Drift Cyclotron Loss Cone Instability Research Articles

Interaction of a high frequency magnetic field with the drift-cyclotron loss-cone mirror instability

The effect of a small amplitude, high frequency field on drift-cyclotron loss-cone instability is investigated. With B1/B0=5% and ωce/ω0=80, the critical characteristic length of plasma density gradient reduces dramatically and is comparable to the finite β stabilization effect.

Warm plasma effects on drift cyclotron loss cone mode

Although plasma density gradients in mirror machines are larger than the theoretical threshold for the drift cyclotron loss cone instability, turbulence is experimentally observed only at low harmonics of the ion cyclotron frequency. The evolution of the drift cyclotron loss cone instability as the loss cone velocity distribution is progressively filled with a warm Maxwellian component is studied. The addition of the warm plasma results in both a lowering of the growth rate and a distortion and narrowing of the frequency spectrum. Under conditions typical of an almost filled loss cone, the numerical result shows that the growth rate is largest for the lower ion cyclotron harmonics and is quite consistent with experimental observations.

Magnetic mirror confinement of laser-produced LiH plasmas

The results of measurement and analysis of the decay of mirror-confined lithium hydride plasmas are reported. The plasmas are produced by laser irradiation of a solid LiH particle suspended at the center of a mimimum-B magnetic field. In the initial state, the ratio of the Li3+ energy to H+ energy is 7 to 1, the mean energy of lithium ions is about 2000 eV, and the electrons are cold. In the subsequent decay of the mirror-confined plasmas from 3×1013 to 1012 cm−3, the H+ lifetime is sufficiently shorter than the lifetime of the Li3+ ions so that the plasma evolves to a lithium plasma. The density decay is faster than classical but quiescent for the first 250 μsec. After that time, there are sudden increases in the plasma decay rate and in the rf emission at the central cyclotron frequency of lithium ions, interpreted as evidence of the onset of the drift cyclotron loss cone instability. The quiescent behavior is correlated with the observation of optical radiation from the plasma between 70 and 250 μsec. The plasma luminosity is interpreted as evidence for the creation of cold plasma by ionization of cold neutral reflux from the walls of the baseball coil. A quasi-linear Fokker–Planck calculation of the ion distributions and the growth and damping of the drift cyclotron loss cone mode yielded results consistent with an explanation of the quiescent decay as the suppression of the instability by the cold plasma.

Identification of Drift-Cyclotron Loss-Cone Instability in a Plasma and Suppression by Radio-Frequency Field

An instability, which appears near the ion cyclotron frequency, is observed in the plasma reflected by one side mirror. The instability is identified as the drift-cyclotron loss-cone instability by comparing the experimentally determined dispersion relation with the theory. It is demonstrated that this instability can be suppressed by the high frequency electric field excited in a resonant cavity. The mechanism of suppression is attributed to the modification of the loss-cone distribution due to the ponderomotive force which pushes back the plasma.

Stabilization of drift cyclotron loss cone instability with additions of small amounts of cool plasma

The effect of the addition of a cooler ion component on the drift cyclotron loss cone instability was investigated using the local approximation linear dispersion relation. The parameters considered were mirror ratio, density gradient, density (including finite β), ratio of cool to hot ion temperature, and type of cool component (Maxwellian or loss cone). Several different regimes of the parameter space were defined and mapped, according to the characteristics of the instability (e.g., source of free energy, critical density gradient, range of unstable wavenumbers, maximum growth rate). The most effective stabilization was found to occur for addition of a small Maxwellian cool component with density ratio (nc/nh) ≳3.4[(ah/n)(dn/dx)]3/2 (R−1/2−R−1)1/2 and with temperature ratio satisfying 1≳ (Tc/Th)3/2(nh/nc), (R−1/2−R−1)−1≳0.43; ah is the hot ion Larmor radius, R is the mirror ratio, nc and nh are the cool and hot ion densities, and Tc and Th are the cool and hot ion temperatures. The stabilization allows a larger critical gradient, (ah/n) (dn/dx) ≈R1/4 (ωci2/ωpi2+me/mi)1/2 , and a larger minimum unstable wavenumber kah≈R1/4(ωci2/ωpi2 +me/mi)−1/2, than that with no cool component. The instability was also studied numerically for (ah/n) (dn/dx) ∼1, and the minimum nc/nh required for this stabilization was consistent with the density ratio used in the 2XIIB experiment at Livermore. For β≳0, even smaller amount of cool plasma is needed for this stabilization.