Skew Quadrupole Effect of Laser Plasma Electron Beam Transport

Abstract

Laser plasma acceleration (LPA) capable of providing femtosecond and GeV electron beams in cm scale distances brings a high interest for different applications, such as free electron laser and future colliders. Nevertheless, LPA high divergence and energy spread require an initial strong focus to mitigate the chromatic effects. The reliability, in particular with the pointing fluctuations, sets a real challenge for the control of the dispersion along the electron beam transport. We examine here how the magnetic defects of the first strong quadrupoles, in particular, the skew terms, can affect the brightness of the transported electron beam, in the case of the COXINEL transport line, designed for manipulating the electron beam properties for a free electron laser application. We also show that the higher the initial beam divergence, the larger the degradation. Experimentally, after having implemented a beam pointing alignment compensation method enabling us to adjust the position and dispersion independently, we demonstrate that the presence of non-negligible skew quadrupolar components induces a transversal spread and tilt of the beam, leading to an emittance growth and brightness reduction. We are able to reproduce the measurements with beam transport simulations using the measured electron beam parameters.

COXINEL transport of laser plasma accelerated electrons

“…Simulated and measured transverse beam shape (Figure 9a,c,d2-3) RMS horizontal (σ x ) and vertical (σ z ) sizes for the low roll angle case. degrades the transport [26]. The simulation is able to well reproduce the shape of the beam along the line (Figure 9a-d4-5).…”

“…Following the proportionality in Equation (1) [26], a change in the tilt converts into an equal effect on the emittance. The roll angle of the quadrupole magnet θ, due to its inherent imperfections, is defined as θ = − 1 2 arctan( a 2 b 2 ), with b 2 the normal quadrupolar term and a 2 the skew quadrupolar term.…”

“…and measurements of the transverse beam shape at the four screens along the line for different θ values. Measured QUAPEVA high and low θ correspond to different shimming configurations [26]. Figure 9a1-d1 presents the transport simulation with "undulator slit" optics without roll angle and the initial beam distribution of Figure 5b.…”

Electron Beam Brightness and Undulator Radiation Brilliance for a Laser Plasma Acceleration Based Free Electron Laser

“…For a typical LPA electron beam of 500 MeV and ε n = 1 mm•mrad at the source, the emittance requirement is satisfied for an operating wavelength λ ≥ 12 nm. However, unlike beam drifts in conventional accelerators where the ε n scales as γε e f f (ε e f f being the effective emittance), a problem arises regarding the transport, where the chromatic emittance ε chrom increases quadratically with the divergence along a distance s [15][16][17][18]. Such dependence can be understood by using the general definition of the normalized emittance expressed as [19]:…”

“…As mentioned in the transport line description, an electron monochromator is placed in the middle of the chicane. This helps reduce the energy spread, decrease the emittance and increase the brightness at the center of the undulator [18,59].…”