Lasers and new methods of particle acceleration Page: 4 of 11
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no breakdown limits since it is already ionized. Further, the plasma can support large longitudinal waves in which
the electrons oscillate with w, = (47re2n/m)1/2, due to the space charge of the immobile ion background (regardless
of the wavelength). One can create a relativistic plasma wave by properly phasing these oscillations such that vd 2 c.
So that, the electron can reach relativistic energies before dephasing from the wave. The accelerating gradient of a
relativistic plasma wave can be expressed as n~ur'" vn' V/cm, where n'"'' is the density of the perturbed wave, n is
the plasma density in cm-3. Relativistic plasma waves can be generated by propagating intense laser beams or intense
particle beams. In the following sections we discuss laser and particle beam-driven acceleration schemes. The laser
wake-field acceleration (LWFA), the plasma beat-wave accelerator (PBWA) and the self-modulated laser-wake-field
accelerator (SMLWFA) concept are illustrated in Figures 1, 2, and 3 respectively.
In the laser wake-field acceleration (LWFA) a single short laser pulse of length L excites a plasma wave of
wavelength A,. In this scheme L ~ Ap. This method requires short, 1 pico-second, laser pulses of ultra high intensity
21018 W/cm2 and could not be tested until chirped-pulse amplification (CPA) was used to create Table-Top Terawatt
(T3) lasers. Several papers on progress in T3 technology based on CPA in solid state lasers were presented at this
The plasma beat-wave accelerator (PBWA) was proposed earlier as an alternative to LWFA because short-
pulse, high-power lasers were not available. This approach employs two long pulse laser beams of slightly different
frequencies w1 and w2 such that wi - w2 w, the frequency of the plasma wave which is to be resonantly excited.
PBWA experiments have been performed in Japan (ILE), the USA (UCLA), Canada (CRL) and France (LULI). The
UCLA experiment observed the highest electron energy gain, -28 MeV [Clayton et al.], with an effective accelerating
gradient of 2.8 GV/m. They plan to continue with PBWA experiments.
short laser pulse
TL -, X/2c
Figure a Schematic of Laser-driven Plasma-Wake acceleration
P requires two
laser beams with
Figure 2: Schematic of Plasma Beat-Wave accelerator (PBWA).
One of the impressive advances reported is in the area of self-modulated laser wakefield acceleration (SML-
WFA). In this method, a laser pulse of length L > X, is subdivided into a series of shorter pulses of length ~ A,/2 by
its interaction with the plasma wave (which it created). This interaction creates a large amplitude (resonantly driven)
plasma wave. This process requires a laser power greater than the critical level required for relativistic guiding of the
laser field. The phase velocity of the guiding plasma wake can become relativistic for high enough plasma electron
densities, for example n, ~ 1019 cm-3.
Experiments on SMLWFA have been performed in Japan (KEK), the US (LLNL, CUOS, NRL) and the
UK (RAL). The latter experiment achieved impressive results: electron energy gains of X44 MeV and accelerating
gradients X100 GV/m. Conventional accelerators are capable of accelerating gradients of - 100 MV/m. This
experiment employed a 2.5 TW, 0.5 picosecond laser, producing an intensity of 1019 W/cm2 and a plasma electron
density of 1019cm-3. The accelerated electrons in this experiment cover a wide range of energies from a few MeV
up to the maximum. The theoretical limit for this experiment was - 70 MeV. The spectrometer was capable of
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Parsa, Z. Lasers and new methods of particle acceleration, article, February 1, 1998; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc706701/m1/4/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.