Charge lifetime measurements at high average current using a ${\mathbf{K}}_2\mathrm{CsSb}$ photocathode inside a dc high voltage photogun

Two ${\mathrm{K}}_2\mathrm{CsSb}$ photocathodes were manufactured at Brookhaven National Lab and delivered to Jefferson Lab within a compact vacuum apparatus at pressure $\sim {10}^{-11}\mathrm{Torr}$. These photocathodes were evaluated using a dc high voltage photogun biased at voltages up to 200 kV, and illuminated with laser light at wavelengths 440 or 532 nm, to generate dc electron beams at currents up to 20 mA. Some conditions produced exceptionally large photocathode charge lifetimes, without measurable quantum efficiency decay, even from the center of the photocathode where operation using GaAs photocathodes is precluded due to ion bombardment. Under other conditions the charge lifetime was poor due to extensive ion bombardment under severely degraded vacuum conditions, and as a result of localized heating via the laser beam. Following beam delivery, the photocathodes were evaluated using a scanning electron microscope to determine surface morphology.

Figure 1

The transfer arm of the BNL ${\mathrm{K}}_2\mathrm{CsSb}$ deposition system holding the stainless steel and aluminum puck. The puck can be biased for QE measurements, heated, and cooled.

Figure 2

Spectral response of the ${\mathrm{K}}_2\mathrm{CsSb}$ photocathodes created at BNL just after deposition.

Figure 3

Schematic of the vacuum suitcase composed of a magnetic sample manipulator, six-way cross with 4.5” CF, gate valve, and vacuum pumps.

Figure 4

Photograph (a) and schematic view (b) of the dc high voltage photogun at JLab.

Figure 5

Schematic of the photogun and beam line at JLab’s Injector Test Stand Facility.

Figure 6

QE maps of ${\mathrm{K}}_2\mathrm{CsSb}$ photocathode #1 using 532 nm light when the photocathode exhibited poor lifetime: (a) before running beam and (b) after running beam from many locations (the numbering denotes the sequential order of the run).

Figure 7

Charge lifetime of ${\mathrm{K}}_2\mathrm{CsSb}$ photocathode #1 versus the radial position of the 532 nm laser spot relative to the electrostatic center of the cathode. The vertical error bars are related to the quality of the exponential fit to the QE decay curves, and the horizontal error bars reflect uncertainty of the laser position relative the electrostatic center.

Figure 8

QE scans following the vacuum event during which greater than 1.5 mA of beam was dumped into a valve for an extended period of time: (a) 440 nm light and (b) 532 nm light. Absence of QE in the wafer center was an indication that photocathode material was sputtered away by ions produced by the extracted beam. Note the high QE spot in (b) at approximate location $x/y=4$, 0 mm was produced after extracting 4 mA over 24 hrs using 440 nm light after the vacuum event.

Figure 9

QE evolution during 200 kV beam delivery from ${\mathrm{K}}_2\mathrm{CsSb}$ photocathode #1: (a) using 440 nm light and (b) 532 nm light. Note QE increasing with extracted charge. The discontinuities are due to static charge buildup on viewers and eventual discharges that move the beam at the Faraday cup.

Figure 10

Beam current versus time using ${\mathrm{K}}_2\mathrm{CsSb}$ photocathode #1 and 532 nm light with laser spot diameter: (a) $500\mu \mathrm{m}$ FWHM and (b) $1050\mu \mathrm{m}$ FWHM. The larger laser spot provided longer lifetime.

Figure 11

QE difference scans of ${\mathrm{K}}_2\mathrm{CsSb}$ photocathode #1 using (a) 440 nm laser at 0.2 W for 12 hrs and (b) 532 nm laser at 0.7 W for 2 hrs. The photocathode was at ground potential, no beam was extracted while the laser was illuminating the photocathode. The QE increased due to the presence of the laser light, at both wavelengths.

Figure 12

The QE at the electrostatic center of ${\mathrm{K}}_2\mathrm{CsSb}$ photocathode #2 versus extracted charge at 10 mA average current, 100 kV cathode bias voltage, and with the anode biased at $+1500\mathrm{V}$ (red) and grounded (blue)

Figure 13

QE evolution at (a) 1 mA and (b) 10 mA equivalent laser power. Continuous lines represent QE measured while delivering beam at 100 kV and data points represent QE measured at low voltage during “laser-heating” runs, with no high voltage and no beam delivered to the Faraday cup. Data points were connected to aid the eye.

Figure 14

(a) Schematic view of the elements used in the ANSYS 14.0 thermal analysis. (b) Close-up cross sectional view of the photocathode and the stainless steel/aluminum puck with 1.3 W of laser light distributed over a 0.5 mm diameter region. The heat is primarily localized within the stainless steel where the temperature reaches $135°\mathrm{C}$.

Figure 15

(a) Gaussian beam widths in the $x$ direction for different focusing solenoid field strengths, for beam extracted from the electrostatic center of photocathode #2. The data were fit with a parabola to determine emittance, as described in the text. (b) Normalized rms emittance measurements versus laser beam size for measurements performed on photocathodes #1 and # 2. For each photocathode-wavelength configuration, the data were fit with a line through the origin. The slope of the line yields the normalized rms emittance per laser size.

Figure 16

(a) QE scan of photocathode #1 prior to transfer to the SEM and, (b) photograph of the photocathode showing the locations evaluated with SEM and EDS.

Figure 17

(a) Surface of photocathode #1 at 400x magnification and (b) 3000x magnification.

Figure 18

EDS spectra taken from $400x$ images of locations 1–4 from photocathode #1. Notice location 1 has a purely stainless steel composition.

Figure 19

(a) QE scan and (b) a photograph of photocathode #2 prior to transfer to the SEM.

Figure 20

SEM images at 3000x magnification using an 8 keV electron beam at photocathode locations that had been illuminated with 2 W laser light, in one case (a) while extracting beam at 10 mA using a laser spot $1050\mu \mathrm{m}$ FWHM and the other case, (b) simply heating the photocathode with a laser spot $500\mu \mathrm{m}$ FWHM without generating beam. The topology of these two locations was very different, and very different from that of photocathode #1.

Figure 21

Chemical assay comparison at locations of photocathode #2 that were illuminated with laser light and from unused locations.