摘要:

Although the atomic force microscope (AFM) resembles superficially the scanning tunneling microscope (STM), its imaging resolution is in general much coarser. For the AFM, long‐range interactions—most notably the van der Waals force—imply that image resolution is set by the macroscopic tip radius rather than by a single atom, as with the STM. Experimentally, we show that van der Waals forces can be measured using an AFM. By immersing tip and sample in an appropriate fluid, we can effectively eliminate the van der Waals force, leading to a marked improvement in AFM image quality.

ISSN:0021-8979    

DOI:10.1063/1.352845

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In order to calibrate a novel type of time‐of‐flight mass spectrometer to be flown in the solar wind, we have continued our investigation of the charge exchange of low energy ions passing thin carbon foils. We analyzed elements with widely different chemical properties: Ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe in the energy range 0.5–3 keV/u were passed through carbon foils with thicknesses between 1.1 and 10 &mgr;g/cm2, and their charge state distributions and residual energies were determined. It was found that (1) the charge state distribution behind the foil is independent of the charge of the incident projectile, (2) isotopes show the same charge exchange properties at equal velocities as we have found previously, (3) at the lowest energies the charge state distribution is no longer a function of the residual energy alone but depends on both residual energy and foil thickness, (4) probable differences in chemical properties between the front and back surfaces of the foil have no detectable influence on the charge exchange properties, and (5) strong electron shell effects manifest themselves when results for different elements are compared.

ISSN:0021-8979    

DOI:10.1063/1.352846

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Sheet electron beams focused by periodically cusped magnetic (PCM) fields are stable against low‐frequency velocity‐shear instabilities (such as diocotron mode). This is in contrast to more familiar unstable behavior in uniform solenoidal magnetic fields. Two rectangular‐cross‐section magnetic configurations capable of focusing in both transverse dimensions are investigated: (i) a closed‐side two‐plane PCM configuration that is topologically equivalent to conventional round‐cross‐section PPM focusing; and (ii) an open‐side configuration that uses ponderomotive PCM focusing in the vertical plane and simplevzByLorentz force focusing in the horizontal plane. Both configurations are capable of stable sheet beam confinement. The open‐side configuration appears more practical both for focusing and for realizing matched (cold) beam conditions in which the beam envelope is free from oscillations. For realistic beams with finite emittance, the existence of a matched cold beam solution implies less emittance growth at beam injection.

ISSN:0021-8979    

DOI:10.1063/1.352847

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Details of an experimental investigation of the gain and saturation intensity of a multichannel extended‐area discharge CO2laser are presented. Measurements were made as a function of several discharge parameters. The gain coefficient was found to be similar in each of the individual discharge regions, having a maximum value of 0.70%/cm. Effects of gas pressure, gas mixture, and input power on gain and saturation parameters were studied. The saturation intensity varied as the square of the total gas pressure. The central interelectrode region of the device, which contained no visible discharge, exhibited a significant level of gain. The investigation reported herein provided important data for the design and optimization of optical extraction configurations.

ISSN:0021-8979    

DOI:10.1063/1.352848

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The temperature dependence of the dc drift of Mach–Zehnder external modulators is evaluated based on Ti diffused LiNbO3with a sputter deposited SiO2buffer layer. From Arrhenius’ plots of the results, activation energies of the drift rate between 25 and 80 °C were found to be about 1 eV. The activation energy seems to depend on the structural parameters of the SiO2layer.

ISSN:0021-8979    

DOI:10.1063/1.352849

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The streaming induced in a short vertical liquid column (L/R≤1,L: length,R: radius) by the vibration of one of the supporting end walls has been utilized in this novel study. Vibration essentially drives a surface flow in the zone away from the vibrating wall, with the return flow in the bulk towards the wall. Preliminary measurements of the surface streaming velocity show that it increases with the frequency and amplitude of vibration and the zone length, and decreases with the viscosity of the zone liquid. This controlled surface streaming has been employed to balance an opposing, steady thermocapillary flow in a model half‐zone of silicone oil. In addition to the evidence gathered through flow visualization, temperature measurements in the zone reveal that the radial temperature gradients set up by the thermocapillary flow are weakened/offset by this balancing.

