Ion Acceleration Independent of the Electric Current Direction in Z-Pinch Plasma

Abstract

Ions accelerated in z-pinch experiments with either positive or negative discharge were measured by using a Thomson Parabola analyzer in order to understand the directional tendencies of the ion acceleration. Ions having energy on the order of MeV were observed in both positive and negative discharges. The velocity and energy of each ion species were measured to be considerably similar in magnitude, in spite of the difference in the polarity of the power supplies. The highest-velocity ions with different charges in each measurement lay on the constant velocity line. The model independent of the current direction should be considered as the main mechanism of ion acceleration in this study. Accelerated ions having energy on the order of MeV have been observed in many studies of high current experiments, particularly in the plasma foci [1, 2] and z pinch systems [3,4]. Charged voltages of the main power supplies of these systems are several tens of kV, thus the understanding of any acceleration mechanism is required. Some acceleration models are discussed as the potential mechanisms of the acceleration, such as an induced electromotive force caused by increase of inductance [5,6] and/or resistivity [7,8], the kinematic effect under the calculated electric fields [9,10], and a displacement current caused by the blocking of the conduction [11]. In all the above theories, the direction of the accelerated ions are assumed to be the same as that of the electric currents. The models of these theories are collectively called the "current-directional electric field model" in this study. Although few experimental observation [3] have reported the reversed acceleration of ions, no comparative experiments have been carried out. Because it is difficult to measure the electric field in the fast and microscopic phenomena of pinch, the nature of the particle acceleration process by the electric field still remains undefined. Experimental observations using another approach are needed. Assuming that the above-mentioned current-directional electric field model is the main mechanism, the reversal of polarity of the power supply should reverse the acceleration direction of ions. Accordingly, either positive or negative power supplies were examined in the same gas-puff z-pinch experiment system in this study.

Transmittance Measurement of Accelerated Ions and Electrons in Divergent Gas-Puff <inline-formula> <tex-math notation="LaTeX">$Z$ </tex-math></inline-formula>-Pinch Plasma

“…These results indicate that ions can be accelerated to have similar energy in both positive and negative discharges. This qualitative tendency in positive and negative discharges is consistent with the previous data shown using Thomson parabola analyzer [13]. The maximum energy of ions in the study is larger than those in the previous study.…”

“…A characteristic ion acceleration has been observed in the divergent gas-puff z-pinch experiment [13]. The results indicated that the acceleration direction of the ions was not affected by the discharge current direction.…”

Emission of high-energy ions in the divergent gas-puff z-pinch plasma

“…High-energy ions up to 1 MeV have been observed in the divergent gas-puff z-pinch experiment. As the ion acceleration has also been observed in the experiment of current reversal, the acceleration has been attributed to some mechanism independent of the current direction 1 . The result indicates that the acceleration mechanism is not the electromagnetic induction, and the understanding of the mechanism is desired.…”

High-energy electron acceleration in the gas-puff Z-pinch plasma

“…In both of the positive and negative discharges, the xray was generated only from the anode surface, so it was considered that the electrons were accelerated by the induced electromagnetic force at the pinch time. Although ion acceleration independent of current direction has been observed in the divergent gas-puff z-pinch experiment [2], the acceleration mechanism may be different between the ions and electrons.…”

“…These ions have been believed to be accelerated by the induced electromagnetic force at the pinch time. On the other hand, ion acceleration independent of the current direction has been observed in the divergent gas-puff z pinch [2].…”