4 research outputs found
Non-modal stability analysis and transient growth in a magnetized Vlasov plasma
Collisionless plasmas, such as those encountered in tokamaks, exhibit a rich
variety of instabilities. The physical origin, triggering mechanisms and
fundamental understanding of many plasma instabilities, however, are still open
problems. We investigate the stability properties of a collisionless Vlasov
plasma in a stationary homogeneous magnetic field. We narrow the scope of our
investigation to the case of Maxwellian plasma. For the first time using a
fully kinetic approach we show the emergence of the local instability, a
transient growth, followed by classical Landau damping in a stable magnetized
plasma. We show that the linearized Vlasov operator is non-normal leading to
the algebraic growth of the perturbations using non-modal stability theory. The
typical time scales of the obtained instabilities are of the order of several
plasma periods. The first-order distribution function and the corresponding
electric field are calculated and the dependence on the magnetic field and
perturbation parameters is studied. Our results offer a new scenario of the
emergence and development of plasma instabilities on the kinetic scale.Comment: 6 pages, 5 figure
Positive and negative streamers in ambient air: modeling evolution and velocities
We simulate short positive and negative streamers in air at standard
temperature and pressure. They evolve in homogeneous electric fields or emerge
from needle electrodes with voltages of 10 to 20 kV. The streamer velocity at
given streamer length depends only weakly on the initial ionization seed,
except in the case of negative streamers in homogeneous fields. We characterize
the streamers by length, head radius, head charge and field enhancement. We
show that the velocity of positive streamers is mainly determined by their
radius and in quantitative agreement with recent experimental results both for
radius and velocity. The velocity of negative streamers is dominated by
electron drift in the enhanced field; in the low local fields of the present
simulations, it is little influenced by photo-ionization. Though negative
streamer fronts always move at least with the electron drift velocity in the
local field, this drift motion broadens the streamer head, decreases the field
enhancement and ultimately leads to slower propagation or even extinction of
the negative streamer.Comment: 18 pages, 10 figure