ISSN:0021-8979    

DOI:10.1063/1.352850

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Plasma source ion implantation is a process in which a target is immersed in a plasma and a series of large negative‐voltage pulses are applied to it to extract ions from the plasma and implant them into the target. A Monte Carlo simulation model is developed to study the energy and angle distributions of ions at the planar target for higher pressures of the neutral gas. Cross sections of the charge exchange and momentum transfer that depend on the ion energy are taken into account precisely. The energy and angle distributions of N2+at the target during the sheath edge evolution for the different pressures are determined.

ISSN:0021-8979    

DOI:10.1063/1.352851

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Ion species and their ratios in nitrogen, oxygen, and argon plasmas in the plasma source ion implantation process have been determined with a simple and low‐cost measurement system. The measured ion species ratio in the nitrogen plasma was used as an input parameter for the computer simulation code transport and mixing from ion irradiation to predict the atomic composition‐depth profile. Comparison between the code results and data derived from Auger analysis for a nitrogen‐implanted Ti‐6Al‐4V alloy showed good agreement. In this article, the design, performance, and possible future improvements regarding the resolution of this measurement system will be discussed.

ISSN:0021-8979    

DOI:10.1063/1.352852

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The resistive hose instability can disrupt propagation of self‐pinched beams in dense gas. To reduce growth of the instability, beams can be conditioned prior to propagation. The objectives of beam conditioning are to center the beam in order to reduce initial transverse perturbations which seed the hose instability, and to ‘‘tailor’’ the beam emittance in order to detune the head‐to‐tail coherence of the instability. Emittance tailoring can be performed by transporting the beam through a ‘‘passive ion‐focused regime’’ (IFR) cell, a low‐pressure gas cell that induces a head‐to‐tail taper of the beam radius. The radius taper is then converted to an emittance taper by passing the beam through a thick exit foil which scatters the beam. Beam centering can be accomplished by transporting the beam through either: (i) a passive IFR cell that is narrow enough to provide wall guiding, or (ii) a laser‐ionized ‘‘active’’ IFR cell, or (iii) a wire cell in which the centering is provided by a current‐carrying wire. Axisymmetric particle simulation studies of IFR tailoring cells, alone and in tandem with each of these types of centering cells, and also the effect of supplementary focusing lenses and conducting foils are reported. The parameter choices that are conducive to effective beam conditioning are discussed. The emphasis is on conditioning configurations and beam parameters that have actually been tested in experiments with the Advanced Test Accelerator and SuperIBEX accelerator.

ISSN:0021-8979    

DOI:10.1063/1.352853

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electron behavior in an electron cyclotron resonance microwave discharge maintained by the TM01mode fields of a cylindrical waveguide has been investigated via a Monte Carlo simulation. Since this discharge has high degree of ionization (≥1%), a self‐consistent simulation of the plasma dynamics is achieved through the use of the ponderomotive and grad B(−&mgr;∇∥B) forces. Accumulation of negative charges on the boundary surface sets up a sheath whose influence is also taken into account. The time averaged, spatially dependent electron energy distribution (EED) is computed self‐consistently by integrating electron trajectories subjected to the microwave fields, the divergent background magnetic field, the space charge field, and the sheath field, and taking into account electron–electron collisions and collisions with the neutral hydrogen atoms. The EED is characterized by two electron temperatures with the population of the tail increasing for decreasing pressure. At low pressures (∼0.5 mTorr), the sheath potential is on the order of 100 V and decreases with increasing pressure. This observation suggests a pressure range for operation of reactors for diamondlike carbon film deposition.

ISSN:0021-8979    

DOI:10.1063/1.352824

出版商:AIP    

年代:1993

数据来源: AIP