The Neptune lab at UCLA has had the demonstration and improvement of plasma beat-wave acceleration (PBWA) as a primary objective since its founding as a collaboration between the PBPL and Prof. Chan Joshi's laser-plasma group. An ongoing series of experiments utilizes the relativistic (12-14 MeV), low-emittance electron beam from the Neptune rf photoinjector and the terawatt MARS CO2 laser to probe laser-generated relativistic plasma waves with electron bunch injection, continuing both to demonstrate improved PBWA results and to elucidate the mechanisms responsible for plasma-wave creation and guiding. (These experiments build on UCLA's experience with PBWA since its first demonstration by Prof. Joshi in 1993.) Since any accelerator created in a plasma is limited in length to a few centimeters by the diffraction of the drive laser, staging must be implemented to reach energies of interest to high-energy physics using laser-plasma techniques. Both injection and extraction of a high-quality electron beam are thus of great importance to further development of the PBWA. The combined facilities and expertise of the beam and laser physics groups at UCLA put the PBPL at the forefront of groups working on this challenging problem.
The most recent stage of PBWA experiments took place between September 2001 and October 2002, and was designed to examine the details of timing and injection. In these runs, a beat-wave was produced in hydrogen gas at a density of 10^16 per cc by injecting high-power CO2 laser pulses, 100-200 ps in length, with wavelengths of 10.3 and 10.6 um. The two-frequency laser pulse both produces the plasma and creates a relativistic plasma wave with a wavelength of 340 um. Diagnosis of the wave was accomplished using collinear Thomson scattering from a green probe beam that was much smaller than the wave dimension, allowing the wave fields to be mapped axially and longitudinally. It was discovered that the location of the peak plasma fields was more than one Rayleigh length upstream from the vacuum laser focus, an effect due to ionization-induced refraction in the plasma. The plasma brightness was also found to be strongly correlated to the local plasma wave energy.
An electron bunch of length 10 ps at an energy of 12 MeV was injected into this plasma wave from the Neptune photoinjector and measured after the plasma chamber in a magnetic spectrometer. Since the plasma wavelength at this density is ten times shorter than the electron bunch length, one must expect 100% energy spread in the output. The electron bunch is automatically temporally synchronized with the laser pulse within tens of nanoseconds, because the photocathode drive laser is used to switch one stage of the CO2 laser amplification, but must be further synchronized at the picosecond level for injection into the short-lived beat wave. This is accomplished using electron-beam controlled transmission of the CO2 light through a 1-mm thick germanium crystal. In the absence of an electron beam, CO2 is transmitted readily through the germanium, but when the electron beam impacts the germanium a long-lived plasma is created which blocks transmission of the laser light; hence laser light detected downstream of the germanium indicates that the laser arrived at the germanium before the electron bunch. During the experiment, we scan the picosecond timing of the laser beam using a movable delay line; the time window in which the electron beam and laser beam overlap is the region in which the germanium changes from opaque to transparent. This determination was made without the final stage of amplification, since laser pulses of over 100 J would destroy the germanium, so additional delay has to be added to compensate for the change in refractive index due to the inverted medium in the final amplifier. The experimentally-obtained value for this delay was 120 ps, with an overall timing uncertainty between electron bunch and laser pulse of 20 ps (about 10% of the laser pulse length).
Diagnosis of the electron beam's maximum final energy is complicated by the low flux of high-energy electrons, with most injected particles remaining unaccelerated--a result of the electron beam's large size compared to the plasma accelerator. A phosphor screen on the spectrometer output was used for energies up to 15 MeV, but low numbers of electrons are expected up to 50 MeV. Surface-barrier detectors (SBDs) are used to record incident electrons at these higher energies, but as they are also sensitive to other ionizing radiation, including x-ray photons, they must be carefully shielded and compared with various null tests to remove spurious signals.
In initial experiments conducted at resonant pressure, using a short (~100 ps) CO2 laser pulse, electron energy gain reached 10 MeV, considerably less than expected. It was found that the ionization-induced refraction inferred from Thomson scattering limited the acceleration length to about 1 cm by defocusing the laser, a result consistent with the amount of energy gain. Various channel-guiding mechanisms have been proposed for overcoming diffraction in laser-plasma experiments; in the PBWA it was possible to achieve guiding by simply using a longer laser pulse (>220 ps) with a short rise time and long fall. With the laser pulse length longer than the characteristic time for ion motion, an ion channel is created during the first part of the laser pulse that then guides the later part of the pulse. This self-guiding effect, verified with PIC simulation, resulted in electron energy gain of 35 MeV when longer pulses were used, indicating an accelerator more than three times longer than in the short-pulse case.
These results have been accepted for publication in Physical Review Letters; highlights were presented at PAC2003.
Using the knowledge gained in the first phase of the PBWA experiment, we plan to investigate the extent to which electron beam quality can be preserved during beatwave acceleration by injecting highly compressed and tightly focused beams into the accelerator, which will lead to much smaller energy spreads at the output. This desire has motivated several years of basic beam-physics experiments in the PBPL, seeking to understand the effects of magnetic and ballistic compression on beam phase space, as well as work on IFEL bunching that will build on the current IFEL experiment.