Electrostatic Ion - Cyclotron Waves in a Two - Ion Component Plasma

The excitation of electrostatic ion-cyclotron (EIC) waves is studied in a single-ended Q machine in a two-ion-component plasma (Cs+ and K+). Over a large range of relative concentrations of Cs+ and K+ ions, two modes are excited with frequencies w , z Q,,, and w2 t QK+, where the Q's are the respective cyclotron frequencies. The results are discussed in terms of a fluid theory of electrostatic ion cyclotron waves in a two-ion component plasma. Electrostatic ion-cyclotron (EIC) waves in single-ion component plasmas have been studied for a number of years in the laboratory [1]-[8]. In recent years further work has been motivated by observations in the polar magnetosphere of electrostatic waves with frequencies slightly above the first few harmonics of the H + cyclotron frequency [9], [lo]. The earth's ionosphere, however, generally contains a mixture of several positive ion species, e.g., H', O', NO', with concentrations varying according to altitude. The problem of EIC wave excitation in such a case was first studied by Kindel and Kennel [ 111 using the kinetic theory developed earlier by Drummond and Rosenbluth [ 121. There have been a few experiments on EIC waves in multi ion-species plasmas. Ono et al. [13] studied the parametric excitation of EIC waves in a He+-Ne+ plasma. A pump electric field was coupled into the plasma by an external RF oscillator, and the propagation of the resulting decay waves was investigated. Sugai et al. [14] launched waves in the ion cyclotron frequency range into a He+-Ne+ plasma using an electrostatic antenna. They concentrated their studies on the EIC wave characteristics in the presence of light minority ions with small concentrations ( 1 percent). One experiment has been performed in which the EIC waves were excited internally by passing a current through a plasma which contained K+ and Baf ions [15]. This work was mainly concerned with the question of ion heating by EIC waves in a two-ion-species plasma. We report here some preliminary results on EIC wave excitation in a steady-state plasma containing variable concentrations of Cs' and K+ ions. Our experiments were conducted in a single-ended Q machine [16] in which an alkali metal plasma is produced by contact ionization of cesium or potassium atoms (or both) on a hot tantalum plate ( Tp = 2200K). The plasma column typically has densities in the range ne = 5 X lo9 ~ m ~ 5 X 10" ~ m ~ , and temperatures T, = T, = 0.2 eV, and is confined by a longitudinal magnetic field variable up to about 4 kG. Two independent atomic beam ovens (K and Cs) are used simultaneously to produce plasmas with a mixture of K+ and Cs+ ions. Estimates for the individual ion concentrations were obtained by monitoring the change in overall plasma density (using a Langmuir probe) as the neutral atomic flux from only one oven was varied at a time. We are developing a mass spectrometer for more accurate determinations of the ion concenManuscript received December 14, 1987. This work was supported by The authors are with the Department of Physics and Astronomy, UniIEEE Log Number 8820934. the U.S. office of Naval Research and by NASA. versity of Iowa, Iowa City, IA 52242-1479. trations to permit detailed studies of the EIC mode excitation as the concentrations are varied. The ion-cyclotron instability was excited by drawing an electron current to a small (8-mm diameter) metallic disk (thus with a radius a few times the gyroradius of the heavy-ion species but smaller than the plasma column), located on the axis of the plasma column, biased at a few volts above the plasma potential. This disk is located 8 mm in front of an electrically floating, cold end plate, 88 cm from the hot plate. The instability is detected by observing the fluctuations in the exciter disk current. Typically, EIC wave amplitudes of A n / n = 30-40 percent are observed. The work described in this paper was primarily concerned with the basic question of what type of EIC modes might be expected in a plasma containing roughly comparable (neither component being a small fraction of the total) concentrations of K+ and Cs+ ions. Typically, we observe, for example, that as Cs+ ions are introduced into an initially pure K+ plasma, there is a relatively large range of Cs+ concentrations for which both K+ and Cs' modes are observed. As the Cs+ concentration is further increased, a point is reached when the K+ modes disappear. A similar behavior is observed starting from a pure Cs+ plasma. Fig. 1 shows the spectrum of fluctuations of the current to the exciter biased at + O S V, under the conditions B = 2500 G, ne = 2 x 10" ~ m ~ , and roughly 80 percent K+ and 20 percent Cs+ ion composition. We identify the two largest spectral peaks (Cs l and K1) as corresponding to the Cs+ and K+ EIC waves which, apparently, are simultaneously excited in the plasma. The second and third Cs+ EIC wave harmonics are also present. The ratio of the observed frequencies for the K1 mode and the Cs l is consistent with the inverse ratio of their masses, QK+/Qcs+ = M c , / M K = 3.4, where the Q's are the ion gyrofrequencies. The K+ and Cs+ EIC mode frequencies are 20 percent higher than their corresponding gyrofrequencies of fK+ = 97.4 kHz and&+ = 28.6 kHz, at B = 2500 G. A more definitive identification of the EIC modes was made by obtaining a frequency spectrum, as in Fig. 1, for various values of the magnetic field. The observed frequencies versus B are shown in Fig. 2. As can be seen, we are able to track many spectral peaks as the magnetic field is varied. In addition to the individual Cs+ and K+ EIC modes, a low-frequency peak labeledf* is also present which corresponds to the difference between the K1 peak and the Cs3 peak. The combination of this "beat frequency" at around 9 kHz (at 2500 G ) with the various Cs+ and K+ cyclotron harmonics results in additional spectral peaks at Cs l + f*, Cs2 + f*, and K1 & C s l . At present, we have not identified the nature of these additional features in the EIC wave spectrum. We note, however, that these additional peaks are only observed at the lower total plasma densities investigated ( 10" ~ m ~ ) . These complex spectra were not observed when the total density was increased by about a factor of 10, even though the relative ion concentrations were similar to those of the case presented here. In summary, we see that, at least under the conditions discussed above, it is possible to excite independent EIC modes corresponding to each ion component present in the plasma. This result can perhaps be most easily understood on the basis of a simple fluid analysis of EIC waves in a plasma consisting of electrons and two positive ion species. The analysis proceeds along much the same lines as for EIC waves in a plasma containing positive ions, negative ions, and electrons [17]. We use the continuity and momentum equations for the electrons, light ions (15) and heavy ions (H ) with a uniform magnetic field B2. The plasma is taken to be uniform and charge quasi-neutral, ne = nL + nH, with cold ions TL = 0093-3813/88/0600-0396$01 .OO