摘要:

Negative ion sources and the mechanisms for negative ion production are reviewed. Several classes of sources with surface origin of negative ions are examined in detail: surface‐plasma sources where ion production occurs on the electrode in contact with the plasma, and ‘‘pure surface’’ sources where ion production occurs due to conversion or desorption processes. Negative ion production by backscattering, impact desorption, and electron‐ and photo‐stimulated desorption are discussed. The experimental efficiencies of intense surface negative ion production realized on electrodes contacted with hydrogen‐cesium or pure hydrogen gas‐discharge plasma are compared. Recent modifications of surface‐plasma sources developed for accelerator and fusion applications are reviewed in detail.

ISSN:0034-6748    

DOI:10.1063/1.1144048

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The ratio of electrons to negative ions extracted from Penning surface‐plasma sources such as the 8X source is low even before any further steps are taken to suppress the electrons. For the 8X source thee−/H−ratio is typically four or five to one for H−operation and nine to one for D−operation. Because the coextractede−present a power‐loading problem to the 8X‐source extraction system, methods to dissipate and/or reduce the power in thee−beam must be developed before extracting a dc H−or D−beam. Thus, we systematically varied the geometry of a cylindrical collar installed in the near‐extraction region of the 8X source. The observed dependence of the extractede−and H−currents on the collar radii and lengths suggests that a conical collar would provide superior electron suppression. The conical collar that we tested lowered thee−/H−ratio to 0.9/1 without reducing the extracted H−current.

ISSN:0034-6748    

DOI:10.1063/1.1144049

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A 12‐channel system for resolving either the &ohgr; orkspectra of plasma Langmuir turbulence is developed. The direct detection system is operated with a scattering linewidth of order 100 GHz at scattering angles ranging from 10−1to 10−3rad. Stray light rejection of up to 13 orders of magnitude is provided by an ammonia‐filled gas cell. It is possible to vary the absorption frequency width from 20 MHz to 5 GHz keeping absorption in the ammonia line center constant. The &ohgr; andkspectra of Langmuir turbulence excited in the plasma by a high‐current relativistic electron beam are measured via the technique described.

ISSN:0034-6748    

DOI:10.1063/1.1144050

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

For energetic x rays (hv≥3 keV), ring coded‐aperture imaging gives better signal‐to‐noise ratio than equivalent‐resolution pinhole cameras for inertial confinement fusion (ICF) targets. We have created a time‐resolved ring coded‐aperture microscope by combining a 4×3 array of annular apertures with a gated microchannel‐plate x‐ray imager. The new instrument can produce 500‐ps duration sequences of images with a temporal resolution of 80 ps and a spatial resolution of 5 to 6 &mgr;m. In demonstration experiments, coded images of imploded targets directly driven by the Omega laser at the Laboratory for Laser Energetics showed the formation and dissolution of the target cores with improved detail. Contour plots of the images indicate that the laser illumination pattern is imprinted in the imploded core region. The gated ring‐aperture microscope will be useful in detailed studies of ICF target hydrodynamics.

ISSN:0034-6748    

DOI:10.1063/1.1144051

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The design and operation of a new vaporizing assembly used forQ‐plasma sources are described in this article. The vaporizing system, consisting of two ovens and an effuser, is used to vaporize and direct atoms onto a hot ionizer plate. The hot plate, which may be of tantalum, tungsten, or rhenium, singly ionizes the atoms on contact. The main advantages of this new assembly, compared to previous designs, are the production of higher plasma densities and the control of the radial plasma profile. The heaters of the two atomic beam ovens are independently controlled and monitored so that the gradients of the radial plasma profile can be modified. Plasma production with two ion species can also be performed.

ISSN:0034-6748    

DOI:10.1063/1.1144052

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this article we describe a simple fast pulse gas valve developed for use in a plasma discharge experiment. The valve delivers 1017–1019molecules per pulse varied by changing the voltage on the electromagnetic driver power supply. Valve pulse widths are observed to be less than 300 &mgr;s full width at half maximum with a rise time of less than 100 &mgr;s resulting in a maximum gas flow rate of ∼1022molecules per second. An optical transmission technique was used to determine the mechanical opening and closing characteristics of the valve piston. A fast ionization gauge (FIG) was used for diagnosis of the temporal character of the gas pulse while the total gas throughput was determined by measuring the change in pressure per pulse in a small test chamber with a convectron tube gauge. Calibration of the FIG was accomplished by comparing the net change in pressure in a large chamber as measured by the FIG to the net change in pressure in a small test chamber as measured by the convectron tube gauge.

