  A self-consistent hybrid model of standing and moving striations was
developed for low-current DC discharges in noble gases. We introduced the
concept of surface diffusion in phase space described by a tensor diffusion in
the nonlocal Fokker-Planck kinetic equation for electrons in the collisional
plasma. Electrons diffuse along surfaces of constant total energy between
energy jumps in inelastic collisions with atoms. Numerical solutions of the
1d1u kinetic equation for electrons were obtained by two methods and coupled to
ion transport and Poisson solver. We studied the dynamics of striation
formation in Townsend and glow discharges in Argon gas at low discharge
currents using a two-level excitation-ionization model and a full chemistry
model, which includes stepwise and Penning ionization. Standing striations
appeared in Townsend and glow discharges at low currents. Moving striations
were obtained for glow discharges at currents exceeding a critical value. They
originate at the anode and propagate towards the cathode. We have obtained two
types of moving striations with the two-level and full chemistry models, which
resemble the s and p striations previously observed in the experiments.
Simulations indicate that processes in the anode region could control moving
striations in the positive column plasma. The developed model helps clarify the
nature of standing and moving striations in DC discharges of noble gases at low
discharge currents and low gas pressures.
