Plasma sources for future plasma wakefield accelerator based electron/positron collider

Abstract

The recent experimental progress in plasma wakefield accelerator (PWFA) experiments [I. Blumenfeld et al., Nature 445, 741 (15 February 2007)] make it possible to contemplate the application of the PWFA to a future electron positron linear collider. One of the main parameter of such a collider is the luminosity. The luminosity determines the number of occurrence of a given process with a given cross section, per unit time. For example, current designs call for luminosities in the1034 cm−2s−1. To reach such luminosities, particle beams with average powers in the 10 MW range are necessary. In a plasma wakefield accelerator, a plasma works as an energy transformer to convert a relatively high-current, low-energy bunch train to a proportionately lower-current, higher-energy bunch train. This transformation is continued until the desired particle energy is reached to do physics. Microwave powered structures have a demonstrated ability to produce bunch trains with parameters of interest to the PWFA with high efficiency. Such structures operate most efficiently when the bunches are spaced tightly together, e.g. by a few ns. Consequently, the plasma source must be stable and reproducible on the ns time scale. Assuming an energy transfer efficiency from a PWFA drive bunch to the accelerated witness bunch of 35% implies that about 6.5 MW of beam power is left in the plasma. For a beam with an initial energy of 25 GeV, a final energy of 250 GeV and an accelerating gradient of 25 GeV/m, the total plasma length is 9 m, corresponding to an energy deposition of ≈720 kW/m. The energy will be deposited in a very small volume of plasma, typically a radius of about 100 µm for densities of interest. Therefore, while heat evacuation at a large radius may not be a serious issue, thermal shock waves can be expected. These waves will modify the gas and plasma density for the following bunches. The beam and plasma source parameters will be presented, and the main issues with the plasma source discussed.