Start-to-end simulation of the shot-noise driven microbunching instability experiment at the Linac Coherent Light Source

The shot-noise driven microbunching instability can significantly degrade electron beam quality in x-ray free electron laser light sources. Experiments were carried out at the Linac Coherent Light Source (LCLS) to study this instability. In this paper, we present start-to-end simulations of the shot-noise driven microbunching instability experiment at the LCLS using the real number of electrons. The simulation results reproduce the measurements quite well. A microbunching self-heating mechanism is also illustrated in the simulation, which helps explain the experimental observation.

Figure 1

The rf cavity structure wake function (top) and the resistive wall wake function (bottom) used in the simulation.

Figure 2

A schematic plot of LCLS accelerator layout [17].

Figure 3

Transverse rms size (top) and rms bunch length (bottom) evolution through the accelerator.

Figure 4

Simulated electron beam longitudinal phase space (top) and current profile (bottom) at injector 135 MeV (left), after bunch compressor BC1 (middle), and after DL2 (right). Bunch charge is 180 pC; the laser heater was off.

Figure 5

Measurement (left) and simulation (right) of the final longitudinal phase space distribution with the laser heater off. Beam current is 1 kA, with bunch charge 180 pC. The bunch head is to the right.

Figure 6

The final current profile (left) and bunching factor (right) of the Fig. 5 longitudinal phase-space distribution with the laser heater off.

Figure 7

Measurement (left) and simulation (right) of the final longitudinal phase space distribution with the laser heater at 19 keV. Beam current is 1 kA, with bunch charge 180 pC. The bunch head is to the right.

Figure 8

The final current profile (left) and bunching factor (right) of the Fig. 7 longitudinal phase-space distribution with the laser heater at 19 keV.

Figure 9

Measurement (left) and simulation (right) of the final longitudinal phase space distribution with the laser heater off. Beam current is 500 A, bunch charge 180 pC. The bunch head is to the right.

Figure 10

The final current profile (left) and bunching factor (right) of the Fig. 9 longitudinal phase-space distribution with the laser heater off.

Figure 11

Measurement (left) and simulation (right) of the final longitudinal phase space distribution with the laser heater at 19 keV. Beam current is 500 A, bunch charge 180 pC. The bunch head is to the right.

Figure 12

The final current profile (left) and bunching factor (right) of the Fig. 11 longitudinal phase-space distribution with the laser heater at 19 keV.

Figure 13

Final slice rms energy spread after the undulator as a function of laser heater induced extra energy spread. The top plot is the measurement from Ref. [17], while the bottom is from simulation.

Figure 14

Evolution of the slice energy spread along the machine at (a) before BC1, (b) before BC2 and (c) after undulator. The red curves are with laser heater off, and the green curves are laser heater at 3 keV. Beam current is 1 kA.

Figure 15

Final longitudinal phase space distribution from the simulation with laser heater at 3 keV. This is to compare with Fig. 5 where the laser heater was off. A stronger microbunching effect is observed with very small heating.