Focusing Magnetic Field Research Articles

A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO+

A crossed electron–ion beam experimental set-up has been upgraded for the study of electron impact ionization and dissociation of molecular ions by means of ionic product detection. Both the experimental set-up and the data analysis procedures are described in detail for the estimation of (i) absolute cross sections, (ii) kinetic energy release distributions (KERD) and (iii) anisotropies of angular distributions. Absolute cross sections are obtained separately for dissociative excitation (DE) and for dissociative ionization (DI). A double focusing magnetic field analyser is used for the observation of product velocity distributions, in the laboratory frame, at selected electron energies. The KERD in the centre of mass frame is calculated from the measured velocity distribution as well as the anisotropy of the angular distribution with respect to the initial orientation of the molecular ions. Results are reported for dissociative ionization and dissociative excitation of CO+ to C+ and O+ fragments in the energy range from about 5 eV to 2.5 keV. Absolute cross sections for DE at maximum, i.e. for an electron energy around 35 eV, are found to be (9.69 ± 2.08) × 10−17 cm2 and (6.24 ± 1.33) × 10−17 cm2, for C+ and O+, respectively, and the corresponding threshold energies are found to be (8.5 ± 0.5) eV and (14.8 ± 0.5) eV. The DE process leading to C+ production is seen to dominate at low electron energies. For DI, the absolute cross section is found to be (12.56 ± 2.38) × 10−17 cm2 around 125 eV and the corresponding threshold energy is (27.7 ± 0.5) eV. KERDs, which extend from 0 to 24 eV both for C+ and O+, exhibit very different shapes at low electron energy but similar ones above 100 eV, confirming the role observed respectively for DE and DI. The groups of states contributing to the different processes are identified by comparing present energies thresholds values and the KERDs with theoretical values. Anisotropies are estimated to be in the range 3–6% for both C+ and O+.

Investigation of the chaotic dynamics of an electron beam with a virtual cathode in an external magnetic field

The effect of the strength of the focusing magnetic field on chaotic dynamic processes occurring inan electron beam with a virtual cathode, as well as on the processes whereby the structures form in the beamand interact with each other, is studied by means of two-dimensional numerical simulations based on solving a self-consistent set of Vlasov-Maxwell equations. It is shown that, as the focusing magnetic field is decreased,the dynamics of an electron beam with a virtual cathode becomes more complicated due to the formation andinteraction of spatio-temporal longitudinal and transverse structures in the interaction region of a vircator. The optimum efficiency of the interaction of an electron beam with the electromagnetic field of the vircator isachieved at a comparatively weak external magnetic field and is determined by the fundamentally two-dimensional nature of the motion of the beam electrons near the virtual cathode.

Magnetic-based microfluidic platform for biomolecular separation

A novel microfluidic platform for manipulation of micro/nano magnetic particles was designed, fabricated and tested for applications dealing with biomolecular separation. Recently, magnetic immunomagnetic cell separation has attracted a noticeable attention due to the high selectivity of such separation methods. Strong magnetic field gradients can be developed along the entire wire, and the miniaturized size of these current-carrying conductors strongly enhances the magnetic field gradient and therefore produces large, tunable and localized magnetic forces that can be applied on magnetic particles and confine them in very small spots. Further increases in the values of the generated magnetic field gradients can be achieved by employing miniaturized ferromagnetic structures (pillars) which can be magnetized by an external magnetic field or by micro-coils on the same chip. In this study, we demonstrate magnetic beads trapping, concentration, transportation and sensing in a liquid sample under continuous flow by employing high magnetic field gradients generated by novel multi-functional magnetic micro-devices. Each individual magnetic micro-device consists of the following components: 1. Cu micro-coils array embedded in the silicon substrate with high aspect ratio conductors for efficient magnetic field generation 2. Magnetic pillar(s) made of the magnetic alloy NiCoP for magnetic field focusing and magnetic field gradient enhancement. Each pillar is magnetized by its corresponding coil 3. Integrated sensing coil for magnetic beads detection 4. Microfluidic chamber containing all the previous components. Magnetic fields of about 0.1 T and field gradients of around 300 T/cm have been achieved, which allowed to develop a magnetic force of 3 x 10(-9) N on a magnetic particle with radius of 1 mum. This force is large enough to trap/move this particle as the required force to affect such particles in a liquid sample is on the order of approximately pN. Trapping rates of up to 80% were achieved. Furthermore, different micro-coil designs were realized which allowed various movement modes and with different step-sizes. These results demonstrate that such devices incorporated within a microfluidic system can provide significantly improved spatial resolution and force magnitude for quick, efficient and highly selective magnetic trapping, separation and transportation, and as such they are an excellent solution for miniaturized mu-total analysis systems.

