Analysis of the fringe field region of magnetic quadrupole lenses: field measurements and ion optical calculations

An investigation into the minimisation of the duodecapole fringe field components of magnetic quadrupole lenses has been conducted at Melbourne. As part of this program, a detailed field mapping of the fringe field region of the Melbourne magnetic quadrupole lenses has been conducted (G.R. Moloney, D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 54 (1991) 24). The field mapping technique measures the radial magnetic field component, B_r over the three cylindrical coordinates, r, and z. We have also conducted a program of modifying the pole tip termination of the lenses to reduce the duodecapole field components. Complete magnetic field measurements for the modified lens pole tips have been presented previously (G.R. Moloney, D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 77 (1993) 35–38). This paper presents an Ion Optical analysis of the effects of the fringe field components on the beam optics of the Melbourne Microprobe systems. The optical effects are calculated with a new computer code developed at Melbourne. Standard numerical ray tracing is performed utilising Runge-Kutta techniques.     

Multipole lenses and their application in nuclear microprobe lens systems

In principle, aberrations of nuclear microprobe lens systems can be corrected with multipole lenses. This paper discusses multipole lenses and how they may be used to correct divergence dependent aberrations in a quadrupole probe forming lens system. Also discussed is the grid shadow method which provides a practical means for experimental study of the properties of the multipole lenses as well as a potential means for adjustment of the multipoles to achieve correction. Results are presented showing the effect of sextupole, octupole and duodecapole lenses in simplified lens systems that consisted of a single magnetic quadrupole and a single magnetic multipole. Further results are presented that show the grid shadow method applied to the magnetic quadrupole Russian quadruplet of the Melbourne Proton Microprobe. In this case the system was found initially to suffer from large parasitic aberration, probably octupole, which was later significantly reduced by replacement of some damaged pole tips.     

Design of the new superconducting microprobe system in Munich

At the Munich 15 MV tandem accelerator a new two-stage microprobe system is under construction. It will demagnify a beam spot by a factor 200 in both transversal directions for ions up to an energy of 200 MeV · q²/A. The second stage of the system is a superconducting quadrupole doublet lens, with overlayed 2n-poles, n = 4, 6 and 8. The multipole components of this lens have been determined by a ray tracing program with a special treatment of the fringing fields. As a result of these calculations, the overall geometrical aberrations at the focal point are 180 nm horizontally and 280 nm vertically at full angular acceptance (image coordinates ±9 and ±20 mrad, respectively). A main part of the superconducting lens is a central ceramic tube which defines the exact positions of the poles. Superconducting NbTi foils are used to shape the field to the desired one. Superconducting current loops automatically serve for a suppression of all odd harmonic field components.     

Power absorption,plasma parameters and wave structure in inductive RF plasma source with low value external magnetic field

The efficiency of radio-frequency (RF) power absorption, RF magnetic field structure and plasma parameters were measured in cylindrical inductive RF plasma sources 20 cm in diameter and 22, 32, 53 cm in length with a low value external magnetic field. The experiments were carried out in argon at pressures of 13–140 mPa. The RF power supply changed from 200 W to 800 W. The spiral antenna was used for sustaining the discharge. It was shown that efficiency of RF power absorption depended nonlinearly on the external magnetic field values. At maximal values of the RF power absorption efficiency, the axial distributions of longitudinal B_z and azimuthalBcomponents of RF magnetic field manifested the formation of the partially standing wave with a half wavelength close to 8 cm. At the same conditions, the axial dependence of the radial RF magnetic field component B_r differed drastically. It was concluded that the B_z and Bamplitudes were largely determined by the RF field of Trivelpiece-Gould wave, while B_r amplitude represented the radial RF field of the helicon wave.     

