FLASHForward: plasma wakefield accelerator science for high-average-power applications

Abstract

The FLASHForward experimental facility is a high-performance test-bed for precision plasma wakefield research, aiming to accelerate high-quality electron beams to GeV-levels in a few centimetres of ionized gas. The plasma is created by ionizing gas in a gas cell either by a high-voltage discharge or a high-intensity laser pulse. The electrons to be accelerated will either be injected internally from the plasma background or externally from the FLASH superconducting RF front end. In both cases, the wakefield will be driven by electron beams provided by the FLASH gun and linac modules operating with a 10 Hz macro-pulse structure, generating 1.25 GeV, 1 nC electron bunches at up to 3 MHz micro-pulse repetition rates. At full capacity, this FLASH bunch-train structure corresponds to 30 kW of average power, orders of magnitude higher than drivers available to other state-of-the-art LWFA and PWFA experiments. This high-power functionality means FLASHForward is the only plasma wakefield facility in the world with the immediate capability to develop, explore and benchmark high-average-power plasma wakefield research essential for next-generation facilities. The operational parameters and technical highlights of the experiment are discussed, as well as the scientific goals and high-average-power outlook. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

Keywords: electrons; high-average power; plasma wakefield acceleration.

Figure 1.

Schematic of the FLASH SRF front-end supplying the FLASHForward experimental beamline with high-average-power electron beams. Also shown are the adjoining FLASH1 and FLASH2 FEL beamlines. The FLASHForward beamline occupies the same experimental interlock area as that of FLASH2. (Online version in colour.)

Figure 2.

Schematic of the FLASHForward experimental beamline, highlighting major sections and components. (Online version in colour.)

Figure 3.

(a) Longitudinal phase-space and (b) slice energy spread (red), normalized transverse emittance (black, grey), and peak current (blue) of the witness bunch after a propagation distance of approximately 15 cm simulated in OSIRIS 3D [–22]. A typical FLASHForward 1 GeV drive beam with 0.1% relative energy spread, σ_z = 25 μm rms length, σ_x,y = 6 μm rms width, 2 μm normalized transverse emittance, a charge of 500 pC, and a peak current of 2.4 kA was chosen to drive the wake. The plasma electron density at the peak of the ramp was 4 × 10¹⁷ cm⁻³ (ten times higher than that of the flat-top) with a downramp length of approximately 100 μm. (Online version in colour.)

Figure 4.

(a) Longitudinal phase-space and (b) slice energy spread (red), normalized transverse emittance (black, grey), and peak current (blue) of the witness bunch after a propagation distance of approximately 10 cm simulated in HiPACE [27]. A typical FLASHForward 1 GeV drive beam with 0.2% relative energy spread, σ_z = 40 μm r.m.s. length, σ_x,y = 5 μm r.m.s. width, 2 μm normalized transverse emittance, and a charge of approximately 500 pC was bisected and trimmed by the scraper, with the parameters optimized to provide a loaded wakefield at the point of the witness. The plasma electron density of the flat-top acceleration region was 5 × 10¹⁶ cm⁻³. (Online version in colour.)

Figure 5.

Schematic of the 10 Hz macro-pulse and MHz bunch train structure available to FLASHForward for high-power experimentation. (Online version in colour.)

Figure 6.

Average-power capabilities (as a function of the number of bunches per second their drivers can deliver) of state-of-the-art LWFA (blue squares) and PWFA (red triangles) facilities. The white circles represent the state-of-the-art or next generation of photon science and collider facilities. By implementing certain infrastructural changes to FLASHForward, PWFA research would move into the region of phase space necessary for future facilities. (Online version in colour.)

Figure 7.

Schematic of the proof-of-principle effective MHz experimental set-up illustrating two driver-witness bunch-pairs with variable separation interacting with two distinct plasmas generated by consecutive high-voltage discharges. (Online version in colour.)

Figure 8.

Schematic of the proposed beamline scheme for separation of driver and witness bunches in high-power multi-bunch trains. (Online version in colour.)

Figure 9.

Schematic of the proposed beamline modifications necessary to use the FLASHForward X-TDS for characterization of MHz PWFA bunch trains. (Online version in colour.)