Nonlinear coupling between breathing and quadrupole-like oscillations in the transport of mismatched beams in continuous magnetic focusing fields

A nonlinear analysis of the transport of breathing beams considering nonaxisymmetric perturbations is performed. It is shown that large-amplitude breathing oscillations of an initially round beam may couple nonlinearly to quadrupole-like oscillations, such that the excess energy initially constrained to the axisymmetric breathing oscillations is allowed to flow back and forth between breathing and quadrupole-like oscillations. In this case, the beam develops an elliptical shape with a possible increase in its size along one direction as the beam is transported. This is a highly nonlinear phenomenon that occurs for large mismatch amplitudes on the order of 100% and is found to be particularly relevant for space-charge-dominated beams with K≳k0ϵ, where K is the beam perveance, k0 is the vacuum phase advance per unit axial length, and ϵ is the emittance of the beam. A simple model based on mapping techniques is used to clarify the mechanism that leads to the energy exchange between the modes and is tested against results from direct integration of the envelope equations.

Formation of a self-magnetic cusp in a highly bunched relativistic annular electron beam

The formation of a self-magnetic cusp that develops in a highly bunched, relativistic, annular electron beam as the beam current approaches a critical current limit is demonstrated. The self-magnetic cusp is calculated within the framework of a fluid equilibrium model for a beam of periodic, azimuthally symmetric, relativistic bunched annular disks propagating in a perfectly conducting cylindrical pipe with a uniform magnetic focusing field. Magnetic cusp formation may play an important role in determining the operational current limit of high-current bunched relativistic annular beam experiments.

Production of intense beams of singly charged radioactive ions

An apparatus for the production of intense beams of singly charged radioactive ions operating in on-line regime is proposed. The radioactive atoms are produced in a uranium-graphite (UC) target bombarded with neutrons. The neutron flux is generated by a graphite neutron converter, which is bombarded with protons. The atoms of the produced isotopes are ionized in the electron beam generated with the electron gun and the ions of interest are extracted in a separator. The apparatus consists of the following parts. (1) Rotating converter dissipating a substantial power of proton beam. (2) UC target placed in a graphite container at high temperature. The atoms of radioactive isotopes can be extracted with a flow of noble gas. (3) Triode electron gun with ionization channel is placed inside the solenoid forming a focusing magnetic field. The cathode of the electron gun is a spout of the graphite container. The atoms of radioactive isotopes are carried with gas flow through the spout into the electron beam. (4) Correction coil located near the gun matches the electron beam with the ionization channel. (5) The first anode has a potential of 1–4 kV with respect to the cathode, and the second anode has some lower potential than the first anode and it is the tube of ionization channel. (6) Electron collector dissipates the electron-beam power. (7) Uranium-graphite target, the gun, the ionization channel as well as solenoid are located on an isolated platform with potential of 30–60 kV with respect to ground. The beam of singly charged ions from the ionization channel passes the collector, goes through the extractor, acquires energy of 30–60 keV, and gets transported to the separator where the required species are selected.

Electron-beam focusing in 1:1 electron projection lithography system

Factors influencing the transferred image resolution in 1:1 electron projection lithography system are considered. The influence of Coulomb interaction between electrons, gradient of magnetic field, and angle between focusing electric and magnetic fields on the electron-beam focusing is estimated. In the presence of a magnetic-field gradient in the focusing system, the radius of an electron’s orbit is changed with a change of value of magnetic field B. As a result, after completing a whole turn, the center of electron’s orbit moves in the direction perpendicular to B and the magnetic-field gradient, which results in distortion of the transferred image. Coulomb interaction between electrons during their flight from the emitter toward the e-beam resist-coated wafer also results in diffusion of the transferred image and may be considered a crucial parameter that limits the high-resolution capability in the case of high emission currents. The presence of the angle between the direction perpendicular to the wafer and emitter planes and the magnet’s field direction results in an overall displacement of the transferred image and a variation of cyclotron orbit radius that depends on the distance between the emitter and wafer.

