HIGH-ENERGY PHYSICS
Terawatt laser accelerates electrons
Roland Roux
Researchers from Ecole Polytechnique (Palaiseau, France), CNRS Université Paris Sud (Orsay, France), and Imperial College (London, England) have reported the acceleration of electrons by the wake field of a 100-TW ultrafast neodymium-doped glass laser. Ac cording to the researchers, this represents the first time an injected beam of electrons has been accelerated by a laser wake-field accelerator (LWFA).1
The experiment was based on a laser system located at the Laboratoire pour l`Utilisation des Lasers Intenses (LULI). The setup involves a series of cascaded amplifiers and chirped pulse amplification and produces an average output power of 100 TW in 400-fs pulses at 1057 nm (see Laser Focus World, March 1998, p. 16). The 80-mm-diameter output beam is injected into a pulse compressor and focused in a gas-filled chamber by a 1.4-m-focal-length 30° off-axis parabola. A continuous-wave 3-MeV electron beam is injected into the plasma, and the accelerated electrons are measured by magnetic spectrograph.
A series of 250 shots have been performed, most of them with the laser energy in the 4- to 9-J range. With a 1.5-J central spot energy, an output power of 3.5 TW was achieved with 4 ¥ 1014 W/cm2 maximum intensity, 1.5-GV/m electric field, and 1.6 MeV of maximum linear gain with a tail in the high-energy channels.
The combination of high-peak-power lasers and plasmas can produce very high electric fields, which, under certain conditions, will accelerate electrons to the point at which their velocities approach the speed of light (relativistic electrons). The team of multidisciplinary researchers has been investigating these conditions for several years, hoping to come up with a compact high-energy accelerator. Such systems are an important research tool for developing an understanding of the physics of elementary particles. And higher and higher energy accelerators are required to push beyond the current limited knowledge of the fundamental components of matter.
Accelerator design
Linear accelerators stimulate particles using high electric fields but are inherently limited to fields of about 1 MV/m--beyond which electrical breakdown occurs inside the metal cavities of the accelerator. To increase the energy level further, the length of the accelerator can be increased--such as at the LEP accelerator at CERN in Geneva, Switzerland, which has a circumference of 27 km--but high costs limit the practicality of this approach. Hence, the design of smaller, high-performance accelerators requires the generation of much stronger electric fields while avoiding the breakdown limitation phenomenon.
One proposed solution to this problem involves the combined use of lasers and plasmas. In a high-peak-power laser beam the transverse electric field easily reaches 100 GV/m, but unfortunately these transverse electric fields cannot be used to accelerate particles efficiently to high energies. A plasma, however, is able to convert this transverse field to a longitudinal one that is well suited to relativistic particle acceleration.
Two mechanisms have been considered for producing the electron plasma wave (EPW)--a laser beat-wave accelerator (LBWA) and a laser wake-field accelerator. In an EPW, the high electric field due to the charge separation between electrons and ions is created by ponderomotrice force (Fp), which, in an inhomogeneous electromagnetic field, pushes the electrons toward the slight field zones while the much heavier ions remain practically motionless (see Fig. 1 on p. 32). In 1994, the Ecole Polytechnique team was successful with the LBWA and produced a maximum energy gain of 1.4 MeV, leading to an electric field of 0.7 GV/m over 2.8 mm of characteristic length.
Particle acceleration by a wake field that excites the plasma wave with a single very short laser pulse is, in principle, the simplest method and more efficient than the LBWA method. It does require high laser power and extremely short pulses, however. In this case, the ponderomotive force is related to the ultrashort laser-pulse envelope--the leading edge of the pulse pushes the electrons forward while the trailing edge pushes them backward (see Fig. 2 on p. 34). In the wake of the pulse the electrons then oscillate at the plasma frequency. Maximum efficiency is obtained when the pulse duration is on the order of half the plasma period.
The optimum pulse duration is a function of the electron density. For densities larger than 1017 electrons/cm3, this corresponds to subpicosecond pulse duration.
A variant of this method--the self-modulated laser wake field--has been tried at the Rutherford Laboratory (Appleton, England) and has produced electrons with energy up to 100 MeV. Say the LULI researchers, "All the experiments during the past few years have addressed particle acceleration by combining laser and plasma. Now, they must look into the prospect of obtaining accelerators delivering beam particles with small divergence and high intensity."
REFERENCE
1. F. Amiranoff et al., Observation of Laser Wakefield Acceleration of Electrons, (April 1, 1998). Laboratoire pour l`Utilisation des Lasers Intenses, Ecole Polytechnique, CNRS, Palaiseau, France.
FIGURE 1. Beating between two laser beams with slightly different frequencies, w1 and w2, generates an inhomogeneous structure with intensity E2 and pushes electrons out of the high-intensity zones. The whole picture moves at the group velocity of the incident electromagnetic waves.
FIGURE 2. The envelope of a short laser pulse pushes the electrons forward and backward. A plasma wave is generated in the wake of the pulse.