Two of the key issues of a krypton fluoride (KrF) laser driver
for inertial fusion energy are the development of long life,
high transparency pressure foils (to isolate vacuum in the electron
beam diode from a working gas in the laser chamber), and the
development of durable, stable, optical windows. Both of these
problems have been studied on the single-pulse e-beam-pumped
KrF laser installation GARPUN. We have measured the transport
of electron beams (300 keV, 50 kA, 100 ns, 10 × 100 cm)
through aluminum-beryllium and titanium foils and compared them
with Monte Carlo numerical calculations. It was shown that
50-μm thickness Al-Be and 20-μm Ti foils had equal
transmittance. However, in contrast to Ti foil, whose surface
was strongly etched by fluorine, no surface modification nor
fatal damages were observed for Al-Be foils after ∼1000
laser shots and protracted fluorine exposure. We also measured
the 8% reduction in the transmission of CaF2 windows
under irradiation by scattered electrons when they were set
at 8.5 cm apart from the e-beam-pumped region. However an applied
magnetic field of ∼0.1 T significantly reduced electron
scattering both across and along the laser cell at typical pumping
conditions with 1.5 atm pressure working gas. Thus the
e-beam-induced absorption of laser radiation in optical windows
might be fully eliminated in an e-beam-pumping scheme with magnetic
field guiding.
Transport of electron beams and stability of optical windows
in high-power e-beam-pumped krypton fluoride lasers
