High voltage dc gun for high intensity polarized electron source

The high intensity polarized electron source is a critical component for future nuclear physics facilities. The electron-ion collider requires a polarized electron gun with higher voltage and higher bunch charge compared to any existing polarized electron source. At Brookhaven National Laboratory, we have built an inverted high voltage direct current (HVDC) photoemission gun with a large cathode size. We report on the performance of GaAs photocathodes in a high gradient with up to 16 nC bunch charge. The measurements were performed at a stable operating gap voltage of 300 kV—demonstrating outstanding lifetime and robustness. We observed obvious lifetime enhancement by biasing the anode. The gun also integrated a cathode cooling system for potential application on high current electron sources. The various novel features implemented and demonstrated in this polarized HVDC gun will open the door toward future high intensity-high average current electron accelerator facilities.

Figure 1

Cross-sectional view of the BNL HVDC gun.

Figure 2

dc gap geometry and field distribution. The optimized parameters are listed.

Figure 3

The field gradient of the HVDC gun. The most important parts as dashed red boxed are zoomed in. The main high gradient locations are labeled: 1: Pierce nose; 2: anode; 3: HV trip-point shield; 4: grounded trip-point shield; and 5: ceramic. (a) The field gradient map on the dc gap. (b) The field gradient on the HV ceramic feethrough.

Figure 4

The potential and gradient along the ceramic feedthrough, comparison between with and without triple-point shields (TPS). The position 0 cm is the ground side of the feedthrough, toward the 24 cm of high voltage electrode side.

Figure 5

HVPS conditioning process to reach 350 kV.

Figure 6

The left figure is the cross view of the modified plug with the HV feedthrough and electrodes. The FC72 flow path is shown in red arrow. The right figure is the picture of the modified plug in a transparent acrylic container to test the grease uniformity.

Figure 7

The HVDC gun test beam line. All the diagnostics are labeled.

Figure 8

The dynamic vacuum distribution in the gun chamber. In the simulation, we assume the stainless steel outgassing rate is $5\times {10}^{-13}\mathrm{mbar}\mathrm{L}/\mathrm{s}/{\mathrm{cm}}^2$. The beam dump pressure was assumed to be $1\times {10}^{-8}\mathrm{mbar}$ for this simulation.

Figure 9

Space charge limit for the EIC polarized gun with a 6 mm diameter laser spot. Blue dots are measurement and the red curve is the fitted curve.

Figure 10

Comparison of QE decay between unbiased anode and biased anode operation. The peak and average current in both cases were the same—3.5 A and $37.5\mu \mathrm{A}$, respectively. The $\tau$ is the decay time using exponential fitting. The negative $\tau$ means the QE had slightly increased in 8 h.

Figure 11

Left: fractional amount of time spent on a sample cathode for different operation modes. The percents in the bracket indicate the percentage of overall operation time from this cathode. Middle: QE scan of the cathode before any beam was extracted. Right: QE scan after the cathode was used as per the left pie chart. The colorbar represents QE in percent (%). Note the scale change on the colorbar for “After beam” scan compared to “Before beam” scan.

Figure 12

molflow+ simulation of the beam line, assuming a $1\times {10}^{-9}\mathrm{Torr}$ vacuum in the beamstop.

Figure 13

Comparison of various gun from different facilities around the world. The sloped line shows the average current contour level as labeled. The ball diameter is representative of the peak current of the gun. The red dashed line at EIC R&D shows the maximum achieved peak current of 8 A.