摘要:

Weak axial variations inB(z) or &fgr;(z) in Penning‐Malmberg traps cause some particles to be trapped locally. This causes a velocity‐space separatrix between trapped and passing populations, and collisional separatrix diffusion then causes mode damping and asymmetry‐induced transport. This separatrix dissipation scales with collisionality asv1/2, so it dominates in low collisionallity plasmas. The confinement lifetime in the “CamV” apparatus was dominated by a weak magnetic ripple with &dgr;B/B∼ 10−3, and it appears likely that the ubiquitous (L/B)−2lifetime scalings and other applied asymmetry scalings represent similar TPM effects. TPM transport will limit the containment of large numbers of positrons orp¯s, since TPM loss rates generally scale as total chargeQ2, independent of length. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635151

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The thermal fluctuation spectrum of the signal received on a patch electrode is examined and it is shown that the spectrum shows both the modes of the plasma and a continuous spectrum related to the independent‐particle motions of plasma electrons. Modes whose axial phase velocity are more than 3–4 times the electron thermal speed are lightly Landau‐damped and are clearly separated from the continuum. Long wavelength modes are “acoustic” in nature. If the axial phase velocity of a mode becomes less than 1–2 times the electron thermal speed, then the mode becomes strongly Landau‐damped and it merges into the continuum. The mode velocities are of the order ofwpa, whereais the plasma radius, so that the plasma radius must be at least several deBye lengths in order to have lightly damped modes. In general, the spectrum is a mixture of a continuous spectrum together with a finite number of modes which are Landau‐damped by varying amounts, depending on their phase velocity relative to the electron thermal speed. Only in the extreme limit,wpa<<vthdoes the continuous spectrum tend to a Gaussian of widthk vth, characteristic of independent particles. The effect of the “load impedance” on the measurements is also discussed. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635152

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

For the first time, high amplitude (&Dgr;n/n≈ 40&percent;), high Q (up to 100,000) BGK modes have been controllably excited in a plasma. The modes are created by sweeping an excitation voltage downwards in frequency, thereby dragging a phase space “bucket” of low density into the bulk of the plasma velocity distribution. The modes have no linear limit, and differ markedly from plasma waves and Trivelpiece‐Gould modes. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635153

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Computational studies and experimental measurements of plasma injection into a Malmberg‐Penning trap reveal that the number of trapped particles can be an order of magnitude higher than predicted by a simple estimates based on a ballistic trapping model. Enhanced trapping is associated with a rich nonlinear dynamics generated by the space‐charge forces of the evolving trapped electron density. A particle‐in‐cell simulation is used to identify the physical mechanisms that lead to the increase in trapped electrons. The simulations initially show strong two‐stream interactions between the electrons emitted from the cathode and those reflected off the end plug of the trap. This is followed by virtual cathode oscillations near the injection region. As electrons are trapped, the initially hollow longitudinal phase‐space is filled, and the transverse radial density profile evolves so that the plasma potential matches that of the cathode. Simple theoretical arguments are given that describe the different dynamical regimes. Good agreement is found between simulation and theory. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635154

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

It has long been known that asymmetric electric and magnetic fields produce radial transport in Malmberg‐Penning traps, and much work has been done to understand this transport. Our approach is to apply a variable frequency electric asymmetry to a low density population of electrons and to measure the resulting radial particle flux &Ggr; as a function of radiusr. The low particle density eliminates many plasma modes (which have their own frequency dependence) and allows us to focus on the transport physics. The usual azimuthalE×Bdrift is maintained by a biased central wire, and this arrangement also allows us to independently vary the drift frequencywRby adjusting either the axial magnetic fieldBzor the bias of the central wire &fgr;cw. Up to forty wall sectors are used in order to apply an asymmetry consisting of a single fourier mode (n, l, w), wherenis the axial wavenumber,lis the azimuthal wavenumber, andwis the asymmetry frequency. In the current experiments, we varyw,n, &fgr;cw, andBz. Aswis varied, the particle flux shows a resonance similar to that predicted by resonant particle theory. The peak frequency of this resonancefpeakincreases withwRand varies withn, in qualitative agreement with theory, but when quantitative comparisons are made the experimental values forfpeakdo not match those predicted by theory. Instead, the dependence offpeakon &fgr;cw,Bz, andrfollows simple empirical scaling laws: for inward directed flux,fpeak(MHz) ≈ [−R&fgr;cw(V)/rBz(G)]1/2, whereRis the wall radius, and for outward directed flux,fpeak(MHz) ≈ 0.8[−&fgr;cw(V)/Bz(G)]1/2. These results may provide guidance for the construction of the correct theory of asymmetry‐induced transport. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635155

