Multivariate optimization of a high brightness dc gun photoinjector

We have conducted a multiobjective computational optimization of a high brightness, high average current photoinjector under development at Cornell University. This injector employs a dc photoemission electron gun. Using evolutionary algorithms combined with parallel computing resources, the multivariate parameter space of the photoinjector was explored for optimal values. This powerful computational tool allows an extensive study of complex and nonlinear systems such as the space-charge dominated regions of an accelerator, and has broad areas of potential application to accelerator physics and engineering problems. In the present case, the optimized injector is simulated to deliver beam of very high quality (e.g., a rms normalized emittance of 0.1 mm mrad for 0.1 nC, and 0.7 mm mrad for 1 nC bunches). The field strengths of the active elements of the injector are moderate and technically practical. The relevance of these results to various novel linac-based accelerator proposals is pointed out.

Figure 1

Schematic of the dc gun injector layout.

Figure 2

Normalized rms emittance versus bunch length at the end of the injector for (top) 80 pC and (bottom) 0.8 nC bunch charges.

Figure 3

Beam evolution in the injector for 80 pC bunch charge: normalized transverse (top) and longitudinal (bottom) rms emittances (dashed line) and sizes (solid line) versus position in the injector.

Figure 4

Beam evolution in the injector for 0.8 nC bunch charge: normalized transverse (top) and longitudinal (bottom) rms emittances (dashed line) and sizes (solid line) versus position in the injector.

Figure 5

Normalized rms emittance versus voltage in the gun for (top) 80 pC and (bottom) 0.8 nC bunch charges. The average bunch length corresponding to these calculations was (top) 0.8 mm and (bottom) 0.9 mm.

Figure 6

Axial electric field in the dc gun for 750 kV voltage.

Figure 7

Initial distribution profiles corresponding to minimum emittance at the end of the injector for (left) 80 pC and (right) 0.8 nC cases.

Figure 8

80 pC: emittance sensitivity (solid curve) to the longitudinal profile changes (top) and the corresponding profile shapes (bottom). Data points show the minimum normalized rms emittance obtained after the injector settings were reoptimized for each particular longitudinal tail parameter value.

Figure 9

0.8 nC: emittance sensitivity (solid curve) to the longitudinal profile changes (top) and the corresponding profile shapes (bottom). Data points show the minimum normalized rms emittance obtained after the injector settings were reoptimized for each particular longitudinal tail parameter value.

Figure 10

The minimum transverse normalized rms emittance from injector as a function of the effective thermal energy from the photocathode for (top) 80 pC and (bottom) 0.8 nC bunch charge, respectively.

Figure 11

Transverse normalized rms emittance versus bunch length for various charges in the injector (nC).

Figure 12

Longitudinal rms emittance versus bunch length for various charges in the injector (nC).