[en] A novel experiment to investigate single-surface multipactor on a dielectric surface was developed and tested. The compact apparatus consists of a small brass microwave cavity in a high vacuum system. The cavity is ∼15 cm in length with an outer diameter of ∼10 cm. A pulsed variable frequency microwave source at ∼2.4 GHz, 2 kW peak excites the TE₁₁₁ mode with a strong electric field parallel to a dielectric plate (∼0.2 cm thickness) that is inserted at midlength of the cavity. The microwave pulses are monitored by calibrated microwave diodes. An electron probe measures electron current and provides temporal measurements of the multipactor electron current with respect to the microwave pulses. Phosphor on the dielectric surface is used to detect multipactor electrons by photoemission. The motivation of this experiment is to test recent theoretical calculations of single-surface multipactor on a dielectric

[en] We use Monte Carlo simulations and analytical calculations to derive the condition for the onset of multipactor discharge on a dielectric surface at various combinations of the bias dc electric field, rf electric field, and background pressures of noble gases, such as Argon. It is found that the presence of a tangential bias dc electric field on the dielectric surface lowers the magnitude of rf electric field threshold to initiate multipactor, therefore plausibly offering robust protection against high power microwaves. The presence of low pressure gases may lead to a lower multipactor saturation level, however. The combined effects of tangential dc electric field and external gases on multipactor susceptibility are presented.

[en] Experiments have been performed demonstrating the feasibility of direct implantation of laser-ablated metal ions into a substrate. Initial experiments implanted iron ions into silicon substrates at pulsed, bias voltages up to negative 10 kV. Implantation of Fe ions into Si was confirmed by cross-sectional transmission electron microscopy and x-ray photoelectron spectroscopy. The 7.6 nm depth of damage layers below the Si surface is slightly less than predicted by code calculations for a maximum, effective ion energy of about 8 keV. The ion depth of penetration is limited by the overlying Fe film as well as the slow rise and fall of the voltage. [copyright] 2001 American Institute of Physics

[en] The condition for mutual, or peer-to-peer, locking of two magnetrons is derived. This condition reduces to Adler's classical phase-locking condition in the limit where one magnetron becomes the ''master'' and the other becomes the ''slave.'' The formulation is extended to the peer-to-peer locking of N magnetrons, under the assumption that the electromagnetic coupling among the N magnetrons is modeled by an N-port network.

[en] The paper presents a systematic evaluation of current crowding and spreading resistance in thin film contacts, based on the exact field solution that contains very large contrasts in dimensions and resistivity. It is found that current crowding becomes more severe as the interface specific contact resistivity decreases, the resistivity ratio of the contact electrode to the thin film decreases, or the thickness of either the contact or the thin film decreases. The current transfer length $L_{\text{T}}$ from our exact field solution is compared to that of transmission line model (TLM), $L_{\text{TLM}}={{\left({\mathrm{\rho <!--\; ??\; -->}}_{\text{c}}/{\mathrm{\rho <!--\; ??\; -->}}_{\text{sh}}\right)}^1}^{/2}$, where ${\mathrm{\rho <!--\; ??\; -->}}_{\text{c}}$ is the interface specific contact resistivity, and ρ _sh is the sheet resistance of the thin film under contact. It is found that, if ${\mathrm{\rho <!--\; ??\; -->}}_{\text{c}}$ is small, $L_{\text{T}}$ is bounded by the smaller of the two dimensions—thin film thickness and contact size. As ${\mathrm{\rho <!--\; ??\; -->}}_{\text{c}}$ increases, $L_{\text{T}}$ increases, but saturates at a constant value, determined by the smaller of the two dimensions—contact size and $L_{\text{TLM}}$. The total contact resistance is decomposed into three components: the interface resistance due to ${\mathrm{\rho <!--\; ??\; -->}}_{\text{c}}$, the spreading resistance due to current crowding, and the resistance due to the contact electrode. Unambiguously identified, each component is explicitly evaluated and compared in detail. (paper)

