No abstract

We have demonstrated storage of plasmas of the charged constituents of the antihydrogen atom, antiprotons and positrons, in a Penning trap surrounded by a minimum-B magnetic trap designed for holding neutral antiatoms. The neutral trap comprises a superconducting octupole and two superconducting, solenoidal mirror coils. We have measured the storage lifetimes of antiproton and positron plasmas in the combined Penning-neutral trap, and compared these to lifetimes without the neutral trap fields. The magnetic well depth was 0.6 T, deep enough to trap ground state antihydrogen atoms of up to about 0.4 K in temperature. We have demonstrated that both particle species can be stored for times long enough to permit antihydrogen production and trapping studies.

Abstract. We study ion acoustic damping effects on parametric decays of right-handpolarized electromagnetic waves. We do this because ion beams have been observed in a variety of space environments, and consequently, these waves can exist in those places. Damping effects are incorporated into the model by adding to the longitudinal component of the equation of motion a collision-like term. Like for left-hand-polarized waves, the effect of damping is twofold. On the one hand, damping decreases the maximum growth rate of the existing instabilities while increasing the instability range, and on the other hand, it destabilizes regions that are stable in the absence of damping. Thus, for lowfrequency pump waves, to 0 << tOci , and for low/3 = vt/v• (tOci is the ion gyrofrequency, and vt and v• are the thermal and the Alfv•n velocities, respectively), where the only parametric instability is a decay instability, damping destabilizes the frequency range between to = 0 and the threshold of the decay instability. As/3 increases, two new parametric instabilities develop: One of them is a modulational instability, and above some threshold value of the pump wave amplitude, there is also a beat wave instability. The decay instability is also possible for pump wave amplitude above some threshold. For even larger/3 the decay instability is no longer possible. In all cases, damping effects reduce the growth rate of the existing instabilities and destabilize regions which are stable in the absence of damping. These results are in agreement with those obtained by Vasquez [1995]. It is also shown that for large-frequency pump waves, electron/ion whistler waves, the decay instability reappears even for large/3 and has very large growth rates.

[1] Ion beam-plasma interactions are the source of wave activity in several space environments. Therefore the study of these waves and their parametric decays are very important in space physics. Thus we study parametric decays of right-hand polarized proton beam-plasma waves including the effect of the beam. It is shown that there are new instabilities due to the beam and the associated ion acoustic waves. The branch of the linear dispersion relation corresponding to the beam has negative energy so that whenever this branch participates in a decay, the corresponding instability should lead to an explosive instability. Some explosive behavior has indeed been found in simulation results involving right-hand polarized waves. Since the free-energy source of the linear beamplasma instabilities is the beam energy, one expects the beam speed to decrease as the waves grow and experience nonlinear decays. Therefore we study the effect of varying beam velocity in the parametric decays. It is shown that there are instabilities which depend on the beam velocity in such a way that they are stabilized as the beam decelerates.