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Feb 2000

Volume 71, Issue 2, pp. 335-1239

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back to top FUNDAMENTAL PHENOMENA FOR ION SOURCES

Selection of the powdery metal hydride best for producing H− by thermal desorption

H. Kawano, A. Tanaka, S. Sugimoto, T. Iseki, Y. Zhu, M. Wada, and M. Sasao

Rev. Sci. Instrum. 71, 853 (2000); http://dx.doi.org/10.1063/1.1150311 (3 pages) | Cited 3 times

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To find the metal hydride best for producing H− by thermal desorption, the desorption rates of H−, H2 and electron (e−) directly emitted from a powdery sample (∼1 mg) of NaH, LiH, MgH2, CaH2, SrH2, TiH2, ZrH2, KBH4, LiAlH4, or NaAlH4 heated up to ∼1000 K were determined simultaneously with a special system. Theoretical analysis of the experimental data thus achieved yields the following results. (1) Both H− and e− are emitted from those active spots (mainly consisting of alkali or alkali earth metal) created by thermal decomposition (e.g., LiH→Li+H2/2). (2) The active spots are readily destroyed and reconstructed by admission of H2 and by stopping the admission, respectively. (3) The work function (ϕ) of activated NaH is very low (∼2 eV), but NaH is rapidly depleted owing to its thermal instability. (4) Among the ten hydrides, CaH2 is concluded to be the best as a source material for thermal desorption of H− because activated CaH2 (ϕ≃5 eV) is most stable and strongest in H− ( ∼ 10−12 A or 10−10 A/cm2 after mass analysis) around 900 K. © 2000 American Institute of Physics.
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68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Selection of the substrate metal best for thermal positive ionization

H. Kawano, H. Mine, M. Moriyama, M. Tanigawa, and Y. Zhu

Rev. Sci. Instrum. 71, 856 (2000); http://dx.doi.org/10.1063/1.1150312 (3 pages) | Cited 2 times

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To select the substrate metal best for producing thermal positive ions from incident sample molecules, a beam of diatomic molecule (MX) such as LiCl, NaBr, or TlCl was directed with a constant flux ( ∼ 1012–1014 molecules cm−2 s−1) onto a polycrystalline thin wire (∼0.01–0.02 cm in diameter) of refractory metal (Nb, Mo, Ta, W, Re, Ir, or Pt) heated in vacuum ( ∼ 10−7–10−5 Torr), and the total emission current ( ∼ 10−10–10−8 A) of the positive ion (M+) was measured as a function of wire temperature (∼900–2300 K). The experimental data newly achieved were analyzed by our theoretical model to determine the ionization efficiency (β+) and also the work function (ϕ+) effective for the ionization. Among the seven metals, Re is concluded to be best for the ionization because Re is highest in ϕ+( ∼ 7.5 eV), thereby making it possible to produce M+ efficiently (β+ = 1) and stably (within ±1%) from those 30 elements (M) whose ionization energy is less than ∼6.5 eV. © 2000 American Institute of Physics.
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79.40.+z Thermionic emission
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
73.30.+y Surface double layers, Schottky barriers, and work functions

Influence of electron injection on electron cyclotron resonance plasma properties and reflected mode electrons (abstract)

V. P. Ovsyannikov, F. Ullmann, and G. Zschornack

Rev. Sci. Instrum. 71, 859 (2000); http://dx.doi.org/10.1063/1.1150145 (1 page)

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The injection of an additional strong focused electron beam from a special designed electron gun into a magnetic electron cyclotron resonance (ECR) confinement field is studied. The electron gun uses a cathode with a long lifetime and resistiveness providing high emission current densities with electron currents up to 50 mA and voltages up to 4 keV. A sequence of aluminum foils is used to investigate the trajectories of the electrons in the magnetic field without plasma. The high density electron beam passes through the foils, welds them, and prints its image into the foils. Details of this technique are described in Ref. 1. Using this technique we see that before the electrons enter the sextupole region the beam moves along the magnetic straight lines preserving its structure. Only a central beam passes through the sextupole region, thereby changing its form due to the interaction with radial components of the magnetic field. A new operation method at our 14.5 GHz ECR ion source is based on so-called reflection mode electrons (RMEs) analogous to a known electron beam ion source operation regime.2 The basic idea is that electrons, which traveling from the cathode in a strong axial field, meet an anticathode potential, are reflected from it, move back to the cathode, and will be reflected again and so on. It can be supposed that the electrons will make reflections up to the moment when the anode aperture of the gun is fulfilled and the electrons will be collected on the anode electrode. Investigations are performed extracting nitrogen ions using the RME beam. As a result we got a clear increase in the beam current of the extracted ions (e.g., at 10 mA electron injection an increase of the current of N5+ ions up to 400%) and a shift of the measured ion charge state distribution to higher mean ionization stages. Measured x-ray spectra from a neon loaded plasma show for the case of RME operation increasing energy shifts to the high energy side of the spectra, i.e., the mean ionization degree of the ions in the plasma increases. They also increase the intensity of the neon K x rays (more than 100% increase for RME injection of Ee=4 keV and Ie = 10mA) indicating that for the same operation parameters the mean density of energetic electrons rises at RME injection, i.e., there are more electrons with energies high enough to ionize K-shell electrons in neon. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
52.40.Mj Particle beam interactions in plasmas
29.27.Ac Beam injection and extraction

