Abstract
Control of coherent synchrotron radiation (CSR)-induced emittance growth is essential in linear accelerators designed to deliver very high brightness electron beams. Extreme current values at the head and tail of the electron bunch, resulting from strong bunch compression, are responsible for large CSR production leading to significant transverse projected emittance growth. The Linac Coherent Light Source (LCLS) truncates the head and tail current spikes which greatly improves free electron laser (FEL) performance. Here we consider the underlying dynamics that lead to formation of current spikes (also referred to as current horns), which has been identified as caustics forming in electron trajectories. We present a method to analytically determine conditions required to avoid the caustic formation and therefore prevent the current spikes from forming. These required conditions can be easily met, without increasing the transverse slice emittance, through inclusion of an octupole magnet in the middle of a bunch compressor.
Highlights
Recent advances in free electron laser (FEL) facilities have seen the peak brightness increase by several orders of magnitude
Control of coherent synchrotron radiation (CSR)-induced emittance growth is essential in linear accelerators designed to deliver very high brightness electron beams
Extreme current values at the head and tail of the electron bunch, resulting from strong bunch compression, are responsible for large CSR production leading to significant transverse projected emittance growth
Summary
Recent advances in free electron laser (FEL) facilities have seen the peak brightness increase by several orders of magnitude. A more uniform current pulse improves FEL performance immensely through increased pulse energy, increased peak power, and greater control over the spectral bandwidth [17,22] This improvement has been verified experimentally at the Linac Coherent Light Source (LCLS) [17], where collimating the head and tail of the bunch successfully limits CSR-induced emittance growth and improves FEL performance, at the cost of removing 40% of the bunch charge [17,25]. The alternate method we present in this paper, establishes the conditions needed to ensure the current horns cannot form, without the need to collimate This is done through manipulating the longitudinal phase space in the low energy bunch compressor to inhibit the particle trajectory caustics from forming, which would otherwise result in the double horn current profile.
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