谐波垫补原理及其在加速器常规磁铁设计与磁场垫补中的应用

加速器常规磁铁磁场质量评估通常表示为工作气隙好场区内高阶磁场占主磁场的相对含量,它通常要求在10^-4量级内。这些高阶磁场来源于磁极的有限长度与宽度和极面设计参数选择产生的若干过大的系统高阶磁场偏差及加工与材料的不理想等因素产生的若干过大的随机高阶磁场偏差。这些偏差均会导致磁场质量变坏。谐波垫补法针对每个过大的高阶磁场利用磁极形位改变对磁场的扰动效应产生一与现存过大高阶磁场大小相当但符号相反的高阶磁场,用来抵消现存的过大的高阶磁场,达到根本改善磁场质量的目的。本文分析了谐波垫补原理,并具体分析了它们在二极磁铁和四极磁铁设计与磁场垫补中的应用。     

Measurements of distribution of self-bias potential on an RF-plane electrode in plasma etching devices

The self-bias potential (V_dc) induced on an RF-powdered electrode (153 mm Ø) in a plasma is measured using electrical probes which are buried in, de-insulated from, and RF-connected to the electrode. The configuration of the probes allows to study the distribution of V_dc discretely on the electrode. The potential is homogeneous in the absence of external magnetic field. In the presence of a homogeneous magnetic field parallel to the electrode, it is reduced and a monotonous gradient takes place in its distribution due to the plasma shift induced by E × B drift. When the magnetic field is rotated along the axis of the RF-electrode at a frequency less than 50 Hz, the distribution, which is almost identical to the one in a static field, rotates with the magnetic field. On the coordinate system rotating with the magnetic field, the probes are regarded to be rotating. The potential distribution is obtained as a continuous function of the azimuthal angle. Thus the rotation of the field provides information for the experimental interpolation.     

Magnetic ordering in superconducting YBa2(Cu1−xFex)3Oy oxide

Magnetic susceptibility measurement and the zero-field muon-spin-relaxation experiment have been made for the YBa₂(Cu_1−xFe_x)₃O_y system. The susceptibility has been measured in the field strong enough for the magnetic flux to penetrate the superconductor and the resultant temperature dependence has demonstrated the field cooled effect and a cusp at low temperatures below T_c. The cusp has been observed when the high field has been applied. The dynamical depolarization rate of the muon obtained by the muon-spin-relaxation experiment has shown the maximum at the same temperature as the cusp. The results suggest that the spin glass freezing of localized magnetic moment takes place at the temperature of the cusp, which is consistent with the previous Mössbauer effect experiments. The magnetic phase diagram for the system has been obtained.     

Analysis of wall conditioning by using plasma impact desorption in the tandem mirror GAMMA 10

A method for evaluating wall condition by using plasma impact desorption (PID) technique has been developed and successfully applied to the tandem mirror GAMMA 10 as a monitor for wall conditioning. A magnetically shielded quadrupole mass spectrometer was installed in the vacuum chamber of the GAMMA 10 central cell. The behaviour of the partial pressure of various gas molecules desorbed by ICRF-heated plasma discharges were analyzed. The predominant increase of the partial pressure due to PID (ΔP_PID) was hydrogen (M = 2) and a small amount of impurity as CO (M = 28), CH₃ (M = 15), H₂O (M = 18) and CO₂ (M = 44) was observed in the wall-conditioning discharges. The reduction of hydrogen pressure as well as ΔP_PID of the above impurities was observed with the progress of wall conditioning. This behavior has a good correlation with the increase of partial pressures due to electron-impact desorption measured at the same period. The relation between ΔP_PID and the charge-exchange flux was investigated.     

Thermodynamic properties of nonstoichiometric urania-gadolinia solid solutions in the temperature range 700–1100° C

Partial molar thermodynamic quantities for urania-gadolinia solid solutions of compositions U_1−yGd_yO_2+x, with y values of 0.04 to 0.27, have been obtained using a solid electrolyte galvanic cell technique. The measurements were made for O/M ratios ranging from near stoichiometry to 2.20, and for temperatures ranging from 700 to 1100°C. The results for pure UO_2+x are in accordance with data reported earlier. The oxygen potentials for U_1−yGd_yO_2+x are higher than for pure UO_2+x and increase positively with increasing Gd content or excess oxygen. They can be represented as a function of the mean U valence, except at the stoichiometric composition. Both the partial molar entropy and enthalpy increase negatively with increasing Gd content or excess oxygen.     

