摘要:

An enhanced supersonic carbon source produces carbon atoms in their C(3Pj) electronic ground states via laser ablation of graphite at 266 nm. The 30 Hz (40±2) mJ output of a Nd‐YAG laser is focused onto a rotating graphite rod with a 1000 mm focal length UV‐grade fused silica plano‐convex lens to a spot of (0.5±0.05) mm diameter. Ablated carbon atoms are subsequently seeded into helium or neon carrier gas yielding intensities up to 1013C atoms cm−3in the interaction region of a universal crossed beam apparatus. The greatly enhanced number density and duty cycle shift the limit of feasible crossed beam experiments down to rate constants as low as 10−11–10−12cm3 s−1. Carbon beam velocities between 3300 and 1100 m s−1, with speed ratios ranging from 2.8 to 7.2, are continuously tunable on‐line andinsituwithout changing carrier gases by varying the time delay between the laser pulse, the pulsed valve, and a chopper wheel located 40 mm after the laser ablation. Neither electronically excited carbon atoms nor ions could be detected within the error limits of a quadrupole‐mass spectrometric detector. Carbon clusters are restricted to ∼10% C2and C3in helium, minimized by multiphoton dissociation, and eliminating the postablation nozzle region. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146061

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Intense H−beams have been extracted from a large multicusp plasma source operated with cesium seeding. The H−beams were accelerated up to 100 keV by a single‐stage or a two‐stage electrode system. Spatial profiles of the beams are measured calorimetrically and the beam divergence angle is obtained from half of thee‐folding width. A minimum beam divergence angle of 5 mrads is achieved at a H−current density of 30 mA/cm2with a beam energy of 100 keV. The ratio of acceleration current to H−current increases abruptly when a H−current saturates in the space charge limited region. This enhancement is mainly due to secondary electrons caused by the intersection of H−beams with an extraction grid. When the operating gas pressure decreases, the ratio of the acceleration current to the H−current decreases. This is related to a stripping loss of H−ions in the electrodes. A beam divergence angle reaches a minimum when a ratio ofVacctoVextis set at an optimum value of 1.6 in the single‐stage acceleration. This ratio is almost the same as that in the double‐stage acceleration, where the optimum ratio ofEaccl/Eextis 1.5. In the optimumEaccl/Eextratio the divergence angle is not affected byVacc2. The divergence angle can be reduced by changingVacc2even if the ratio ofEaccl/Eextis not optimized. The beam steering effect by permanent magnets buried in an extraction grid is observed in nine beamlets experiments. A simple calculation of a single particle trajectory gives a good approximation of the beam deflection angle. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146424

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The use of both a small discharge volume and magnetic mirror fields allow sequential ionization and an increased efficiency of this ion source. The extracted ion current can reach 12 mA at an extraction voltage=3 kV and discharge pressure=20 mTorr. In this paper a description of the techniques which are used for measuring the important parameters to evaluate this ion source is presented. These parameters are: beam profile, beam emittance, energy spread, and charge spectra of the beam. The source is found to have a beam emittance of 150 mm m rad, and energy spread of 30–60 eV, and a content of Ar+4reaching &bartil;20% of the total extracted ion current. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146062

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An electron cyclotron resonance plasma with large diameter, uniform, and high electron density is produced using a multislot antenna. The uniformity of the plasma is within 5% over 20 cm in diameter. The electron density of the helium plasma in front of a substrate is 7×1010cm−3, while the electron density without the substrate is higher than the cutoff density for 2.45 GHz. The effect of the magnetic field configuration on plasma uniformity is investigated. Both of theRwave (electron cyclotron wave) andLwave are found to be excited in the plasma. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146063

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A fast reciprocating probe system with a long drive shaft was incorporated into a multi‐junction lower hybrid (LH) wave launcher on JT‐60U in order to investigate an improved coupling mechanism of the radio frequency wave to the core plasma. The system has been operated reliably over a horizontal scan of 25 cm in 1.5 s using a compact pneumatic cylinder drive and springs. A double probe measurement provided the scrape‐off layer plasma profile between the last closed flux surface and the first wall with the spatial resolution of 1−2 mm measured with a laser displacement gauge. The profiles of the electron densityneand temperatureTewere in good agreement with those obtained with a triple probe method. During the LH wave injection with good coupling to the core plasma, an increase in the localTewas observed in front of the LH launcher mouth. The localnewas  (7−10)×1016 m−3, consistent values needed for the good coupling. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146064

