Since their invention, lasers have played important roles in research at particle accelerators. This link is set to continue growing ever closer, as Chan Joshi explains.

Accelerating with lasers

Lasers are now being used directly to produce medium-energy (100 MeV–1 GeV) electron beams (Leemans and Esarey 2009). Indeed, laser particle acceleration has grown into a distinct subfield of research since the first Laser Acceleration of Particles Workshop at Los Alamos in 1982. A short but intense laser pulse propagating through plasma can excite a wave in space-charge density, also called a wake, behind the pulse. The longitudinal electric field of this wake can be tens of giga-volts per metre, which is large enough to capture some of the plasma electrons and accelerate them (figure 3). However, the wake propagates at a phase velocity that is equal to the group velocity of the laser pulse in the plasma. Since the group velocity of a photon packet in a medium is always less than the speed of light, the accelerating electrons continuously dephase with respect to the wake. A combination of beam loading and dephasing leads to a quasi-monoenergetic beam of electrons whose energy increases as the plasma density is decreased. The transverse spread of the electrons can be a few microns and the emittance less than 1 mm mrad. Several groups are embarking on research programmes to demonstrate the coherent amplification of undulator radiation, with the eventual goal of demonstrating a tabletop, extreme-ultraviolet FEL based on a laser-wakefield accelerator (LWFA).

Although a laser-based plasma accelerator operating at the energy frontier is at this stage far into the future, the US Department of Energy (DoE) has funded the construction of a research facility called BELLA at LBNL whose goal is to demonstrate a 1 m-scale 10 GeV LWFA that can then be staged multiple times to give high energies (CERN Courier January/February 2010 p8).

An alternative approach is to use a laser pulse to produce an accelerating electromagnetic mode directly in a miniature photonic band-gap structure or a slow wave structure in a plasma medium. It is too early to say what the eventual architecture of a high-energy accelerator based on these concepts would look like but the research is fascinating in its own right.

A bright future

In the future we are likely to see even greater merging of lasers and accelerators. Laser CS has been proposed as a method for generating polarized positrons for a future e+e– collider using a high-finesse laser cavity in conjunction with an electron storage ring operating at a few giga-electron-volts. In this proposal the electron micro-bunches collide with (the circularly polarized) laser photons circulating in the cavity to produce the CS photons. These polarized multimega-electron-volt photons then collide with a target of high atomic number (Z) to produce a copious number of polarized positrons via pair production (Araki et al. 2005).

A CS-based gamma–gamma collider would be a natural second interaction region for any future e+e– collider because cross-sections for some reactions are larger for gamma–gamma collisions than for e+e– collisions (Telnov 1990). With a proper choice of laser wavelength and intensity, much of the electron energy can be converted into the gamma-ray photon and, with a net yield of about one photon per electron, the final luminosity of a gamma-gamma collider can be comparable to that of an e+e– collider (Kim and Sessler 1996). While the peak power (1 TW) and the pulse width (1 ps) required for the laser used in a gamma–gamma collider are easily obtained today, the repetition rate of such lasers is still a couple of orders of magnitude lower than in state-of-the-art lasers. There is reason for optimism, however, because diode-pumped solid-state lasers appear promising for achieving the high average powers needed.

Other possible uses of laser CS photons are for nuclear spectroscopy, where the transition energies are in the multimega-electron-volt range, as mentioned above, and for the detection of hidden fissionable materials via the observation of nuclear resonance fluorescence (NRF). If the line-width of the CS photons can be made to be less than that of nuclear transitions, then such a source could revolutionize nuclear spectroscopy much in the same way that tunable lasers have transformed atomic spectroscopy. An example of an ambitious CS source is MEGa-ray (mono-energetic gamma-ray), now under development at LLNL. It uses a state-of-the-art 250 MeV, X-band accelerator to generate an extremely bright beam of electrons at an effective repetition rate of 1 kHz, together with a high average power, picosecond laser to generate high fluxes of narrow-bandwidth mega-electron-volt photons for NRF (Gibson et al. 2010). A kilo-joule-class nanosecond laser end-station is proposed at the LCLS facility to generate matter of high energy-density that will then be probed by the highly directional X-rays from the FEL.

Laser cooling normally conjures up images of cooling atoms of low thermal energy. However, at a number of places, laser cooling has already been demonstrated on high-energy beams. For example, experiments at GSI, Darmstadt, have used laser cooling on C3+ ions at around 1.5 GeV, leading to an unprecedented momentum spread of 10–7. Laser cooling has been proposed as a method for achieving beams of ultra-low emittance for future e+e– linear colliders (Telnov 2000).

There is no doubt that lasers will play an ever increasing role in accelerators, and vice versa.

About the author

Chan Joshi, University of California Los Angeles.

Further reading

R Akre et al. 2008 PRST-AB 11 030703.

S Araki et al. 2005 CLIC Note 639. C Bemporad et al. 1965 Phys. Rev. 138 1546.

D L Burke et al. 1997 Phys. Rev. Letts 79 1626.

A L Cavalieri et al. 2005 Phys. Rev. Letts. 94 14801.

M Cornacchia et al. 2001 SLAC-PUB-8950. D J Gibson et al. PRST-AB, to be published.

K J Kim and A Sessler 1996 SLAC Beam Line Spring/Summer 17.

W Leemans and E Esaray 2009 Physics Today 62 44.

R W Schoenlein et al. 1996 Science 274 236.

R L Sheffield et al. 1988 Nucl. Inst. Meth. A272 222.

T Shintake 1992 Nucl. Inst. Meth. A311 453.

V I Telnov 1990 Nucl. Inst. Meth. A294 72.

V I Telnov 2000 Nucl. Inst. Meth. A455 80.

Source: CERN COURIER WEBSITE