Surface-Emission Studies in a High-Field RF Gun based on Measurements of Field Emission and Schottky-Enabled Photoemission

We report on investigations into the fundamental surface emission parameters, the geometric field enhancement factor ($\beta$) and the work function ($\varphi$), by making both field emission and Schottky-enabled photoemission measurements. The measurements were performed on a copper surface in the Tsinghua University $S$-band RF gun in two separate experiments. Fitting our data to the models for each experiment indicate that the traditionally assumed high value of $\beta (\approx 50–500)$ does not provide a plausible explanation of the data, but incorporating a low value of $\varphi$ at some sites does. In addition, direct measurements of the surface conducted after the experiment show that $\beta$ is on the order of a few, consistent with our understanding of the electron emission measurements. Thus we conclude that the dominant source of electron emission in high gradient RF cavities is due to low $\varphi$ sites, as opposed to the conventionally assumed high $\beta$ sites. The origin of low $\varphi$ at these sites is unclear and should be the subject of further investigation.

Figure 1

The role of geometric perturbations (parametrized by $\beta$) vs work function perturbations (parametrized by $\varphi$) and their relative weights should be considered in determining the onset of field emission.

Figure 2

Schematic of the experimental setup for FE and photoemission measurements at Tsinghua University.

Figure 3

Fowler-Nordheim plot of normalized FE data from the $S$-band RF gun with linear fit. Applying traditional analysis, both the field enhancement factor $\beta =130$ and the emission area $A_e=4{\mathrm{nm}}^2$ were extracted from the slope of the fitted line using $\varphi =4.6\mathrm{eV}$, bulk work function of copper.

Figure 4

Schottky-enabled photoemission data from the $S$-band RF gun. Long pulse (3 ps). $h\nu =3.1\mathrm{eV}$, $E_{\mathrm{peak}}=55\mathrm{MV}/\mathrm{m}$, $\mathrm{\text{injection phase}}=80°$, $E_{\mathrm{accel}}=54\mathrm{MV}/\mathrm{m}$. Data with linear fit.

Figure 5

Schottky-enabled photoemission data from the RF gun. Short pulse (0.1 ps). $h\nu =3.1\mathrm{eV}$, $E_{\mathrm{peak}}=55\mathrm{MV}/\mathrm{m}$, $\mathrm{\text{injection phase}}=80°$, $E_{\mathrm{accel}}=54\mathrm{MV}/\mathrm{m}$. Data with linear fit.

Figure 6

$E_{\mathrm{peak}}=50\mathrm{MV}/\mathrm{m}$, $\mathrm{\text{injection phase}}=80°$, $E_{\mathrm{accel}}=25\mathrm{MV}/\mathrm{m}$. Schottky-enabled photoemission, near threshold. The plot becomes nonlinear at laser energy above 0.4 mJ, indicative of the multiphoton emission regime. Data with laser energy $<0.4\mathrm{mJ}$ with linear fit, data with laser energy $>0.4\mathrm{mJ}$, quadratic fit.

Figure 7

$\beta$ vs $\varphi$ plotted using the slope of the FN plot for the Tsinghua data (Fig. 3). Each point on the plot represents a possible combination of field enhancement factor and local work function which match the data.

Figure 8

Emitting area $A_e$ computed from the intercept of the FN plot (Fig. 3) using two different values of $\beta$. The squares indicate the two points we chose for the purpose of comparison.