  Spherical quartz stones of around 1 cm in diameter have been exposed to
anodic arc discharges in a helium atmosphere at 300 Torr. The arc current
flowing between the graphite electrodes was set either in continuous DC mode
(30-150 A) or in pulsed mode at 2 Hz (220 A peak). The ablation rate in each
sample was systematically measured after several seconds of arc plasma
treatment. Optical emission spectroscopy (OES) diagnostics and 2-D fluid
simulations of the arc discharge have shed light on the heat flux transport and
the heating mechanisms of the quartz crystals. A linear correlation is found
between the absorbed power density and the resulting rate of penetration, which
yields a maximal value of 15 cm/h for approximately 150 W/cm2. The linear fit
on the slope provides a specific energy of 40 kJ/cm3. The incident energy flux
onto the sample surface promoted a phase transition from crystalline to glassy
silica, as characterized via Raman spectroscopy. This study points out the
strong potential of arc plasma technology for geothermal drilling applications.
