摘要:

The propagation of intense, relativistic electron beams in air is subject to the resistive hose instability. Conditioning the beam prior to injecting it into the air can extend its range by reducing the hose growth rate and by reducing the initial spatial perturbations that seed the hose instability. Experiments have been performed using the SuperIBEX accelerator (Ipeak=10–30 kA,E=4.5 MeV, 40 ns full width at half‐maximum) to develop conditioning cells that suppress the hose. This paper describes the performance of an active wireB&thgr;cell that is used in conjunction with an ion focused regime (IFR) cell. The IFR cell detunes the instability by producing a head‐to‐tail radius taper on the beam. The wire cell maintains this radius taper while producing an emittance taper that is necessary to suppress the hose growth. In addition, the wire cell reduces the initial beam perturbations through the anharmonic centering force associated with the wire current and its azimuthal magnetic fieldB&thgr;. The ability of theB&thgr;cell to reduce the beam offset with a minimal increase in the beam radius gives it several advantages over the use of a simple, thick scattering foil to perform the radius taper to emittance taper conversion. The SuperIBEX beam propagation distance, in terms of the betatron oscillation scale length, was extended to ∼10&lgr;&bgr;using these cells. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363384

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An intense relativistic electron beam injected into dense gas characteristically propagates in a self‐pinched mode but is susceptible to the resistive hose instability. This convective instability typically leads to large amplitude beam motion and the disruption of propagation. Theory and computation suggest that, although resistive hose cannot be completely suppressed, its convective growth can be reduced by varying the average betatron oscillation frequency from head to tail in the beam pulse. We report here on experiments designed to implement this variation by tailoring the beam emittance using an ion‐focused regime ‘‘conditioning’’ cell. Conditioning effectiveness is assessed by using measured beam quantities to evaluate a detuning parameter &eegr;(t). This information is correlated with beam propagation measurements to determine the optimum conditioning for resistive hose suppression.

ISSN:0021-8979    

DOI:10.1063/1.363385

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We apply directly a perturbative expansion to thehoppingmodel of Feinberg. The first three harmonics of the space‐charge field are presented, considering the contributions from the higher‐order terms at large modulation depth. The dependence of the harmonics on spatial frequency, modulation depth and applied field strength are discussed. An analytical solution of the two‐wave coupling equations is given in this case. The mixing gain &ggr; is also discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363386

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Focused bulk ultrasonic waves have been generated in aluminum plates by surface irradiation with ring‐shaped laser light. The waves are detected by a piezoelectric transducer. Compression and shear peak amplitudes drop quickly when the detector is moved away from the epicenter. This shows that strong focusing exists at the epicenter as the result of constructive interference of the waves generated by different parts of the ring. The focusing persists when the radius of the laser light is scanned over a large range, indicating that the elastic disturbance concentrates in depth along the ring’s central axis. Numerical simulations are presented for comparison. The ‘‘pencil‐like’’ acoustic wave structure is used to observe a sample plate with an artificial flaw. Strong new features including compress‐shear mode conversion at the site of the flaw are observed. These features are used to locate the flaw within the sample. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363387

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The experimental results obtained in an Ar+I2dc discharge during a transient process in which the iodine concentration decreases with time are presented. The following quantities have been measured during the process: electron energy distribution function, electron density and temperature, plasma potential and floating potentials. The temporal evolution of such quantities is analyzed. A transition from electronegative to electropositive plasma is observed. The floating potential values measured during the process are compared with those obtained from a theoretical model, obtaining a quite good consistency between both values. The floating potential is shown to be a powerful tool in the analysis of electronegative plasmas. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363388

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Breakdown in low pressure radio frequency (rf) plasmas is investigated with particle‐in‐cell simulations in a model gas based on argon. A one dimensional model with a realistic rf wave form is used in a range of pressures between 1 and 50 mTorr. Dynamic scaling and electron generated secondaries are introduced in order to simulate breakdown realistically, and it is possible to follow the discharge development from a few initial electrons to the steady state plasma withne∼109cm−3. The results confirm that breakdown is controlled by the resonant multiplication of secondary electrons known as the multipactor. For the pressures investigated, however, ionization is the main source of new charges and a model based on a typical electron trajectory is introduced to account for the growth in the number of electrons in the discharge gap. The model is in good agreement with the simulation results. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363389

