摘要:

The effect of cesium vapor partial pressure on the plasma parameters has been studied in the dc hybrid negative ion source “CAMEMBERT III.” The cesium vapor pressure was varied up to10−5&hthinsp;Torrand was determined by a surface ionization gauge in the absence of the discharge. The negative ion relative density measured by laser photodetachment in the center of the plasma extraction region increases by a factor of four when the plasma is seeded with cesium. However the plasma density and the electron temperature (determined using a cylindrical electrostatic probe) are reduced by the cesium seeding. As a result, the negative ion density goes up by a factor of two at the lowest hydrogen pressure studied. The velocity of the directed negative ion flow to the plasma electrode, determined from two-laser beam photodetachment experiments, appears to be affected by the cesium seeding. The variation of the extracted negative ion and electron currents versus the plasma electrode bias will also be reported for pure hydrogen and cesium seeded plasmas. The cesium seeding leads to a dramatic reduction of the electron component, which is consistent with the reduced electron density and temperature. The negative ion current is enhanced and a goes through a maximum at plasma electrode bias lower than 1 V. These observations lead to the conclusion that the enhancement of pure volume production occurs in this type of plasma. Possible mechanisms for this type of volume process will be discussed. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56360

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

There is a considerable interest in the creation of vibrationally excitedH2molecules because of their application toH−volume-plasma sources, plasma chemistry etc. It was shown previously that a low-voltage discharge inCs-H2mixture might be used as an effective source of vibrationally excitedH2molecules. In the present communication, it is shown that vibrational distribution function (VDF) ofH2molecules may be very significantly improved if vibrational excitation occurs in a moving flux ofH2molecules penetrative through the discharge.The following theoretical model is considered. Hydrogen flux flows in a flat channel and passes through two sections one after another. The first section (I) is a discharge one, where low-voltageCs-H2discharge occurs and vibrationally excitedH2molecules are created. In the next section (II), the excited hydrogen flows in a plane channel, which walls are unheated. Because of a large value of the mean vibrational energy ofH2molecule, very significant intensification ofH2high vibrational level population takes place in the section II. This intensification is of the order of103−104for several vibrational levels. The increase of vibrational level population occurs due to the diffusion of vibrational quanta, by v-v exchange, up to high vibrational levels. VDF, obtained at the exit of a section II, is significantly improved for the purposes ofH−generation by means of dissociative attachment. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56361

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We compare the negative ion fraction obtained for scattering of hydrogenous ions (Hx+,x=1–3) from highly oriented pyrolytic graphite (HOPG), with that obtained for scattering from a polycrystalline graphite surface. In contrast to the HOPG surface, which has a negative ion yield of the order of 1–2&percent;, the polycrystalline sample has yields of up to 30&percent;. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56353

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A new negative ion source extraction model has been formulated and implemented which explicitly considers the motion of positive ions and the volume generation of negative ions. It is found that (1) for high-beam currents, the beam current is limited by a transverse space charge limit, meaning that an increase in negative ion density at the extraction sheath will result in a lower beam current (this result is universally observed at high beam currents); (2) there is a saddle point with a potential barrier preventing most volume produced negative ions from being extracted (the combination of 1 and 2 indicates that most of the negative ions being created do not find their way into the beam); (3) introduction of cesium may cause, most importantly, an increase in the transverse space charge limit (there is an abundance of experimental data supporting this effect); (4) cesium may also result in an increase in the fraction of volume produced negative ions which are extracted; (5) cesium may also result in a reduction of extracted electrons by dint of a less negative bias on the plasma electrode with respect to the adjacent plasma, thus allowing the transverse space charge limit budget to be taken up wholly by the ions. (The combination of 3–5 represents the way an actual increase in the beam current can be achieved); (6) a strong ion time scale sheath instability due to violation of Bohm criteria produces an anomalous ion temperature that increases with beam current routinely seen in experiments; and (7) introduction of cesium may result in a reduction in this instability. These insights may lead to improvements in volume negative ion sources. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56363