Production of intense 48Ca ion beam at the U-400 cyclotron

V. B. Kutner, S. L. Bogomolov, A. A. Efremov, A. N. Lebedev, V. Ya. Lebedev, V. N. Loginov, A. B. Yakushev, and N. Yu. Yazvitsky

Rev. Sci. Instrum. 71, 860 (2000); http://dx.doi.org/10.1063/1.1150313 (3 pages) | Cited 4 times

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Production of the intense accelerated 48Ca ion beam is the key problem in the experiments on synthesizing of new heavy nuclei. For this purpose an axial injection system with the electron cyclotron resonance (ECR)-4M ion source was created for the U400 cyclotron. The task was to achieve an accelerated beam with an intensity of 0.5 pμA of 48Ca5+ at the 48Ca consumption of ∼0.5 mg/h. To solve this problem, a new method for the solid material feed into the ECR source was developed. The combination of a micro oven with a hot tantalum sheet inside the discharge chamber allowed the production of intense beams of ions of metals with relatively low melting point. The present article describes the method, technique, and experimental results on the production of 48Ca ion beam at the U-400 cyclotron from the ECR-4M ion source. The analysis of the working substance balance in the ion source including the ion beam extraction and material regeneration is performed. The analysis based on the experimental data has shown that the efficiency of Ca atom transformation into ion beam is close to that obtained for the gases such as Ar, and the intensity of 48Ca5+ constitutes about 20% of the extracted ion beam. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
29.27.Ac Beam injection and extraction
29.20.dg Cyclotrons

Effect of a biased probe on the afterglow operation of an ECR4 ion source

C. E. Hill, D. Küchler, F. Wenander, and B. H. Wolf

Rev. Sci. Instrum. 71, 863 (2000); http://dx.doi.org/10.1063/1.1150314 (3 pages) | Cited 1 time

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Various experiments have been performed on a 14.5 GHz ECR4 in order to improve the ion yield. The source runs in pulsed afterglow mode, and provides currents ∼120 eμA of Pb27+ to the CERN Heavy Ion Facility on an operational basis. In the search for higher beam intensities, the effects of a pulsed biased disk on axis at the injection side were investigated with different pulse timing and voltage settings. No proof for absolute higher intensities was seen for any of these modifications. However, the yield from a poorly tuned/low-performing source could be improved and the extracted pulse was less noisy with bias voltage applied. The fast response on the bias implies that increases/decreases are not due to ionization processes. A good tune for high yield of high charge states during the afterglow coincides with a high plasma potential. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
52.50.Gj Plasma heating by particle beams
29.27.Ac Beam injection and extraction
52.80.Hc Glow; corona
29.20.-c Accelerators

Comparison of electron cyclotron resonance plasma characteristics discharged by 7.0, 8.0, and 9.4 GHz

Y. Kawai, T. Saburi, S-H. Kim, Y. Fujii, and T. Suzuki

Rev. Sci. Instrum. 71, 866 (2000); http://dx.doi.org/10.1063/1.1150315 (3 pages) | Cited 3 times

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Argon electron cyclotron resonance plasma characteristics discharged by 7.0, 8.0, and 9.4 GHz microwaves are measured at 0.013 and 0.080 Pa in a cylindrical chamber. The plasma densities and electron temperatures were found to be almost independent of the microwave frequency at 0.013 Pa, whereas clear differences were seen at 0.080 Pa. At 0.080 Pa, the plasma densities were observed to increase with the increase in the discharge frequency. Electron temperatures at 0.080 Pa also appeared to become higher with the increase in discharge frequency. © 2000 American Institute of Physics.
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52.50.Dg Plasma sources
52.80.Pi High-frequency and RF discharges

Production of highly charged ions in electron cyclotron resonance ion sources using an electrode in two modes

S. Biri, L. Kenéz, A. Valek, T. Nakagawa, M. Kidera, and Y. Yano

Rev. Sci. Instrum. 71, 869 (2000); http://dx.doi.org/10.1063/1.1150316 (3 pages) | Cited 1 time

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One of the most known ways to obtain higher beam intensities in electron cyclotron resonance (ECR) ion sources is to install an electrode (usually disk) into the plasma chamber. We found that a majority of the groups observed the beam intensity improvement by supplying a suitable biased voltage to the electrode and an electron current was injected into the plasma. A few groups observed the enhancement, however, when the electrode operated at floating potential—without being an electron donor. In spite of the great success of the “biased disk” method, the mechanism is still not completely clear. In this contribution, as a step toward of understanding, we examined the above mentioned two modes. The experiments were performed at the 18 GHz RIKEN and at the 14.5 GHz ATOMKI ECR ion sources. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
07.77.Ka Charged-particle beam sources and detectors

