Orbital resonances and chaos in a combined rf trap
- T-15, MS K717, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
The rf combined trap for confining a non-neutral plasma by a combination of rf fields and a uniform magnetic field is introduced. The particle motion is studied in such a trap in which the rf fields are given by the TM{sub 011} mode. The aim is to determine limits on a focus at the center for low angular momentum particles. Such a focus allows densities in excess of the local Brillouin limit. The motion is described in terms of an effective potential consisting of the ponderomotive potential V{sub pon}(r,z) plus a radial potential m{Omega}{sub c}{sup 2}r{sup 2}/8 due to the magnetic field B{sub z} plus a centrifugal potential p{sub {theta}}{sup 2}/2mr{sup 2}. Near the origin r=0, z=0, the equations reduce to a pair of Mathieu equations for the r and z motions. For certain parameters the system is {ital quasispherically symmetric}, i.e. the effective ponderomotive frequencies {Omega}{sub r}, {Omega}{sub z} in r and z are equal. In this case the Brillouin limit for a uniform density plasma is shown to equal the usual cylindrical limit. Numerical integration of the ponderomotive equations is shown for parameters near those giving quasispherical symmetry. There is resonance for {Omega}{sub r}/{Omega}{sub z}=1, giving m=2, n=2 islands. The islands limit the size of the focus at the origin, even for p{sub {theta}}=0. For particles with large effective energy E{sub eff}, there can also be chaos, depending on the elongation L/a of the rf cavity. It is found that the deleterious effect of islands and chaos on the focus is minimized by having the elongation L/a close to unity and having particles trapped deeply in the ponderomotive well. {copyright} {ital 1997 American Institute of Physics.}
- OSTI ID:
- 544557
- Journal Information:
- Physics of Plasmas, Vol. 4, Issue 5; Other Information: PBD: May 1997
- Country of Publication:
- United States
- Language:
- English
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