Seeding, Controlling, and Benefiting from the Microbunching Instability

Abstract

Advanced light sources using relativistic electrons rely on coherent emission from high-density (compressed) beams. These beams, typically produced by photoinjected linear accelerators, can suffer from uncontrolled microbunching instabilities that are difficult to manage, since a complete understanding of their growth due to space charge and other wakefields is lacking. Here we present the first systematic measurements of microbunching instability using electron beams premodulated in a controlled fashion. By comparing beams having various modulation depths and wavelengths with unmodulated beams, we are able to benchmark, for the first time, the analytical calculations for the microbunching instability. In addition, our results give a proof of principle demonstration of a longitudinal space charge amplifier (LSCA), where a specific beam density pattern develops and grows. A potential application of this particular LSCA scheme is for controlling waveforms and enhancing the spectral content of linac-based sources of coherent terahertz radiation.

Comment on “Seeding, Controlling, and Benefiting from the Microbunching Instability”

“…1 in Ref. [1]). Along this path the longitudinal space charge (LSC) drives the small initial LDM into a deep EM that manifests itself in the zero-phasing energy spectra [4,5] and can be mistakenly inferred as a deep LDM.…”

“…[1,2] the authors discuss microbunching studies using experimental conditions similar to those in Refs. [3][4][5][6].…”

“…[1] and [5]) used the same facility (Source Development Lab), parameters (beam energy, charge, size, length), and technique (zero phasing [7]), and relied on the uncompressed bunch energy spectrum as the "initial condition." The observed modulation can be either attributed to a longitudinal density modulation (LDM) or to an energy modulation (EM) in the longitudinal phase space (or their combination).…”

“…[1], on the contrary, it is assumed that the EMs are induced by LSC effects along the very first (after the gun) linac rf structure, and the phase space is "basically frozen" thereafter (due to higher beam energy) until the chicane's entrance where the EM turns into the LDM and such a bunched beam, still being "frozen," travels down the linac to the spectrometer dipole. Consequently the authors interpret the zero-phasing energy spectra as the true bunching.…”

Interaction of an Ultrarelativistic Electron Bunch Train with aW-Band Accelerating Structure: High Power and High Gradient

“…In fact, this measurement only requires an integrating energy meter, such as a Golay cell, pyroelectric detector, bolometer, or diode detector. In general, two-beam interferometry can be used for tuning drive bunch train separation for wakefield accelerators [32][33][34] and microbunch trains for FELs [23,35]. The duration of the RF pulse excited by a single bunch is finite (τ = 3.4 ns in this case), so that the total number of bunches that can constructively overlap is ceiling(τ /T b ) − 1, where T b is the bunch spacing.…”