The status of normal conducting RF (NCRF) guns, a summary of the ERL2005 workshop

The 32nd Advanced ICFA Beam Dynamics Workshop on Energy Recovering Linacs (ERL2005) was held at Jefferson Laboratory, March 20–23, 2005. A wide range of ERL-related topics were presented and discussed in several working groups with Working Group 1 concentrating upon the physics and technology issues for DC, superconducting RF (SRF) and normal conducting RF (NCRF) guns. This paper summarizes the NCRF gun talks and reviews the status of NCRF gun technology. It begins with the presentations made on the subject of low-frequency, high-duty factor guns most appropriate for ERLs. One such gun at 433 MHz was demonstrated at 25%DF in 1992, while the CW and much improved version is currently being constructed at 700 MHz for LANL. In addition, the idea of combining the NCRF gun with a SRF linac booster was presented and is described in this paper. There was also a talk on high-field guns typically used for SASE-free electron lasers. In particular, the DESY coaxial RF feed design provides rotationally symmetric RF fields and greater flexibility in the placement of the focusing magnetic field. While in the LCLS approach, the symmetric fields are obtained with a dual RF feed and racetrack cell shape. Although these guns cannot be operated at high-duty factor, they do produce the best quality beams. With these limitations in mind, a section with material not presented at the workshop has been included in the paper. This work describes a re-entrant approach which may allow NCRF guns to operate with simultaneously increased RF fields and duty factors. And finally, a novel proposal describing a high-duty factor, two-frequency RF gun using a field emission source instead of a laser driven photocathode was also presented.

Trace and mineral elements in royal jelly and homeostatic effects

Royal jelly from Apis mellifera is a highly active natural biological substance and is probably one of the most interesting raw substances in natural product chemistry. Trace elements play a key role in the biomedical activities associated with royal jelly, as these elements have a multitude of known and unknown biological functions. For this reason concentrations of 28 trace (Al, Ba, Sr, Bi, Cd, Hg, Pb, Sn, Te, Tl, W, Sb, Cr, Ni, Ti, V, Co, Mo) and mineral (P, S, Ca, Mg, K, Na, Zn, Fe, Cu, Mn) elements were systematically investigated in botanically and geologically defined royal jelly samples. In addition, concentrations of 14 trace elements were measured in the associated honey samples – honey being the precursor of royal jelly. Concentrations of K, Na, Mg, Ca, P, S, Cu, Fe, Zn, Al, Ba and Sr in royal jelly were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), while concentrations of Bi, Cd, Hg, Pb, Sn, Te, Tl, W, Sb, Cr, Mn, Ni, Ti, V, Co and Mo in royal jelly were determined by double focusing magnetic sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). In the honey samples, trace and mineral element concentrations strongly depended on botanical and geological origin, and substantial variation was found. In contrast, the concentrations of trace and mineral elements were highly constant in the associated royal jelly samples. The most important results were the homeostatic adjustments of trace and mineral element concentrations in royal jelly. This effect was evidently produced in the endocrine glands of nurse bees, which are adapted for needs of bee larvae. In conclusion, this research yielded a surprising and completely new finding – that royal jelly, as a form of lactation on the insect level, shows the same homeostatic adjustment as mammalian and human breast milk.

Simulation of ion noise in microwave tubes by two-dimensional particle-in-cell method

The ion noise in microwave tube is studied using two-dimensional particle-in-cell method. The fluctuation of the ion amount, which is the source of ion noise of the tube, is observed in the simulation process. It is found that the sufficient ions amount in drift tube and beam envelope scalloping appear to be responsible for ion noise after changing the gas pressure, magnetic focusing field, and drift tube length. A great number ions escape radially and their amount is about half of the beam electron when ion noise occurs. The number of secondary electron is less than that of the ion and beam electron; it affects the ion noise very slightly. There are two ways can be used for depressing the ion noise. The first is adjusting or increasing the magnetic focusing field in drift tube. The second is creating static electric field gradient to make the ions leave the interaction space quickly.

Nanosecond hybrid Modulator for the fast-repetitive driving of X-band, gigawatt-power microwave source

Results of tests of a fast-repetitive nanosecond modulator with a peak output power of 3 GW are presented. It comprises of a type S-5N charging driver, a power compressor based on a pulsed forming line, and a gas spark gap. The modulator has been designed for the formation of high-current electron beams and high-power microwave generation in a pulsed-repetitive regime. Excitation of 10-GHz microwave pulses /spl sim/1 ns long in a relativistic backward-wave oscillator with an elongated periodic slow-wave system has been studied. Optimization of the regime of interaction between electromagnetic fields and particles provided an average power of microwave radiation in the burst-repetitive mode (1 s; /spl sim/700 Hz) of up to 2.5 kW at a focusing magnetic field (/spl sim/0.6 T) below the cyclotron resonance region. The peak output power of the oscillator exceeded 2 GW.

