Dynamics of potentials of a plasma jet heated by neutral beams

Experiments in AMBAL-Yu mirror system [1] revealed an unexpected and intere sting phenomenon: a substantial decrease in the density of the target plasm a jet during the injection of high-energy hydrogen atoms. The AMBAL-Yu is a classical mirror with ninimum-B. Beams of 16 keV hydrogen atoms were injected perpendicularly to the axis. Before the injection, the device was filled with a injected perpendicularly to the axis. Before the injection, the device was filled with a plasma with a density of 2.4 x 10(1 4) cm(-3) and with characteristic electron and ion temperatures of about 10 eV. The plasma jet was produced by an are source with a slot discharge cha nnel [2], which was located outside the mirror near its end system. The pla sma jet generated by this source entered the mirror cell along the magnetic field lines. The jet was subsonic upstream and supersonic downstream from the exit side of the mirror cell. Due to the injection of atomic beams, a p opulation of hot ions with an average energy of 6 keV and density up to 1.1 x 10(13) cm(-3) was generated in a plasma volume of 31. At the same time, the density of the target plasma jet decreased significantly (by a factor o f 2.5). Here is one of the first explanations of the phenomenon. A cold ion is repl aced by a hot one with the larger Larmor radius due to charge-exchange of a high energy atom on a cold ion. Therefore, the total density of ions incre ases on the periphery and decreases in the center. As the end is a conducto r the electrons in the center flow out of the mirror along the magnetic fie ld lines, and on the periphery they flow inside. The development of two-flo w instability and the transport of plasma across the magnetic field lines a re possible. But the measurements carried out with a cesium low-energy atom analyzer [3] did not show any significant transverse plasma flow. Another variation of the explanation is the following. As hot ions in the occupied area heat electrons directly due to collisions, then a subsonic electron fl ow is accelerated. The electrons leave the hot ion region more quickly, and the flow from the gun is not changed. A longitudinal electric field appear s at the boundary that accelerates the ions (note that the longitudinal ele ctric field here has another sign according to the right model described be low). The plasma flow is accelerated, the plasma density decreases. But thi s model does not explain many experimental results.