Abstract:
Cosmic rays (CRs) in space are considered accelerated ions originating from collisionless
shocks. Collisionless shocks are shock phenomena caused by the interaction
between charged particles and electromagnetic fields. The interaction of solar wind
with interstellar magnetic fields is one example to form collisionless shocks. Since the
solar wind consists of various species of ions, it may form various species of CRs. The
particle acceleration mechanism in collisionless shocks is unclear, even though various
numerical simulation studies and laboratory-scale experiments have been conducted. In
this study, we performed a laboratory-scale experiment using a pulsed-power discharge
to understand the compositional effect of mixed gas plasma on collisionless shocks.
To generate fast plasma flow using the pulsed-power discharge, Tapered Cone Plasma
Focus Device (TCPFD) was used. The discharge experiments were performed for various
compositions of He and Ar gases at the controlled pressure of 0.5 Pa. The ratio of the
number density of Ar and He in the mixed gas plasma was controlled by the flow rate of
each gas. The plasma flow’s self-emission and ion current waveform was measured using
a streak camera and a Faraday cup. The experimental results showed that the velocity
of the mixed gas plasma decreases and the peak time the bulk of ions takes to arrive at
the detector increases with the increase of the Ar percentage in the mixture. Since Ar
is heavier than He, the increase in the Ar percentage means an increase in the average
mass of the mixed plasma, it affects the velocity and the peak time of the ion current.
We conclude that the composition of the mixed gas affects both the velocity and arrival
time of the plasma flow generated by the TCPFD.