Increasing the space-charge limit and other effects of cesium seeding in hydrogen negative ion sources

J. H. Whealton, M. Bacal, J. Bruneteau, and R. J. Raridon

Rev. Sci. Instrum. 71, 872 (2000); http://dx.doi.org/10.1063/1.1150317 (3 pages) | Cited 2 times

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The role of cesium seeding in increasing the negative ion current in volume sources is described. By a reduction in the local plasma potential the current of extracted electrons is vastly reduced. As a result, cesium increases the fraction of the transverse space-charge limit available to the ions by as much as a factor of 3. In addition, cesium can increase the total space-charge limit by injection of Cs+ into the presheath—a newly recognized phenomenon consistent with experimental measurements and determined from application of a Double–Vlasov model for negative ion extraction. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Dg Plasma sources

Beam instability excited by the magnetic filter

Hiroshi Naitou, Kazuo Ohi, and Osamu Fukumasa

Rev. Sci. Instrum. 71, 875 (2000); http://dx.doi.org/10.1063/1.1150318 (2 pages) | Cited 6 times

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By the one-dimensional electrostatic particle simulation, the ion beam instability is observed in the plasma divided by the magnetic filter (MF). The strength of the MF is selected to influence only electron dynamics; ions move freely across the MF. There are grounded walls at the left and right ends of the system. Particles hitting the walls are absorbed there. The high temperature and high density plasma (main plasma) faces the low temperature and low density plasma (subplasma) across the MF located at the center of the system. The averaged space potential of the main plasma is higher than that of the subplasma. Due to the potential gap at the MF, ions in the main plasma are accelerated into the subplasma. Depending on the extent of the asymmetry of the system, steady or the periodic (dynamic) state manifests. For the periodic state, high density clumps get into the subplasma and excite the strong ion beam instability. The new clump comes into the subplasma when the old clump reaches the wall. © 2000 American Institute of Physics.
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52.40.Mj Particle beam interactions in plasmas
52.65.Rr Particle-in-cell method

Relation between vapor Cs and adsorbed Cs in H− ion source

M. Ogasawara, T. Morishita, and A. Hatayama

Rev. Sci. Instrum. 71, 877 (2000); http://dx.doi.org/10.1063/1.1150319 (3 pages) | Cited 1 time

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The relation between gaseous Cs density and the coverage of Cs on the plasma grid surface is determined in the existence of the effect of Cs deposit on the cold surface. An equation for the deposit of Cs on the cold surface in the ion source is obtained by considering the saturation of the deposit. The Cs coverage is expressed as a function of gaseous Cs density in the volume of the ion source by considering the relation τθ≪τa, where τθ is the time scale of the Cs adsorption to the plasma grid surface and τa is that of the Cs adsorption to the cold surface. The coverage varies with the slow time scale through the variation of the gaseous density related to the deposit of Cs on the cold surface. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
52.50.Gj Plasma heating by particle beams

Numerical simulation of cesium cooling effects in H− ion source

T. Morishita, M. Ogasawara, and A. Hatayama

Rev. Sci. Instrum. 71, 880 (2000); http://dx.doi.org/10.1063/1.1150320 (3 pages) | Cited 2 times

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Cesium volume reactions are included in a two-point model numerical code for a high power hydrogen negative ion source. The energy balance equation for the electron temperature and rate equations of Cs and Cs+ are included in our code to investigate the electron cooling and volume effects by cesium seeding. Cesium density in the ion source is taken as a variable for the calculation. Cesium is ionized over 95% in the driver region of the ion source. The electron temperature begins to decrease at cesium density 1011 cm−3 and the electron density increases because of the ionization of cesium. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
52.50.Gj Plasma heating by particle beams
52.65.-y Plasma simulation

Monte Carlo simulation of negative ion production in the negative hydrogen ion source

M. Uematsu, T. Morishita, A. Hatayama, T. Sakurabayashi, and M. Ogasawara

Rev. Sci. Instrum. 71, 883 (2000); http://dx.doi.org/10.1063/1.1150321 (4 pages) | Cited 15 times

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Two Monte Carlo simulation codes: (a) neutral transport code and (b) negative ion (H−) transport code, have been developed to understand transport phenomena in negative ion sources. In the neutral transport code, Boltzmann equations for hydrogen molecules (H2) and atoms (H) are solved. Three dimensional (3D) spatial distributions of H2, H, and H− production are obtained for a tandem negative ion source. The volume production of H− is limited to the area around the gas inlet in the first chamber and near the plasma grid in the second chamber. On the other hand, distribution of H− surface production is shown to be almost uniform over all the plasma grid surface. In the negative ion code, H− trajectories are calculated by numerically solving the 3D equation of motion for H− ions. The effects of the magnetic filter on the extraction probability of surface produced H− ions are mainly studied. The dependence of the extraction probability on the field strength is small. © 2000 American Institute of Physics.
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29.25.Ni Ion sources: positive and negative
52.65.Pp Monte Carlo methods
52.50.Gj Plasma heating by particle beams
52.25.Fi Transport properties
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