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A spatially scanning, combined vacuum‐ultraviolet (VUV) and visible range spectrometer system for the spectroscopy of tokamak plasmas in the ASDEX‐Upgrade experiment is described. This system is designed to allow flexible observation of about 2/3 of the boundary plasma using VUV (30–200 nm) and visible range spectrometers viewing along a common line of sight which can be scanned during the plasma discharge by means of a rotatable mirror. From successive spectra recorded using intensified, multichannel photodiode detectors and the recorded position data, spatial profiles of the plasma emission can be reconstructed. Because radiation losses from the boundary plasma can largely be attributed to line emission in the VUV spectral region, this instrument finds application in quantitative studies of radiation loss processes as well as to studies of impurity production and transport. Simultaneous observation in the visible spectral range facilitates aninsituabsolute calibration of the VUV instrument by means of the ‘‘branching‐ratios’’ technique. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146065

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The one‐dimensional (1D) position‐sensitive superheated‐liquid‐droplet in‐phantom neutron dosimeter incorporating a sensitive volume emulsion has been fabricated, prepared, and tested. The 1D position‐sensitive superheated‐liquid‐droplet dosimeter (SLDD) is fabricated from a 3/8‐in.‐o.d., 1/4‐in.‐i.d., 20‐cm‐long, PlexiglasTM‐walled tube filled with a mixture of superheated‐liquid FreonTMdroplets and host medium glycerol solution. Washer‐shaped piezoelectric acoustic transducers are positioned at both ends of the tube; they determine the number and positions of the acoustic events when the superheated‐liquid droplets evaporate upon neutron irradiation. The SLDD is sensitive to a wide range of neutron energy, from thermal (0.0253 eV) up to 10 MeV and higher. The SLDD is irradiated with the137Cs and60Co &ggr; sources, as well as a252Cf neutron source to test for its radiation response and spatial resolution. The SLDD based on the Freon‐134a superheated‐liquid droplets operating at 20 °C and 1 atm is found to be ideal for measuring neutron depth dose and relative biological effectiveness dose. This study also proves that the position of the radiation‐induced nucleation acoustic events can be linearly determined from the differences in the transmission times received by the acoustic transducers on the 1D SLDD. The spatial resolution of the neutron depth dose is 1 mm due to the finite response time (1 &mgr;s) of the piezoelectric acoustic transducers. ©1995 American Institute of Physics. 

ISSN:0034-6748    

DOI:10.1063/1.1146066

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A thermocouple method was used to measure the concentration of hydrogen radicals at hydrogen pressures from 0.01 to 30 Torr. The concentration can be calculated from a heat balance at the thermocouple. Under low hydrogen concentration conditions, all terms in the heat balance equation must be estimated accurately. Our results for the measured absolute concentration of the hydrogen radicals at pressures below 1 Torr agreed well with the calculated concentration predicted by a model that is based on the surface dissociation probability at the hot filament and the simulation of diffusion from the filament. With increasing pressure, the hydrogen radical concentration shifted from the surface‐dissociation‐rate‐controlled regime to the diffusion‐controlled regime at the hot filament. The agreement between the heat balance at the thermocouple and that at the hot filament shows that the measurement of the hydrogen radical concentration is of the correct order. Our results show that this measurement technique could be used in studying the growth mechanisms in various deposition processes where hydrogen is a main species. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146067

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A long‐pulsed neodymium‐doped yttrium aluminum garnet (Nd:YAG) frequency‐doubled laser with no Q‐switch was constructed for optical measurements of high‐velocity phenomena. The laser consists of a doubled‐elliptical pump cavity with a Nd:YAG rod and two xenon flash lamps, an intracavity potassium titanyl phosphate crystal, and a high‐voltage electrical‐pulse source. A narrow‐band‐stimulated emission at the frequency‐doubled 532‐nm wavelength was confirmed by a spectrometer. The delay time from a trigger signal and the effective pulse duration were approximately 40 and 65 &mgr;s, respectively. The laser average output power was measured to be larger than 6 kW by a pyroelectric joulemeter. This laser may be used as a flash monowavelength light source, and also as a long‐pulsed single‐mode laser in the visible wavelength region using for example, an intracavity e´talon and/or a brewster plate. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146068

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Reliable phase‐sensitive time‐resolved interferometry with ultrashort pulsed lasers is performed with the use of a closed scanning Michelson interferometer in combination with a fixed Mach–Zehnder interferometer at the front end. The technique is based on measuring the full phase and frequency properties of the pulse distortion of an ultrashort optical pulse introduced by linear or nonlinear interaction with a sample. The necessary stability and reproducibility to perform an interferometric measurement is provided by a commercially available Fourier transform spectrometer enabling time‐resolved measurements from the IR well into the visible part of the optical spectrum. The feasibility of the technique is demonstrated by measuring the distortion introduced by an etalon and a surface‐plasmon polariton. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146069

出版商:AIP    

年代:1995

数据来源: AIP