Electron beam profile imaging in the presence of coherent optical radiation effects

Abstract

High-brightness electron beams with low energy spread at existing and future x-ray free-electron lasers are affected by various collective beam self-interactions and microbunching instabilities. The corresponding coherent optical radiation effects, e.g., coherent optical transition radiation, impede electron beam profile imaging and become a serious issue for all kinds of electron beam diagnostics using imaging screens. Furthermore, coherent optical radiation effects can also be related to intrinsically ultrashort electron bunches or the existence of ultrashort spikes inside the electron bunches. In this paper, we discuss methods to suppress coherent optical radiation effects both by electron beam profile imaging in dispersive beam lines and by using scintillation imaging screens in combination with separation techniques. The suppression of coherent optical emission in dispersive beam lines is shown by analytical calculations, numerical simulations, and measurements. Transverse and longitudinal electron beam profile measurements in the presence of coherent optical radiation effects in nondispersive beam lines are demonstrated by applying a temporal separation technique.

Time-resolved imaging of the microbunching instability and energy spread at the Linac Coherent Light Source

“…(3)] is smaller. Moreover, it is known that intentionally adding a dispersive region ("phase mixing") can reduce the amplitude of MBI [19,22,36,37]. While past studies assumed the electron beam has no energy chirp in the phase mixing chicane, sample images in Fig.…”

“…Microbunching has been studied extensively both theoretically [1][2][3][4][5][6][7][8][9][10][11][12][13] and experimentally [14][15][16][17][18][19][20][21][22][23]. In the standard model for the longitudinal space charge (LSC) flavor of MBI, density modulations in the beam produce LSC fields that modulate the beam energy, and a subsequent dispersive region generates a corresponding amplification in the density modulation.…”

Impact of Non-Gaussian Electron Energy Heating upon the Performance of a Seeded Free-Electron Laser

“…This instability creates both energy and density modulations on the electron beam, increasing the energy spread up to levels that can strongly degrade the FEL gain process. An accompanying and undesired effect is a large coherent optical transition radiation signal at intercepting diagnostic screens, often limiting the utility of beam profile imaging systems [13,14]. The application of a so-called laser heater (LH) [15] at the low energy (≈100 MeV) part of the accelerator has been found [16] to be an efficient way to suppress the microbunching instability.…”

Backward transition radiation in the extreme ultraviolet region as a tool for the transverse beam profile diagnostic

“…Based on the isotropic light emission from scintillators and the resolution studies in Ref. [6], it was proposed to spatially separate scintillation and BTR light by a proper choice of the monitor geometry [7,8], thus avoiding the contribution of intense coherent radiation in profile measurements [9]. Screen monitors utilizing the principle of spatial suppression were developed for the European X-FEL [10] and have been in operation at FLASH2 (DESY, Germany) for about half a year.…”