Comparison of dc and superconducting rf photoemission guns for high brightness high average current beam production

A comparison of the two most prominent electron sources of high average current high brightness electron beams, dc and superconducting rf photoemission guns, is carried out using a large-scale multivariate genetic optimizer interfaced with space charge simulation codes. The gun geometry for each case is varied concurrently with laser pulse shape and parameters of the downstream beam line elements of the photoinjector to obtain minimum emittance as a function of bunch charge. Realistic constraints are imposed on maximum field values for the two gun types. The superconducting rf and dc gun emittances and beam envelopes are compared for various values of photocathode thermal emittance. The performance of the two systems is found to be largely comparable for up to 154 pC per bunch at 1.3 GHz or 200 mA provided low intrinsic emittance photocathodes can be employed.

Figure 1

(a) The parametrized dc gun geometry: the electrode angle $\alpha$, the cathode-anode gap $g$, and the photocathode recess $d$ are the parameters varied by the optimizer. Equipotential lines are shown. (b) The effect of varying the angle $\alpha$ (from 0 to 45°) on the axial electric field for a fixed gap $g=48\mathrm{mm}$. (c) The effect of varying the gap $g$ (from 20 to 120 mm) for a fixed $\alpha =23°$.

Figure 2

(a) The parameters of the SRF cavity geometry, modeled after the 1.3 GHz TESLA design. The radius $r_{\mathrm{cath}}$ and recess $d$ of the photocathode, the angle $\alpha$ of the leftmost cavity wall, the gap $g$ between photocathode and exit pipe, and the exit pipe radius $r_{\mathrm{pipe}}$ are varied, with the equatorial cavity radius left as the free parameter for frequency tuning. Here, lines of constant azimuthal magnetic field are shown. (b) The effect of varying the angle $\alpha$ (from 0 to 25°) on the axial electric field for fixed $g=75\mathrm{mm}$, $r_{\mathrm{pipe}}=1.83\mathrm{cm}$, $d=0$, and $1/r_{\mathrm{cath}}=0$. (c) The effect of varying the gap $g$ (from 15 to 105 mm) for fixed $\alpha =10.5°$, $d=1.67\mathrm{mm}$, $r_{\mathrm{cath}}=15\mathrm{mm}$, and $r_{\mathrm{pipe}}=1.83\mathrm{cm}$.

Figure 3

Empirical data of voltage breakdown as a function of gap for planar electrodes, after 16, including fits used in constraining solutions. The filled area shows allowed values. Here, $s$ is defined as the shortest distance between the cathode and anode.

Figure 4

Injector layout modeled after Cornell ERL electron source. The beam direction is to the left.

Figure 5

Typical axial fields in the injector: (a) the dc and (b) the SRF gun based.

Figure 6

Above: Archetypal laser pulse shapes, variably superimposed to form the requested laser pulse shape. Below: Sample pulse shapes including their superposition parameters.

Figure 7

Optimized emittance for various bunch charges for (a) dc gun and (b) SRF gun-based photoinjectors.

Figure 8

Field profiles for optimized geometries: (a) dc and (b) SRF guns.

Figure 9

(Top row) The optimal shape for the dc gun for two different laser pulse lengths, 10 ps (solid) and 30 ps rms (dashed line). (Bottom row) The optimal shape for the SRF gun.

Figure 10

Beam envelopes for 80 pC ($\mathrm{MTE}=120\mathrm{meV}$) in the dc gun photoinjector. The initial laser pulse is 10 ps rms.

Figure 11

Beam envelopes for 80 pC ($\mathrm{MTE}=120\mathrm{meV}$) in the SRF gun photoinjector.

Figure 12

Examples of the final phase space for 80 pC bunches ($\mathrm{MTE}=0.12\mathrm{eV}$) for dc [(a),(c)] and SRF [(b),(d)] guns.

Figure 13

Emittance at the end of the shortened beam line for (a) dc and (b) SRF guns.

Figure 14

Beam envelopes and emittance in the shortened beam line for (a) dc and (b) SRF guns. The dc gun shows two cases: 10 ps (solid) and 30 ps (dashed line) rms initial laser pulse. The photocathode MTE is 120 meV.

Figure 15

Examples of projected phase space emittance growth from (a) geometric aberrations and (b) projected (slice) phenomena. The dashed line shows an individual slice either in time (for rf-induced emittance) or in energy (for chromatic aberration).