Linear Two-Dimensional Analysis of Parasitic Backward-Wave Oscillation in a Monofilar-Helix Traveling Wave Tube

We investigated the attainable stability of the monofilar-helix traveling wave tube (TWT) with respect to the parasitic backward-wave oscillation under periodic magnetic focusing. We accounted for the effect of the electron beam rotation in the nonhomogeneous focusing magnetic field, magnetic field amplitude and periodicity, and the specific coupling of the electron beam with the synchronous harmonics of the electromagnetic wave existing on the slow-wave circuit. This specific coupling takes place in the monofilar-helix TWT only and decreases its stability with respect to the parasitic backward-wave oscillation in comparison to other types of helix TWT. For the purpose of this analysis, we developed further the linear two-dimensional (2-D) multiwave theory of the helix TWT described in our previous work . In this theory, we account for space charge fields by means of waveguide excitation model based on the direct solution of Maxwell equations. Differently from , the present version of our theory allows for accounting of the coupling of the synchronous 0th spatial harmonic (forward employed for the amplification) and -1st (parasitic backward) harmonic of the circuit wave with the electron beam in the monofilar-helix TWT. By numerical modeling, we obtained quantitative data on the starting conditions for the parasitic backward-wave oscillation in the typical monofilar-helix TWT in the intersection point of the dispersion curves of these two harmonics. The described coupling mechanism does not exist in the bifilar-helix TWT. Therefore, the TWT with the bifilar helix is more stable with respect to the parasitic backward-wave oscillation than the TWT having the monofilar helix. These theoretical results correlate with the existing experimental evidence. The results of the present study, in particular, explain why the backward-wave oscillation suppression by tailoring of the focusing magnetic field has a significant effect in the bifilar-helix TWT but does not work in the monofilar-helix TWT.

Lead isotope ratio measurements in geological glasses by laser ablation-sector field-ICP mass spectrometry (LA-SF-ICPMS)

We have tested the capabilities of LA-SF-ICPMS for in situ Pb isotopic work on geological glasses using a 213 nm Nd:YAG laser system and a double focusing magnetic sector field ICP mass spectrometer. Experiments were done with the MPI-DING and USGS reference glasses, USGS rock powders molten as glass beads and basaltic glasses from Hawaii. In particular, the precision and accuracy of 208Pb/ 206Pb and 207Pb/ 206Pb ratios were investigated for three-spot analyses of glasses with different Pb contents (0.4–40 μg/g) at spot sizes of 12–160 μm. Using the fast and precise electrical scan mode, we obtained for samples with Pb concentration >1 μg/g in-run precisions (1 R.S.E.) of better than 0.1% and 0.2% for spot diameters of 120 and 40 μm, respectively. The LA-SF-ICPMS isotope ratios agree with the high-precision TIMS and MC-ICPMS data within the external precision (R.S.D. = 0.1–0.2%). The LA-SF-ICPMS results for 208Pb/ 204Pb, 207Pb/ 204Pb and 206Pb/ 204Pb are less precise (about 0.5–1%) due to the low abundance of 204Pb and correction from the 204Hg interference. In this paper, we present the first LA-SF-ICPMS Pb isotope data for the MPI-DING (KL2-G, ATHO-G) and USGS reference glasses (BCR-2G, BHVO-2G) which may be useful for microanalytical in situ Pb isotope analysis. The findings of small (<2%), but significant differences in Pb isotope ratios of Hawaiian glasses of different ages, demonstrate that LA-SF-ICPMS is a suitable tool for geochemical research.

