Observation of Negative-Mass Instability in an Energetic Proton Plasma

Abstract

Microinstabilities at and above ion gyrofrequency are present in almost all hot ion plasmas, even those stabilized against hydromagnetic instabilities by magnetic wells. 1 " 3 Microinstabilities are capable of producing hot-ion losses, 4 and therefore are a major threat to progress in generation and confinement of thermonuclear plasmas in many of the present experiments. Since the design of future experiments presumably will rely heavily upon microinstability theory, experimental studies of microinstabilities are of particular importance when the results can be identified with predictions of theory.In this Letter we briefly describe experimentally observed threshold properties of a microinstability in an energetic proton plasma, and we identify these properties with those of a theoretically recognized instability. We have measured the threshold density and instability growth times for a "standard" proton distribution (to be described), and the response of threshold to changes in (1) the magnetic field shape, (2) the energy spread of trapped protons, and (3) the proton distribution in radial oscillation amplitude. We have also measured the dependence of the fundamental mode frequency on changes in radial oscillation amplitude. We show that the observed properties are those of the negative-mass instability 5 ' 6 familiar from accelerator experience, and we exclude interpretation in terms of the driftcyclotron instability, 7 " 9 the other likely candidate. 4 ' 10 A detailed report of these studies is in preparation.The plasma is created in the DCX-1 facility by dissociation of 600-keV H 2 + injected into a magnetic mirror field with a central field value of 10 kG. The experimental instability is the "gyrofrequency mode" of earlier papers. 4 ' 10 It is characterized by reception on electrostatic and magnetic (B z polarization) probes of rf signals near harmonics of the ion gyrofrequency. Descriptions of the experimental facility, details of the proton losses associated with this mode under certain operating conditions, data on the rf spectrum, and previous considerations of mode assignment for this instability are given in the earlier papers.The gyrofrequency mode of instability can be isolated from other unstable modes of this plasma by using gas-collisional dissociation of the molecular-ion beam at fairly high gas pressures (of order 10" 6 Torr), and this was done in these experiments. Helium was used as the background gas, and trapped-proton lifetimes for most of the experiments were 50-75 jusec.The threshold measurements were made in the period of rising density just after the molecular ion beam was turned on. For the energy-spreading experiments, the measurements were also made in steady state, with the density controlled by varying the beam current. For the threshold density, we take values inferred from measurements of the escaping charge-exchange neutrals at the time of first appearance of the rf signals.The "standard" plasma distribution is that established using the normal plasma radius (20 cm) and H 2 + be...

Negative-Mass Instability of the Spin and Motion of an Atomic Gas Driven by Optical Cavity Backaction

“…When a negative-mass oscillator is coherently coupled to a positive-mass oscillator at nearly the same frequency, the two-oscillator system can undergo an instability, where the transfer of energy between them leads to unbounded growth of the amplitudes of both oscillators. This negative-mass instability has been observed in the classical mechanics of trapped plasmas [7] and ion traps [8], and has been suggested to play a role in galactic structure [9].…”

“…When a negative-mass oscillator is coherently coupled to a positive-mass oscillator at nearly the same frequency, the two-oscillator system can undergo an instability, where the transfer of energy between them leads to unbounded growth of the amplitudes of both oscillators. This negative-mass instability has been observed in the classical mechanics of trapped plasmas [7] and ion traps [8], and has been suggested to play a role in galactic structure [9].Negative-mass oscillators also play an important role in quantum science. Glauber proposed that a negative-mass oscillator coupled to a zero-temperature bath would, through the negative-mass instability, function as an ideal quantum amplifier [10].…”