Laser electron accelerators for radiation medicine: a feasibility study

Table-top laser wakefield accelerators (LWFAs), proposed theoretically in 1979, have now generated individual electron bunches in the laboratory with a significant number of electrons having energies up to 10 MeV and beyond with the maximum energy reaching tens of MeV and charge per laser pulse of > 1 nC. The attained electron beam properties have stimulated a discussion about the possible applications of LWFAs to medical radiation treatment, either directly or via conversion to x-rays. Our purpose in this paper is to analyze whether or not such applications are feasible, or can be made feasible with existing laser technology. Clinical electron beam applications require the selection of specific electron energies in the range of 6-25 MeV with a narrow energy bin (deltaE <5 MeV) for depth control, and a beam expansion to as much as 25 cm x 25 cm for various tumor radiation treatments. As a result, we show that present LWFA sources provide a dose rate that falls short of the requirements for clinical application by at least an order of magnitude. We then use particle simulations to evaluate the feasibility of developing an improved LWFA-based medical accelerator. Current LWFA sources require such high peak intensity that laser repetition rate is restricted to < or = 10 Hz. A scheme to lower the threshold and increase the repetition rate of efficient LWFA thus appears essential. We analyze one such scheme. We show that by "seeding" the primary laser pulse with a second, hundred-fold less intense pulse that is shifted downward in frequency by approximately the plasma frequency omegap, LWFA produces a yield of clinically useful electrons per pulse comparable to that provided by an unseeded source, except that the primary pulse energy is now more than one order of magnitude lower than that in current LWFAs. This enables a repetition rate of approximately 100 Hz or more using existing laser technology, and thus dose rates (several Gy/min) in the range required for medical radiation applications.