High field hybrid photoinjector electron source for advanced light source applications

The production of high spectral brilliance radiation from electron beam sources depends critically on the electron beam qualities. One must obtain very high electron beam brightness, implying simultaneous high peak current and low emittance. These attributes are enabled through the use of very high field acceleration in a radio-frequency (rf) photoinjector source. Despite the high fields currently utilized, there is a limit on the achievable peak current in high brightness operation, in the range of tens of Ampere. This limitation can be overcome by the use of a hybrid standing wave/traveling wave structure; the standing wave portion provides acceleration at a high field from the photocathode, while the traveling wave part yields strong velocity bunching. This approach is explored here in a C-band scenario, at field strengths ($>100\mathrm{MV}/\mathrm{m}$) at the current state-of-the-art. It is found that one may arrive at an electron beam with many hundreds of Amperes with well-sub-micron normalized emittance. This extremely compact injector system also possesses attractive simplification of the rf distribution system by eliminating the need for an rf circulator. We explore the use of this device in a compact 400 MeV-class source, driving both inverse Compton scattering and free-electron laser radiation sources with unique, attractive properties.

Figure 1

Basic layout of the compact hybrid C-band photoinjector.

Figure 2

One-quarter section of the C-band hybrid photoinjector from hfss.

Figure 3

On-axis electric field amplitude profile.

Figure 4

On-axis electric field phase distribution.

Figure 5

On-axis normalized magnetic field profile.

Figure 6

The beam energy gain from the cathode at $z=0\mathrm{m}$ up to the end of the beamline with two high-gradient linacs.

Figure 7

Evolution of the rms transverse beam size ${\sigma }_{x,y}$ and normalized transverse emittance ${\epsilon }_n$ along the beamline; beam charge 250 pC, energy 100 MeV.

Figure 8

Evolution of the rms bunch length ${\sigma }_z$ along the beamline; beam charge 250 pC, energy 100 MeV.

Figure 9

(left) Flattop profile in x and y; (right) cut-Gaussian of same radius (0.5 mm) which corresponds to $1\sigma$ in the distribution before the cut.

Figure 10

Normalized transverse emittance for two-beam transverse distributions: Gaussian (red) and uniform (blue).

Figure 11

Behavior of the rms parameters (a) and the rms normalized emittance (b) up to 1 m downstream the cathode.

Figure 12

(a) Slice partition at the cathode, (b) slice transverse phase space, and (c) energy-position correlation.

Figure 13

Modified bunch (a) shape and (b) transverse and (c) longitudinal phase space properties at the emittance minimum ($z=0.52\mathrm{m}$).

Figure 14

Slices analysis for uniform distribution. (a) Slices partition at the cathode position $(z=0\mathrm{m})$ (b) Slices partition at the emittance minimum $(z=0.52\mathrm{m})$.

Figure 15

Demonstration of the ellipsoidal shape. The red grid is the shape of the ellipsoid surface of Eq. (2) with semiaxes $a=b={\sigma }_x\sqrt{5}$ and $c=\gamma {\sigma }_z\sqrt{5}$ and $u=1$ evaluated at the waist position $z=0.64$. The blue dots are the 3D distribution particles at the waist position, too.

Figure 16

Spatial binning of beam density, showing the development of a uniform distribution.

Figure 17

Second order moments (rms length ${\sigma }_z$ and rms energy spread ${\sigma }_E$) of the beam distribution in a $\sim 1.5\mathrm{m}$ long drift space. Comparison between gpt (solid lines, blue for ${\sigma }_z$, and green for ${\sigma }_E$) and custom tracking with ellipsoidal space-charge forces (dashed lines, red for ${\sigma }_z$, and purple for ${\sigma }_E$). The tracking starts at $z=0$ where the beam exits the hybrid photoinjector. (a) Transverse plane. (b) Longitudinal plane.

Figure 18

The beam energy gain from the cathode at $z=0\mathrm{m}$ up to the end of the beamline with eight high-gradient linacs.

Figure 19

Evolution of the rms transverse beam size ${\sigma }_{x,y}$ and normalized transverse emittance ${\epsilon }_n$ along the beamline; beam charge 250 pC, energy 400 MeV.

Figure 20

Evolution of the rms bunch length ${\sigma }_z$ along the beamline; beam charge 250 pC, energy 400 MeV.

Figure 21

rms emittance dilution induced by random alignment errors in a eight-section linac. The input location is $z=0$ and the emittance is normalized by its injection value.

Figure 22

Demonstration of one-to-one steering algorithms. Green squares represent an ideal machine with no misalignments while red triangles represent a linac whose first section is $50\mu \mathrm{m}$ off axis. Introducing the magnetic corrections according to one-to-one procedures, the new trajectory and emittance are given by the black dashed curves. The gold crosses identify the location of the BPMs. (a) Center of mass trajectory. (b) Normalized rms emittance growth.

Figure 23

rms beam sizes ${\sigma }_x$ and ${\sigma }_y$ in ICS system operated at 220 MeV, from gpt start-to end simulation.

Figure 24

rms beam sizes ${\sigma }_x$ and ${\sigma }_y$ in final focus system at 220 MeV from gpt start-to end simulation.

Figure 25

Beam centroid offset and beam envelope near the Compton interaction point, with an initial misalignment of the centroid of laser on the photocathode of 100 microns.

Figure 26

Instantaneous gain length (orange) and laser power (blue) as calculated in genesis. The peak power at saturation is approximately 830 MW.

Figure 27

Instantaneous gain length (orange) and laser power (blue) as calculated in genesis for water window (4 nm) soft x-ray FEL. The peak power at saturation is approximately 4.2 GW.