Beam shaping to improve the free-electron laser performance at the Linac Coherent Light Source

A new operating mode has been developed for the Linac Coherent Light Source (LCLS) in which we shape the longitudinal phase space of the electron beam. This mode of operation is realized using a horizontal collimator located in the middle of the first bunch compressor to truncate the head and tail of the beam. With this method, the electron beam longitudinal phase space and current profile are reshaped, and improvement in lasing performance can be realized. We present experimental studies at the LCLS of the beam shaping effects on the free-electron laser performance.

Figure 1

A schematic of the LCLS machine layout including a collimator in the middle of the first bunch compressor (BC1) and the XTCAV downstream of the FEL undulator. The L2 and L3 are S-band rf linac sections. We also show the measured images at the middle of the BC1, before and after collimation, and a measured example from XTCAV for the collimated beam after FEL lasing.

Figure 2

Simulation examples of a regular beam without collimation, bunch charge 250 pC. Left column: electron beam longitudinal phase space and current profile after BC1 chicane. Middle column: longitudinal phase space before BC2 chicane and L2-linac wake induced energy change. Right column: longitudinal phase space and current profile after BC2 chicane.

Figure 3

Simulation examples of the collimated beam. BC1 collimator is used to truncate the bunch charge from 250 pC to 180 pC. Left column: electron beam longitudinal phase space and current profile after BC1 chicane. Middle column: longitudinal phase space before BC2 chicane and L2-linac wake induced energy change. Right column: longitudinal phase space and current profile after BC2 chicane.

Figure 4

Measurement examples of the collimated beam for hard x-ray FEL case with photon energy 7 keV. The electron beam energy is 12.5 GeV, and other machine parameters can be found in Table 1. The electron beam longitudinal phase space is shown in (a) for lasing-off condition and (b) for lasing-on condition; in (c) the beam current profile and reconstructed FEL power profile are given. In this example the x-ray pulse energy is 4.8 mJ. Data taken on 3/15/2016.

Figure 5

Measurement examples of the final beam without collimation for hard x-ray FEL case. Electron beam energy is 13.5 GeV, and bunch charge is 250 pC (top row) and 150 pC (bottom row), respectively. (a)–(c): bunch charge 250 pC, the electron beam longitudinal phase space for lasing-off condition (a) and lasing-on condition(b), and the beam current profile and reconstructed FEL power profile in (c). (d)–(f): bunch charge 150 pC, same measurement as 250 pC case. In both cases the x-ray pulse energy is $\sim 2\mathrm{mJ}$. Data taken on 11/16–17/2013.

Figure 6

Archived gas detector history to show the recorded FEL pulse energy before and after using the collimation mode. The photon energy is filtered between 7 to 9 keV. The bunch charge for the period before using the collimation mode was 180 pC from the injector to the undulator, while with the collimation mode, the bunch charge was truncated from 250 pC to 180 pC with BC1 collimator. The actual bunch charge for FEL lasing in this plot is the same.

Figure 7

The procedure to retrieve the initial phase space at the undulator entrance. The final measured longitudinal phase space is in (a), and by projecting to time, we obtain the current profile (b). Using the resistive wall wakefield model, the induced energy loss is calculated in (d). Finally the initial phase space (c) is obtained by subtracting the energy loss in (d) from (a). Bunch head is to the left.

Figure 8

The final phase space of the collimated beam at the entrance of the undulator with different setting of the beam current after BC1: (a) 130 A; (b) 180 A. The final beam energy at the undulator entrance is 4 GeV, with current about 1500 A. The bunch head is to the left.

Figure 9

The measured hard x-ray SASE FEL spectra for operating mode without collimation (a) and (b), and with collimation (c). The blue curve is a single-shot example, and the thick red curve is averaged with 100 shots. In (a), measured on 5/23/15, the bunch charge is 150 pC from injector to the undulator, photon energy is 7 keV, bandwidth is $\sim 25\mathrm{eV}$ FWHM, and pulse energy is about 2.7 mJ; in (b), measured on 4/29/15, the bunch charge is 150 pC from injector to the undulator, photon energy is 9.3 keV, bandwidth is $\sim 25\mathrm{eV}$ FWHM, and the pulse energy is 2.6 mJ; in (c), measured on 6/23/16, the charge is truncated with BC1 collimator from 250 pC to 180 pC, photon energy is 7.9 keV, bandwidth is $\sim 15\mathrm{eV}$, and the pulse energy is 4.3 mJ.