The goal of this experiment is to study plasma wakefield acceleration in the context of a high energy accelerator. The plasma wave or wakefield is driven by a single ultra-short (10-40 µm long), ultra-relativistic (28.5 GeV) electron bunch. In this single bunch scheme, the plasma density is adjusted so that, while the core electrons lose energy in driving the plasma wake, the trailing electrons gain energy. For the bunch lengths of interest, the plasma density is in the 1-3.5x10¹⁷ cm^-3 range. The accelerating gradient expected with a bunch with ≈1.8x10¹⁰ e^- is in the 35-45 GV/m range. A unique feature of this experiment is that the plasma is created through field-ionization of a lithium vapor by the large space-charge field of the high peak current (>10 kA) electron bunch. The electron bunch therefore creates its own plasma and accelerating structure. Recent results have shown energy gains in the 4 GeV range over a plasma length of about 10 cm, demonstrating the excitation and sustainability of gradients in the 40 GV/m range over a long distance (M. J. Hogan et al., Phys. Rev. Lett. 95, 054802, 2005). The latest experiments have shown the increase in enery gain with plasma length, with measured gains well in excess of 10 GeV over a plasma legth of only 32 cm. These experiments have also once again shown the reproducibility of the acceleration process. Present experiments aim at doubling the energy of some of the SLAC beam electrons (28.56 or 42 GeV) over a plasma length of the order of one meter.

An extensive list of publications related to these experiments can be found in the Publications page. Find more details about recent results here.

The goal of this experiment is to study plasma wakefields driven by a train a electron bunches rather that one or two bunches. In the multi-bunch plasma wakefield accelerator (MB-PWFA) scheme, a train of N bunches resonantly drives a plasma wave or wake to a large amplitude. The energy of a trailing bunch separated from the last bunch by half the distance between consecutive drive bunches can then be multiplied by N. In these experiments the number of drive bunches in the 45 MeV, 0.2-0.4 pC beam is larger than 100. The train of bunches is created by modulation of the energy of a picosecond-long electron bunch in an inverse free electron laser (IFEL) driven by a powerful carbon dioxide (CO₂) laser pulse with a wavelength of 10.2 µm. After a drift distance of ≈2.5 m the electrons are bunched longitudinally with a spacing equal to the laser wavelength (10.2 µm) and each bunch is ≈1 µm long. The corresponding resonant plasma density is ≈1x10¹⁹ cm^-3. The plasma is created in an ablative capillary discharge. The expected accelerating gradient is in the 7 GV/m range, much larger than the gradient previously measured with the non-modulated electron bunch (35 MV/m, V. Yakimenko et al., Phys. Rev. Lett. 91, 014802 2003).

The goal of this experiment is to study the excitation through Cherenkov radiation of GV/m wakefields in dielectric tubes (J.B. Rosenzweig et al., AIP Conference Proceedings, December 7, 2004, Volume 737, Issue 1, pp. 811-817). Such large wakefields can in principle be excited by the high current (<30 kA), ultra-short (>30 fs) electron bunches now available at SLAC. These GV/m wakefields can be used to accelerate a bunch trailing the drive bunch by the appropriate distance. The advantage of a dielectric wakefield accelerator over a plasma wakefield accelerator is that the process is symmetric for electrons and positrons. However, the wakefield amplitude is limited by damage of the dielectric tube. The first test beam was dedicated to the study of damage as a function of the bunch length (M.C. Thompson et al., Proc. 2005 PAC conference). Further experiments will focus on the study of the emission in vacuum of the THz frequency range wakefields.

The goal of this experiment is to use a train of 1-10 microbunches produced using a method we recently demonstrated (P. Muggli et al., Proc. PAC 07 Conference). In this method a mask is placed in a dispersive region of the beam line (the dogleg in the case of the ATF). The electron bunch is ran off crest along the linac so that it enters the dogleg withan energy/time correlation. Therfore, the mask imprints its pattern on the beam in the correlated energy/space plane in the dogleg. After the dogleg, the energy space correlation is returned to a time correlation and the bunch has a temporal microbunch structure. The pattern of the mask can be designed to produce a pattern appropriate for specific application. For PWFA experiments, the number of drive bunches can be chosen, and a witness bunch can be produced with the appropriate relative phase in the wakefield for maximum acceleration. The plasma density and the bunch train can be adjusted for maximum wakefields or maximum transformer ratio. Multi-bunch PWFAs can be used to multiply the energy of the witness bunch by the number of drive bunches (beyond energy doubling experiments), and to maximize the energy transfer efficiency.

An example of a train of five equidistant drive bunches followed by a witness bunch separted by one and a half time the distance between the drive bunches.