Tunable high gradient quadrupoles for a laser plasma acceleration based FEL

Abstract

Laser Plasma Acceleration (LPA) is capable of producing a GeV beam within a cm accelerating distance, but with a rather high initial divergence and large energy spread. COXINEL aims to demonstrate a compact Free Electron Laser using such a source, where a specific transport line with adequate elements is used, such as tunable high gradient quadrupoles for handling the divergence. An innovative permanent magnet based quadrupole (QUAPEVA) made of two quadrupoles superimposed capable of generating a gradient of 200 T/m is presented. The first quadrupole consists of magnets shaped as a ring and attaining a constant gradient of 155 T/m, and the second one made of four cylindrical magnets surrounding the ring and capable of rotating around their axis to achieve a gradient tunability of ± 46 T/m. Each tuning magnet is connected to a motor and controlled independently, enabling the gradient to be tuned with a rather good magnetic center stability (±10 µm) and without any field asymmetry. The measurements and field optimization of seven quadrupoles with different magnetic lengths are reported. A set of QUAPEVA triplet, installed at COXINEL, achieved good focusing and enabled beam based alignment.

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

“…The laser beam is focused by an off-axis parabola into a gas mixture composed of 99% He and 1% N 2 . The first magnetic element is a triplet of tunable high gradient permanent magnet-based quadrupoles (QUAPEVA) [23,45,46], placed 5 cm from the electron source. The triplet strongly focuses the LPA electron beam and permits to handle the large divergence at an early stage of the transport and mitigate the emittance growth.…”

Skew Quadrupole Effect of Laser Plasma Electron Beam Transport

“…Electrons were produced and accelerated by a Titanium:Sapphire laser focused on a gas jet. A triplet of permanent magnets quadrupoles with tunable gradient (QUAPEVA [57,58]) positioned 5 cm from the source handles the bit of mrad initial divergence of the electron beam via a strong magnetic field, insuring a proper beam focusing. Then, a demixing chicane consisting of four electro-magnetic dipoles accompanied with a removable and adjustable slit placed in the middle stretches the beam longitudinally, sorts electrons by energy, and then selects the energy of interest via the slit.…”

“…Strategies are proposed to mitigate this, such as an adiabatic matching section [46], tailoring focusing profiles [47] or creating a decreasing plasma gradient after the source [48]. Another robust approach consists of capturing the electron beam right after the source via strong focusing magnetic fields as close as possible to the plasma-vacuum interface with conventional accelerator techniques (e.g., the use of FODO with quadrupoles) [34,49,50]. The required gradient in a compact device is challenging.…”

“…Because of the expertise gained in accelerator facilities, the use of permanent magnet quadrupoles (PMQs) appears to be slightly more robust in respect of APL. The required gradient level corresponds to values achieved for prototype systems [49,[56][57][58][59][60][61][62][63][64][65][66][67][68] rather than quadrupoles produced in large amounts. Moreover, special care has to be taken for the design of these PMQs, in particular for their multipolar content in order to minimize the beam spot size while avoiding chromatic aberration effects dominance [56,69].…”

Permanent Magnet-Based Quadrupoles for Plasma Acceleration Sources

“…For a bore radius of 12.5 mm, the achieved gradient can be varied from 50 to 102 T/m. • Seven systems (QUAPEVA) of type (V) have been built in collaboration between SOLEIL and SigmaPhi [52,53] for the COXINEL project with an objective of FEL amplification using an LPA source. The QUAPEVA cosists of Nd 2 Fe 14 B magnets and permendur poles achieving a maximum gradient of 202 T/m and 45% tunability.…”