Progress toward a laser-driven x-ray free-electron laser

The use of x-ray radiation has driven the development of synchrotron sources and, more recently, x-ray free-electron lasers (FELs). These microwave-based facilities are huge and expensive, yet governments are prepared to support them (usually one per nation) because of their great value to industry, academia, and society. However, laser-wakefield accelerators (LWFAs) are now advancing to the point where compact radiation sources could be developed into a new, complementary, or even disruptive technology. In addition to reductions in scale and cost—by a factor of up to 1000—x-ray pulse durations are significantly shorter than those of conventional lasers, which could facilitate probing of ultrafast dynamic processes.

The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) project, based at the University of Strathclyde, is developing laser-plasma accelerators as drivers of radiation sources.¹ The first demonstration of a compact synchrotron source based on an LWFA was recently demonstrated using a conventional undulator and a diverging electron beam.² Our challenge is to develop a compact x-ray FEL. Here we focus on developments to improve the electron-beam properties and discuss the suitability of LWFAs as drivers of FELs. Our work shows that the once-distant prospect of a compact x-ray FEL is now within reach.

On the ALPHA-X accelerator beam line (see Figure 1), electrons are accelerated in a relativistically self-guiding plasma channel formed in a hydrogen-gas jet by a 35fs titanium-sapphire laser pulse with a power of 1J. Electrons are self-injected from the background plasma into the density wake that trails behind the laser pulse (through the combined action of the laser's ponderomotive force and the plasma's restoring force). Measurements of the electron-energy spectra are carried out using a magnetic-dipole spectrometer with a wide energy range (up to 700MeV) and excellent energy resolution (< 0.5%).

Figure 1. The ALPHA-X compact wakefield accelerator, beam-transport system, and undulator.

To date, laser-plasma accelerators have produced electron beams with rms energy spreads typically in the 2–10% range that are usually instrument-resolution limited.^2–5 Our initial results show that monoenergetic electron beams of up to 100MeV can be produced with an rms relative energy spread σ_γ/γ<0.8% (see Figure 2), where γ is the Lorentz factor. We also demonstrate effective beam transport using dipole and quadrupole magnets. We know from simulations that the electron bunches are a fraction of a relativistic plasma wavelength (i.e., ≪λ_p≈6μm for our gas-jet parameters), which implies that electrons see a large variation in potential across their length. However, the Coulomb force of the electron bunch produces its own wake, which has the effect of partially cancelling out the laser-generated wake and thus of reducing the variation in the potential. This results in a marked reduction in the energy spread (see Figure 3).

Figure 2. Electron-energy spectrum measured using a magnetic-dipole spectrometer, in arbitrary units (a.u.). σ_{γ: rms} relative energy spread_.

Figure 3. Simulations showing energy-spectrum compression due to beam loading. ξis an axial coordinate parameter showing that the electron bunch length is about 1 micron. γis the Lorentz factor.

We explore the implications for FEL operation using our 15mm period-magnetic undulator. CCD images of the transverse electron-beam profile show that the divergence is of order 2mrad. Assuming a beam size at the accelerator exit (on the basis of simulations) on the order of a few microns, we can estimate the beam's normalized emittance εn<1πmm mrad. This satisfies one condition for FEL gain, which requires the geometric emittance to be less than the wavelength, i.e., εn<λγ/4π, where λ is the radiation's wavelength (λ≈250nm for our setup).⁶

A more stringent condition exists for the energy spread. For net FEL gain, σ_γ/γ should be less than the FEL gain parameter ρ.⁶ For a 10pC electron bunch in our beam line, we estimate ρ≈0.011 and 0.002 for λ=250 and 8nm, respectively. Comparing this to the measured energy spread (upper limit: σ_γ/γ=0.008), the beam quality is close to (or may already satisfy) the requirement for net FEL gain.

We showed that the quality of the electron beams produced by our laser-plasma accelerator approaches that required for an FEL, which would substantially increase the photon yield. The next step is to pass the matched electron beam through an undulator to realize a compact vacuum-UV FEL. Our current setup should also be able to demonstrate FEL gain in the soft x-ray regime (λ<10nm).

We acknowledge support by the UK Engineering and Physical Sciences Research Council and the European Commision's New and Emerging Science and Technology Activity under the Sixth Framework Pro- gramme ‘Structuring the European Research Area’ program (project EuroLEAP, contract number 028514).