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Effects of cesium vapor injection onH−production in a tandem volume source are studied numerically as a function of plasma parameters. Model calculation is performed by solving a set of particle balance equations for steady-state hydrogen discharge plasmas. Here, the results with a focus on electron temperature and gas pressure dependence onH−volume production are presented and discussed. ConsideringH−surface production due to H atoms and positive hydrogen ions, enhancement ofH−production and pressure dependence ofH−production observed experimentally are qualitatively well reproduced in the model calculation, where stripping loss in the extraction and acceleration regions is taken into account. For enhancement ofH−production, so-called electron cooling is not very effective if plasma parameters are initially optimized with the use of a magnetic filter. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56354

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A code called NIETZSCHE has been developed to simulate the negative ion transport in a plasma source, from their birth place to the extraction holes. The ion trajectory is calculated by numerically solving the 3-D motion equation, while the atomic processes of destruction, of elastic collisionH−/H+and of charge exchangeH−/H0are handled at each time step by a Monte-Carlo procedure. This code can be used to calculate the extraction probability of a negative ion produced at any location inside the source. Calculations performed with NIETZSCHE have allowed to explain, either quantitatively or qualitatively, several phenomena observed in negative ion sources, such as the isotopicH−/D−effect, and the influence of the plasma grid bias or of the magnetic filter on the negative ion extraction. The code has also shown that in the type of sources contemplated for ITER, which operate at large arc power densities (>1 W&hthinsp;cm−3), negative ions can reach the extraction region provided if they are produced at a distance lower than 2 cm from the plasma grid in the case of «volume production» (dissociative attachment processes), or if they are produced at the plasma grid surface, in the vicinity of the extraction holes. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56355

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

An increase of theH−-density in a pulsed low-pressure hydrogen multicusp discharge is observed after the current pulse, i.e. in the “post-discharge.” TheH−-density is determined by laser induced photodetachment at a Langmuir probe. A semiempirical model is used to infer the total density of vibrationally excited molecules in the electronic ground state from a comparison with the measured shape of the transientH−-density in the post-discharge. The total density of vibrationally excited molecules is found to be independent of the discharge current up to 20 amps. The density evolution of vibrationally excited molecules during the discharge is obtained from measurements of the maximumH−-density in the post-discharge as a function of the duration of a single discharge pulse. After 200 &mgr;s the density is saturated. The lifetime of vibrationally excited molecules in the post-discharge is determined by varying the time interval between discharge pulses of short duration. The lifetime is found to be approximately 1 ms. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56356

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The purpose of this experiment is to improve the optics of an intenseH−negative ion beam, to be used in future tokamaks. The emittance diagnostic has been installed in the experimental set-up INCA, to perform an optimization of a negative-ion injector preaccelerator. The first stage of this work consists to test the operation of the electric-sweep scanner, to measure the beam current in the 2D-phase space and to investigate the diagnostic performances. In addition to the measurement ofH−angles, we can scan the angles of the fastH+created along the beam in the transport region and test the efficiency of the electron magnetic trap installed in the second accelerator grid (extraction electrode). We also observe the presence of a neutral component. This analyzer not only provides a diagnostic of the beam divergence but also improves of the knowledge of the beam components. We studied two accelerator configurations. The first one permitted to obtain an optimal beam optics under 15 keV. Then the accelerator geometry was modified to ensure a higher optimum high-voltage. We will show several experimental results performed with two accelerator configurations. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56357

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper we report the first results on a prototype negative ion source for LHD-NBI. The ion source with a multi-cusp configuration consists of the external magnetic filter and has an inner volume of 140 cm in height ×35 cm in width ×25 cm in depth. Two cesium lines are installed to seed cesium vapor into arc plasma for enhancement ofH−current. TheH−ion beam was extracted from an area of120&hthinsp;cm×25&hthinsp;cmand was accelerated via single-stage accelerator. AnH−current of about 4 A with the energy of 73 keV was obtained at the arc power of 250 kW for a duration of 0.3 sec. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56358

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Advanced Tokamak concepts and steady state plasma scenarios require external plasma heating and current drive for extended time periods. This poses several problems for the neutral beam injection systems that are currently in use. The power loading of the ion source and accelerator are especially problematic. The Kamaboko negative ion source, a small scale model of the ITER arc source, is being prepared for extended operation of deuterium beams for up to 1000 seconds. The operating conditions of the plasma grid prove to be important for reducing electron power loading of the accelerator. Operation of deuterium beams for extended periods also poses radiation safety risks which must be addressed. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56362

出版商:AIP    

年代:1998

数据来源: AIP