Electron-beam interaction


The core of FLASHForward facility is the electron beam line. Its purpose is to extract beams from the FLASH2 free-electron laser, transport them with desired properties to the plasma cell, capture and diagnose them after the interaction with plasma, and ultimately transport the beams to the FEL undulators.

The pre-plasma beam line consists conceptually of three sections.

The extraction section begins with two dipoles coupling out the beam from FLASH2, at an angle of four degrees. Fast pulsed dipoles with a rise time of around hundred microseconds will be used, allowing parallel operation with FLASH2. Two quadrupoles and a sextupole are also located in this section, as well as a charge monitor, a beam position monitor, and a beam loss monitor. The overall length of the extraction section is six meters.

A fragment of the matching and final focusing section of the FLASHForward electron beamline.
From left to right: beam position monitor, quadrupole, vertical corrector dipole, horizontal corrector dipole, further quadrupoles.
The electron beams propagates from right to left.

The compression section starts directly after the safety wall separating FLASH tunnel and FLASH2 undulator tunnel. Its overall length is around 22 meters. In addition to several quadrupoles and sextupoles, this section features a reverse 0.8 degree bend, allowing flexible tuning of the longitudinal dispersion (R56). In particular, by choosing a specific combination of quadrupole strengths, the beam line can be made isochronous, that is having zero longitudinal dispersion; however, positive or negative values of R56 are also possible, allowing compression or elongation of the beam in this section. The final components of the beam line are two dipoles, making the next section parallel to FLASH2 with a horizontal separation of 4 meters.

The final section before the plasma target is the so-called matching and final focus section. It is 13 meter long and features nine quadrupoles in total. The first four quadrupoles allow matching the transverse phase-space of the beam to the design optics; The next quadrupole is used to measure the beam emittance and Twiss parameters, while the final four quadrupoles are used to focus the beam down to sizes smaller than 10 micrometers, as required by plasma-wakefield experiments.

The plasma cell is located in a cylindrical vacuum chamber of 500 mm diameter, which is placed closely after the final quadrupole of the matching section. The chamber houses a hexapod with a plasma cell on top. The hexapod allows micrometer-precision alignment of the plasma cell in all 6 dimensions (three coordinates and three rotation angles).

The post-plasma beam line of FLASHForward contains the necessary components to capture as well as diagnose the beams coming out of the plasma. Beam position, charge, energy spectrum, transverse profile, and ultimately longitudinal phase space measurements will be possible.

The ultimate goal for the post-plasma beam line is to install undulators and to demonstrate FEL gain by plasma-accelerated beams.

The experimental chamber of FLASHForward, housing the hexapod and the plasma target.
The final focusing quadrupoles can be seen directly upstream of the chamber (the electron beam propagates from right to left).
On the opposite side of the tunnel, the support structures of the variable-gap FLASH2 undulators are visible (yellow-coloured).