Characterization of Si/GexSi1 − x heterojunction bipolar transistors formed by Ge ion implantation in Si

Epitaxial Si/Ge_xSi_{1 − x} heterojunctions were formed by high dose Ge ion implantation in Si followed by rapid thermal annealing at 1000°C for 10 s. This technique was adopted to fabricate Si/Ge_xSi_{1 − x} heterojunction n-p-n bipolar transistors (HBT) using a self-aligned, double polycrystalline silicon process commonly used for fast Si bipolar transistors. The devices are characterized by a 60 nm wide neutral base with a Ge concentration peak of ≈ 7 at.% at the base-collector junction. Good static and dynamic electrical characteristics are demonstrated and discussed.     

Refractive index and anisotropy measurements in He implanted KTiOPO4 (KTP) optical waveguides

Planar optical waveguides have been formed in Z-cut KTP materials by He⁺ ion implantation. Guided modes's spectra and refractive index profiles n_x, n_xy, n_y and n_z are determined from dark m-lines spectroscopy and by using an IWKB method. The coupling efficiency and light confinement are better with TM (n_z) modes than TE ones. This could be explained by the cristallographic axes orientation with respect to the propagation direction. The anisotropy investigation of the waveguide will end this work.     

Use of reference samples for more accurate RBS analyses

While one of the primary assets of RBS analysis is that it is quantitative without use of reference samples, for certain types of analyses the precision of the method can be improved by measuring RBS spectra of unknowns relative to the RBS spectra of a similar known sample. The advantage of such an approach is that one can reduce (or eliminate) the uncertainties that arise from error in the detector solid angle, beam current integration efficiency, scattering cross-section, and stopping powers. We have used this approach extensively to determine the composition (x) of homogeneous thin films of TaN_x using as reference samples films of pure Ta. Our approach is to measure R=(Tacount)_unknown/(Tacount)_standard and use RUMP to determine the function x(R). Once the function x(R) has been determined, this approach makes it easy to analyze many samples quickly. Other analyses for which this approach has proved useful are determination of the composition (x) of WN_x, SiO_xH_y and SiN_xH_y, using W, SiO₂ and amorphous Si as reference samples, respectively.     

Optimization of the working distance of an ion microprobe-forming system

A high-resolution ion microprobe necessitates the use of a small working distance (the distance from the final quadrupole lens of a probe-forming system to the specimen) in order to produce a large demagnification. But at the same time a small working distance is a source of a number of practical difficulties. We have presented an approach for determining a working distance that provides the best spatial resolution with the main practical limitations taken into account. We used a probe-forming system acceptance as a criterion of optimality. The calculations have revealed the existence of an optimal working distance in a set of common probe-forming systems, but it can be achieved only after changing of a design of a final quadrupole lens. We proposed a possible design of conic lens that allows solving the problem of detectors location and creating a short focus system. Three-dimensional calculations of magnetic field within this lens predicted a good quality of field structure.     

Simultaneous compensation of 2nd-order parasitic aberrations in both principal sections of an achromatic quadrupole lens doublet

Experimental measurements are made of the aberrations of a doublet of achromatic quadrupole lenses. Second-order terms are altered and eliminated. Adjustment is guided by a theoretical model containing dipole and hexapole terms. Fixed hexapole terms of unknown magnitude arising from the inherent imperfect symmetry of lens construction cause the parasitic aberrations. Variable hexapole terms, caused by 2-pole electric excitations, are used to achieve compensation. The variable terms are adjusted to null the total hexapole field of each lens, leaving a small dipole field. Thus compensation of parasitic aberration is achieved at the expense of a small fixed image shift, which may be ignored or compensated by a beam-sweeping system. When a doublet with bore of 1 mm and working distance 10 cm is used to focus 1.2 MeV protons, aberrations less than 0.5 μm over an aperture of 0.4 × 0.5 mm are obtained. Alternative compensation methods require us of 5 separated lenses, 2 quadrupole and 3 hexapole. The field in each such hexapole lens is adjustable in magnitude and azimuth, requiring 6 parameters to be adjusted. A single such lens does not produce exact compensation. A method of compensation using the quadrupole lens structure is superior because construction and mechanical alignment of the 3 separate lenses is not required, and because only 4 parameters must be adjusted.     