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Electrostatic modes in an electron plasma confined in the Penning‐Malmberg trap ELTRAP (Physics Department of Milan University) are investigated, analyzing the electric signals on the electrodes, and using the CCD diagnostics. In different experimental conditions, a well‐defined mode (possibly them= 1 diocotron mode) is found, with a frequency proportional to (Q/B) (Qis the total charge,Bthe magnetic field strength). The amplitude starts to increase at the beginning of the hold phase of the cycle, reaches a maximum, and then decreases to the background noise level. The decrease of the amplitude corresponds to a decrease of the frequency, indicating plasma loss at the wall. A fast rise of the potential barrier easily excites the mode. Measurements performed with increasing ramp times show that the growth rate decreases, and at a long enough ramp time the instability does not arise. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635156

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A theoretical approach is presented for predicting the collision‐based axial loss rate of a plasma confined by a Malmberg‐Penning trap, just after the axial well depth is made shallower. The assumptions used in developing the approach include a square well, uniform plasma properties, a particle mean‐free‐path that is much larger than the plasma dimensions, and a velocity distribution that is Maxwellian except within the regions of direct loss in velocity space. Example predictions are provided, and comparisons with existing theories are made. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635157

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

It is well known that a small fraction of positive ions can destabilize diocotron modes on electron plasmas. However, the historical (and recent) interpretation of experimental results in terms of 2D (or modified 2D) theories of ion‐induced instabilities is apparently erroneous. Here, we experimentally characterize a strong exponential instability with no threshold, obtaining growth rates orders of magnitude larger than predicted. The positive ion population is maintained either by continuous external injection of ions or by ionization of the background gas within hot electron plasmas. In both cases, the observed exponential growth rate &ggr;mis directly proportional to the ion creation rate &ngr;+, i.e., &ggr;m= &kgr;m&ngr;+, with &kgr;m≈ (101–103)/Neform0= 1,2,3. Experimental results also suggest that non‐2D effects, including end confinement fields, are important. This strong instability may have important implications for the anti‐hydrogen creation technique of propelling anti‐protons through trappede+clouds. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635158

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Asymmetry modes (m= 1,kz≠ 0) are diocotron‐like modes in finite‐length plasma columns in Malmberg‐Penning traps. We have investigated the modes with a detailed 3‐d particle‐in‐cell (PIC) drift‐kinetic computer simulation. Although PIC simulations do not employ realistic collisions, the simulations in this case reproduce many of the salient features of the data. Particle transport associated with the damping is seen not to be a direct collisional effect, but rather a feature of orbital dynamics associated with transitions from trapped‐to‐untrapped or untrapped‐to‐trapped state relative to the inversion plane of the asymmetry. In the simulations we observe aB−1dependence of the mode frequencies and aB−0.5dependence of the damping constant for large rigidity. We further observe a steepening of the dependence of the decay constant toB−2as the rigidity of the plasma falls below about 2.0. We have also used the simulations to investigate the modes at small seed amplitudes and observe linear flattening in the mode frequency as the seed amplitude becomes small. In contrast, the decay constant does not flatten for small seed amplitude. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635159

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The expansion of pure electron plasmas due to collisions with background neutral gas atoms in the Electron Diffusion Gauge experiment device is observed. Measurements of plasma expansion with the new, phosphor‐screen density diagnostic suggest that the expansion rates measured previously were observed during the plasma’s relaxation to thermal quasi‐equilibrium, making it even more remarkable that they scale classically with pressure. Measurements of the on‐axis, parallel plasma temperature evolution support the conclusion. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635160

出版商:AIP    

年代:1903

数据来源: AIP