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In the following, we report tests of a semiconductor as electrode material in a pseudospark switch, which is a low pressure gas discharge switch. Polycrystalline SiC or BC disks with central apertures were embedded in molybdenum outer electrodes in a typical single stage pseudospark geometry. The maximum voltage applied was 15 kV. Tests were done with peak currents from 50 up to 500 kA at pulse lengths of a few microseconds. We found no difference in the voltage breakdown or the rise in current when compared to conventional metal electrodes such as molybdenum or tungsten. Fast shutter photographs showed that the discharge burns in an intense column at the center and is distributed very homogeneously over the SiC surfaces. Neither cathode nor anode spots were observed. Small sparks occurred at the metal–carbide interface after current zero, which led to a reignition in the second current half‐wave. An estimation after the tests showed an erosion rate of about 5 &mgr;g/C approximately one order of magnitude lower than in metals. We assume that the current is transported in a thin surface layer. Ion bombardment in the pseudospark discharge heats this layer to more than 2000 K, lowering the specific resistance. The negative temperature–resistance characteristic means that cathode spots are avoided. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363377

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have studied both experimentally and theoretically the generation of damage in GaAs due to ion implantation through mask openings of small dimensions. We show that it is possible to master the generation of damage, i.e., the amorphization phenomenon in the direction perpendicular to the ion beam and close to the mask edges. A theoretical model is used to simulate the ion implantation process and damage accumulation through Ti–Au masks. After comparing the shape of the crystalline/amorphous interfaces as revealed by cross‐sectional electron microscopy with our simulations, this model is used to predict the evolution of the two‐dimensional damage distributions beneath the mask edges as functions of ion beam and implantation mask parameters. Undamaged regions of nanometer dimensions can be preserved even when using masks of reasonable dimensions (100–200 nm). This can be done only by adjusting the ion beam parameters through the accurate simulation of the two‐dimensional damage generation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363378

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The irradiation of crystalline (&agr;‐SiO2) and amorphous (a‐SiO2) silicon dioxide with a stationary electron beam produces characteristic changes in the surface topography. The development of these changes has been investigated using cathodoluminescence spectroscopy and microscopy, scanning probe (atomic force) microscopy, and scanning electron microscopy. Electron irradiation produces a permanent volume increase on (crystalline) &agr;‐SiO2, while in (amorphous)a‐SiO2an initial small volume increase is followed by volume loss as irradiation continues. The observed changes are consistent with electromigration of oxygen under the influence of the electric field induced by charge trapping at preexisting or irradiation‐induced defects. Oxygen enrichment may produce expansion of the surface region due to the formation of peroxy linkage defects. Ina‐SiO2, charges trapped by defects at grain boundaries produce enhanced electric fields which may result in volume reduction at the surface, when critical field strengths are exceeded. The observed volume reductions may be attributed to electron stimulated desorption of constituents, in particular oxygen mass loss, and densification of the surface region associated with the formation of oxygen‐deficient defect centers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363379

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The annealing behavior of electron, proton, and alpha particle irradiated, epitaxialn+pInP solar cells has been characterized using several techniques. Current–voltage measurements were made under simulated 1 sun, AM0 solar illumination and in the dark. The radiation‐induced defect spectra were monitored using deep level transient spectroscopy and the base carrier concentration profiles were determined through capacitance–voltage measurements. The irradiated cells were annealed at temperatures ranging from 300 up to 500 K. Some cells were annealed while under illumination at short circuit while others were annealed in the dark. These experiments produced essentially the same results independent of illumination and independent of the irradiating particle. An annealing stage was observed between 400 and 500 K, in which the radiation‐induced defects labeled H3 and H4 were removed and the carrier concentration recovered slightly. Concurrently there was a small reduction in the junction recombination current and a slight increase in the photovoltaic (PV) output of the cell; however, most of the radiation‐induced defects did not anneal, and the overall PV recovery was very small. A full analysis of the annealing data is given, and a model for the radiation response and annealing behavior of the cells is presented. The results are compared to those reported previously for irradiated, diffused junction InP solar cells. Although the radiation‐induced degradation mechanisms appear to be essentially the same in the two cell types, the recovery of the PV output is found to be quite different. This difference in cell recovery is explained in terms of the defect annealing characteristics in the individual cell types. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363380

出版商:AIP    

年代:1996

数据来源: AIP