Release of nitrogen from SiOxNy films during RBS measurement

We have found that nitrogen atoms are released very rapidly from ultrathin SiO_xN_y films (2.6 nm) during RBS measurement with 500 keV He⁺ ions. The release behavior strongly depends on the preparation technique of the SiO_xN_y films. There is no release from the film prepared by thermal nitridation of SiO₂, while 80% of the nitrogen atoms are released from the film prepared by plasma nitridation at a fluence of 1×10¹⁶ cm⁻². The release cross-section for plasma SiO_xN_y films is of the order of 10⁻¹⁶ cm². This large cross-section cannot be explained by a simple recoil mechanism. The nitrogen release is also observed under irradiation with 5–10 keV electrons though the cross-section is of the order of 10⁻¹⁹ cm². These findings suggest that the observed nitrogen release is an electronic excitation induced process.     

3D-simulation studies of the modified magnetic multipole structure for an electron cyclotron resonance ion source

Experiments have shown that efficient operation of an electron cyclotron resonance ion source requires that the magnetic field fulfills the so-called scaling laws. In most cases the requirements for the radial magnetic field, i.e. the strength of the magnetic multipole are the most difficult to satisfy. This is due to the fact that the multipole is usually produced from permanent magnets, which makes a value of 1.3 T feasible. One possible solution to increase the multipole field is the so-called Modified MultiPole Structure (JYFL-MMPS). This new idea makes it possible to increase the magnetic field at the places where the plasma flux is in contact with the plasma chamber wall. In this article we will present detailed three-dimensional magnetic field simulations of the JYFL-MMPS.     

Contribution to the study of the U---Cs---Mo---I---O system

The reaction in the U---Cs---Mo---I---O system at 1073 K have been studied as a function of oxygen potential. The chemical constitution and the morphology of the phases formed were examined by electron-probe microanalyzer and X-ray diffractometer. The existence of a two-phase field UO_2+x + Cs₂UO₄ was confirmed in the U---Cs---O system. The threshold oxygen potential was determined for the decomposition of CsI by UO_2+x to form Cs₂U₄O₁₂ and gaseous iodine. The effect of molybdenum on the UO_2+x−CsI reaction was also investigated. Predominance of Cs₂MoO₄ over Cs₂UO₄ was verified in a certain range of oxygen potentials. Some assessments and interpertations of the experimental results were made with the aid of thermodynamic calculations.     

Simulation of a low energy beam transport line

A 2.45 GHz electron cyclotron resonance intense proton source and a low energy beam transport line with dual-Glaser lens were designed and fabricated by Institute of Modern Physics for a compact pulsed hadron source at Tsinghua. The intense proton beams extracted from the ion source are transported through the transport line to match the downstream radio frequency quadrupole accelerator. Particle-in-cell code BEAMPATH was used to carry out the beam transport simulations and optimize the magnetic field structures of the transport line. Emittance growth due to space charge and spherical aberrations of the Glaser lens were studied in both theory and simulation. The results show that narrow beam has smaller aberrations and better beam quality through the transport line. To better match the radio frequency quadrupole accelerator, a shorter transport line is desired with sufficient space charge neutralization.     

An RF Technique for Plasma Number Density Measurement

It is well-known that a cylindrical plasma column in a transverse rf electric field is subject to a dipole resonance. The frequency at which this occurs can be used to determine the average electron density across the plasma section. This paper describes the experimental application of the technique to plasmas with and without a confining axial static magnetic field, and gives theoretical curves to assist in rapid reduction of the data. Higher order multipole resonances can also be excited. The feasibility of applying these to determination of the number density profile is discussed.     

The production of neutron-rich heavy nuclei of astrophysical interest at the IRIS facility

Targets of ThC_x and UC_x have been used for production of Fr, Ra(Ac) nuclides of astrophysical interest at the IRIS mass separator on line with the 1 GeV proton synchrocyclotron at LNPI. A fine-grained powder of a target has been prepared using a special technology based on high temperature destruction of the thermostable organometallic compounds of U, Th or Cm (diphtalocyanine). New neutron-rich nuclides have been identified: ²³²Fr (5 ± 1 s), ²³³Ra (30 ± 5 s), ²³⁴Ra (30 ± 10 s). The yields of some nuclides from a CmC_x target have been also determined.