black lines indicate the fitted beam center and rms beam width. Bottom row: horizontal lineouts (black curves) averaged over 40 pixels in the vertical direction; the lineouts are fitted with Gaussian functions shown by the red curves.