Upstream ion wave excitation in an ion-beam–plasma system

Abstract

Plasma normal modes in ion-beam-plasma systems were experimentally investigated previously only for the waves propagating in the downstream (along the beam) direction. In this paper, the ion wave excitation and propagation in the upstream (against the beam) direction in an ion-beam-plasma system were experimentally studied in a double plasma device. The waves were launched by applying a ramp voltage to a negatively biased excitation grid. Two kinds of wave signals were detected, one is a particle signal composed of burst ions and the other is an ion-acoustic signal arising from the background plasma. These signals were identified by the dependence of the signal velocities on the characteristics of the ramp voltage. The velocity of the burst ion signal increases with the decrease of the rise time and the increase of the peak-to-peak amplitude of the applied ramp voltage while that of the ion-acoustic signal is independent of these parameters. By adjusting these parameters such that the burst ion velocity approaches to the ion-acoustic velocity, the wave-particle interaction can be observed.

Observation of double pseudowaves in an ion–beam–plasma system

“…The excited wave signals were detected by measuring the perturbations in the electron density (approximately equal to the ion density perturbation because of the quasi-neutrality) with the planar probe and were monitored on a digital oscilloscope. Different from the previous observations, [27,28] here we observed two kinds of pseudowaves, depending on the beam energy. Figure 3 shows the evolutions of the received signals, detected at 123 mm from the EG (the signals are well separated at this distance), with respect to the beam energy for a given excitation voltage with τ = 3.59 µs and V pp = 16 V. In the case of high beam energy (ε b = 20 eV), the three normal modes (to be identified later) were observed, i.e., the fast beam mode (F), the slow beam mode (S), and the background ion-acoustic mode (IA).…”

“…In the downstream direction, these burst ions consist two groups: the beam and the background, since the original ions moving from the upstream to the downstream direction in the sheath regions contain these two components. This case is opposed to the case for the burst-085201-4 ions in the upstream direction where there is only background group [28] (i.e., no burst ions originating from the beam ions moving from the downstream to the upstream direction, since the ion beam only move from the upstream to the downstream direction). The average velocity of the burst ions can be approximated by [26]…”

“…Different from the experiments in Refs. [27] and [28], here the bias on the target chamber wall was kept at −40 V and the bias on the SG2, V SG2 , was varied to control the beam energy so that larger range of the beam energy (1.4 eV-21 eV) can be achieved. A cylindrical probe (CP) was used to diagnose the plasma parameters in the source chamber, and a planar probe (PP, 10mm diameter) was used both to diagnose the plasma parameters and to detect the wave signals in the target chamber.…”

“…The pseudowaves or burst-ion signals, on the other hand, can be identified from the property that the signal velocities depend sensitively on V pp and τ. [27,28] In order to confirm that the signal P1 and P2 are the burstion signals, but the signal F, S, and IA are the plasma normal modes, we present in Fig. 5 The results show that, the velocities of the F, S, and IA signals do not vary with τ and V pp , indicating that they are the plasma normal modes, whereas the velocities of the P1 and P2 signals decrease with the increase of τ and increase with the increase of V pp .…”

“…Recently, in the wave-excitation experiment conducted in an ion-beam-plasma system, a kind of burst-ion signal was observed and identified to coexist with the plasma normal modes both in downstream [27] and in upstream [28] directions. These burst-ions signals only represent one kind of the pseudowaves that are produced from the background plasma.…”