New developments in laser acceleration of beams

We report experimental results in which ultra-short duration (femtosecond) laser pulses from tabletop lasers are focused to intensities above 10¹⁹ W/cm² onto either gas jets or thin solid-density films. At such extreme electromagnetic field strengths (10¹¹ V/cm), plasmas are formed in which the electrons oscillate relativistically, creating gigabar pressure. The displacement of electrons-but not the heavier ions-from the region of the laser focus drives large space-charge fields (exceeding 1 GeV/cm). For laser pulses that are short compared with a plasma period, this takes the form of a wakefield, which accelerates MeV energy beams of electrons. For pulses long compared with a plasma period, we show that a Coulomb explosion accelerates protons (or other ions) to energy in excess of 10 MeV in well-collimated beams. In both cases, not only is this acceleration gradient up to a thousand times greater than in radio-frequency accelerators, but we also found that their transverse geometrical emittances are at least comparable, e.g., up to 10¹⁰ particles per pulse and divergence angles as low as 1 for electrons and 20 for protons. Additionally, the repetition rate of the electron gun is 10 Hz, a thousand-fold improvement over its past performance. In order to reduce the large electron energy spread, we show experimentally the injection of electrons into a laser-driven plasma wave by use of a separate synchronized laser pulse. Applications of these subpicosecond duration pulses include laboratory astrophysics, cancer radiotherapy, fast-ignitor fusion, radiochemistry, radiobiology, isotope production, and high-current injectors for high-energy and nuclear physics. For instance, with just a single pulse of MeV deuterons, we created a radioisotope in the reaction ¹⁰B(d,n)¹¹C. Similarly, we showed that a single pulse of electrons was sufficient for pulsed radiolysis.