[en] A scalable electron cyclotron resonance (ECR) plasma source was investigated for plasma cathode applications. The rectangular source utilized permanent magnets to establish the ECR condition. The microwave applicator region was windowless, making the source applicable to sputtering environment applications. The source was characterized using primarily two diagnostics: (1) a near-field and far-field Langmuir probe and (2) a downstream electron extraction electrode. Source operation and plasma properties were characterized at low pressures ranging from 0.2 to 5 mTorr and power levels up to 250 W. Evidence of grad-B drift in the plane of the source was observed. Extracted currents agreed well with predictions

[en] Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. To exploit triple point emission, metal-oxide-junction (MOJ) cathodes consisting of dielectric ''islands'' over stainless steel substrates have been fabricated. The two dielectrics used are hafnium oxide (HfO_x) for its high dielectric constant and magnesium oxide (MgO) for its high secondary electron emission coefficient. The coatings are deposited by ablation-plasma-ion lithography using a KrF laser (0-600 mJ at 248 nm) and fluence ranging from 3 to 40 J/cm². Composition and morphology of deposited films are analyzed by scanning electron microscopy coupled with x-ray energy dispersive spectroscopy, as well as x-ray diffraction. Cathodes are tested on the Michigan Electron Long-Beam Accelerator with a relativistic magnetron, at parameters V=-300 kV, I=1-15 kA, and pulse lengths of 0.3-0.5 μs. Six variations of the MOJ cathode are tested, and are compared against five baseline cases. It is found that particulate formed during the ablation process improves the electron emission properties of the cathodes by forming additional triple points. Due to extensive electron back bombardment during magnetron operation, secondary electron emission also may play a significant role. Cathodes exhibit increases in current densities of up to 80 A/cm², and up to 15% improvement in current start up time, as compared to polished stainless steel cathodes

[en] The magneto-Rayleigh-Taylor instability (MRT) of a finite slab is studied analytically using the ideal MHD model. The slab may be accelerated by an arbitrary combination of magnetic pressure and fluid pressure, thus allowing an arbitrary degree of anisotropy intrinsic to the acceleration mechanism. The effect of feedthrough in the finite slab is also analyzed. The classical feedthrough solution obtained by Taylor in the limit of zero magnetic field, the single interface MRT solution of Chandrasekhar in the limit of infinite slab thickness, and Harris' stability condition on purely magnetic driven MRT, are all readily recovered in the analytic theory as limiting cases. In general, we find that MRT retains robust growth if it exists. However, feedthrough may be substantially reduced if there are magnetic fields on both sides of the slab, and if the MRT mode invokes bending of the magnetic field lines.

[en] An electron rotating under a uniform axial magnetic field and a radial electric field exhibits an effective mass that may be negative, positive, or infinite, in response to an azimuthal electric field. This paper reports simulation results that show instability and stability when the effective mass are negative and positive, respectively, depending on the magnitude and orientation of the radial electric field. Thus, the inverted magnetron would have a much faster startup than the conventional magnetron, an important consideration for pulsed operation. When the effective mass is infinite, the electrons hardly respond to an azimuthal ac electric field.

[en] Soft metal gaskets (indium and silver) were used to reduce contact resistance between the wire and the electrode in an aluminum wire Z pinch by more than an order of magnitude over the best weighted contact case. Clamping a gasket over a Z-pinch wire compresses the wire to the electrode with a greater normal force than possible with wire weights. Average contact resistance was reduced from the range of 100-3000 Ω (depending on wire weight mass) to 1-10 Ω with soft metal gaskets. Single wire experiments (13 μm Al 5056) on a 16 kA, 100 kV Marx bank showed an increase in light emission (97%) and emission volume (100%) of the plasma for the reduced contact resistance cases. The measured increases in plasma volume and light emission indicate greater energy deposition in the ablated wire. Additionally, dual-wire experiments showed plasma edge effects were significantly decreased in the soft metal gasket contact case. The average height of the edge effects was reduced by 51% and the width of the edge effects was increased by 40%, thus the gasket contact case provided greater axial uniformity in the plasma expansion profile of an individual wire.