ISSN:0034-6748    

DOI:10.1063/1.1144053

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Time‐resolved electric field profile measurements were made in a driven 15 kHz parallel plate helium glow discharge using optogalvanic spectroscopy. Electric field values in the sheath region were determined from the energy splitting of the helium Rydberg state Stark manifolds due to the local applied electric field as a function of position and phase. The measured field values in the cathode sheath region range from about 800 to 300 V/cm; the measurement accuracy of electric field values are ±25 V/cm. The line of sight laser probe measurements have a spatial resolution of 200 &mgr;m and a temporal accuracy of better than 1% of the driving cycle. The low driving frequency allows for a comparison with the dc discharge approximation. The spatial electric field profile is identical to a dc discharge operating at the same peak current and voltage values and the temporal field profile is symmetric about the current wave form extremum.

ISSN:0034-6748    

DOI:10.1063/1.1144054

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A neutron detector capable of measuring the time‐dependent yield of 14‐MeV neutrons from a D–D plasma producing predominantly 2.5‐MeV neutrons has been developed. The detector consists of a thick polythene recoil proton radiator backed by a graphite foil attached to a large area totally depleted ion‐implanted diode. Protons scattered in the forward direction by 14‐MeV neutrons pass through the graphite foil and are registered in the diode. Recoil protons from 2.5‐MeV neutrons, however, are prevented from reaching the diode by the foil. When operated with a 1.5‐MeV energy bias, the measured neutron detection efficiency for 15‐MeV neutrons is 3.2×10−3per neutron. The corresponding figure for 3.1‐MeV neutrons is a factor of 540 lower. The neutron detector and its laboratory calibration are described, as is its deployment at the Joint European Torus where it serves as a triton burn‐up monitor.

ISSN:0034-6748    

DOI:10.1063/1.1144055

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A newly developed probe, having a 0.05% resolution, has been used to detect errors in the toroidal magnetic field of the NRL modified betatron accelerator. Measurements indicate that the radial field components (errors) are 0.1%–1% of the applied toroidal field. Such errors, in the typically 5 kG toroidal field, can excite resonances which drive the beam to the wall. Two sources of detected field errors are discussed. The first is due to the discrete nature of the 12 single turn coils which generate the toroidal field. Both measurements and computer calculations indicate that its amplitude varies from 0% to 0.2% as a function of radius. Displacement of the outer leg of one of the toroidal field coils by a few millimeters has a significant effect on the amplitude of this field error. Because of uniform toroidal periodicity of these coils this error is a good suspect for causing the excitation of the damagingl=12 resonance seen in our experiments. The other source of field error is due to the current feed gaps in the vertical magnetic field coils. A magnetic field is induced inside the vertical field coils’ conductor in the opposite direction of the applied toroidal field. Fringe fields at the gaps lead to additional field errors which have been measured as large as 1.0%. This source of field error, which exists at five toroidal locations around the modified betatron, can excite several integer resonances, including thel=12 mode.

ISSN:0034-6748    

DOI:10.1063/1.1144056

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

To obtain a large aperture lens with outstanding focal length homogeneity, a concave electrostatic quadrupole lens with a periodic configuration of the electrodes was developed and its performance was numerically studied. By adjusting the azimuthal angle (&thgr;c) of the electrode to an adequate periodic function along the lens axis, the integration of the field gradient along the axis, for a period (∫&fgr;dz/∫dz), becomes equal to an ideal quadrupole field distribution. To verify the usefulness of the lens, the beam spot distortion of the paraxial beam, which had an initial beam diameter of 90% of the lens radius, was calculated. The results revealed that the error of the focal length inside 90% of the lens radius was only 0.012% when the lens has nine period per unit length. Also, by considering the characteristics of the modified Bessel function, a simple formula for the uniformity of the focal length was obtained, and this expression agreed well with the simulation results.

ISSN:0034-6748    

DOI:10.1063/1.1144057

出版商:AIP    

年代:1993

数据来源: AIP