Mobilization of exogenous and endogenous selenium to bile after the intravenous administration of environmentally relevant doses of arsenite to rabbits

AbstractExtending our studies of the effect of arsenite on the metabolism of inorganic selenium (selenite and selenate) to lower doses, we intravenously injected New Zealand white rabbits with aqueous solutions of arsenite, selenite, arsenite + selenite, selenate and selenate + arsenite at 50 µg and 5 µg metalloid per kilogram body weight. Bile samples were collected for 25 min, acid‐digested and analyzed for total arsenic and selenium by double focusing magnetic sector field inductively coupled plasma mass spectrometry. At both dose levels, and in accord with previous observations, an increased mutual biliary excretion of arsenic and selenium was observed regardless of whether selenium was coadministered with arsenite in the form of selenite or selenate. Based on our previous investigations into the in vivo interaction between arsenite and selenite (or selenate), these findings can be rationalized in terms of the biliary excretion of the seleno‐bis(S‐glutathionyl) arsinium ion, [(GS)2AsSe]−. In addition, the treatment of rabbits with 50 µg arsenic per kilogram body weight in form of arsenite alone also resulted in a significantly increased bile selenium concentration compared with bile from untreated animals (p &lt; 0.05), which implies a mobilization of endogenous selenium to bile. Combined, these results establish a causal relationship between the exposure of mammals to arsenite and selenium deficiency. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Control of beam halo-chaos via exterior magnetic field switching control*

In this paper, the magnetic field parametric switching method for controlling t he beam_halo chaos in the period_focusing channel(PFC) is studied. The law of t he parametrical switching control is presented . The simulation results demonstr ated that the chaotic envelo pe of the ion beam can be controlled well by this method. Then the method is app lied to the multi_particle model with the period_focusing magnetic field channel . The PFC simulation results show that beam halo_chaos for five different kinds of beams can be controlled well. The beam halos and its regeneration can be eliminated. This method is easy to realize in experiment.The research offers a valuable reference for the design of the PFCs in the high_current linea r proton accelerators.

Three-dimensional nonlinear theory of travelling wave tubes and simulation

A three-dimensional (3D) nonlinear theory of travelling wave tubes (TWTs) is developed, which includes a fundamental radio frequency (RF) and harmonics. When the instantaneous bandwidth exceeds an octave, the harmonic is generated and the mutual coupling between the harmonic and the fundamental RF can be observed in TWTs due to nonlinear interaction between the electron beam and the RF. At low frequencies the harmonic has an obvious effect. Based upon Tien's disc model, a plastic 3D super-particle model is proposed to improve the nonlinear analysis of TWTs. Numerical results employing a periodic magnetic focusing field are presented.

Physical effects at formation and transportation of intensive electron beams with high convergence

Physical effects at formation and transportation of intensive electron beams caused by influence of transversal thermal electron velocities and disturbance of magnetic focusing fields axial symmetry have been considered.

High-power subnanosecond 38-GHz microwave pulses generated at a repetition rate of up to 3.5 kHz

An original relativistic backward tube (BWT) for a 38 GHz range is developed and tested. The BWT is capable of generating stable pulses of ∼250 ps duration and a peak power of ∼250 MW in trains with a length of up to 1 s at a repetition rate of 1–3.5 kHz. The BWT design implements an inhomogeneous slow-wave structure of increased cross section with a band reflector. A pulsed electron beam (∼270 keV, ∼2 kA, 0.9 ns) was injected by a high-current accelerator based on a high-voltage generator with an inductive energy store, a semiconductor current interrupter, and a pulse-shaping hydrogen-filled discharge gap. A focusing magnetic field of 2 T was generated by a cooled pulsed solenoid power-supplied from a special stabilized current source.

Nonlinear time-domain analysis of coupled-cavity traveling-wave tubes

A time-dependent nonlinear analysis of a coupled-cavity traveling-wave tube (CCTWT) is presented. The coupled-cavity structure is modeled by a set of equivalent circuit equations where the equations for currents and voltages are coupled to the nearest neighbor cavities. Input and output coupler models as well as sever cavities are included in the formulation. The electron dynamics are treated using the three-dimensional Lorentz force equations although the RF field representation is an analytic model based on cylindrically symmetric geometry. The magnetic focusing fields are also cylindrically symmetric and can be either a solenoid or a periodic permanent magnet stack. The space-charge fields are found by mapping charge to a two-dimensional grid (r, z) and solving Poisson's equation by a finite difference grid formulation. The circuit and Lorentz force equations are integrated in time in a self-consistent manner. The formulation is capable of treating multiple drive frequencies and the associated intermodulation products as well as oscillations and backward wave instabilities. Hence, the model can be used to perform stability analyses. Furthermore, the cavity parameters can be varied to model dynamic velocity tapering for efficiency enhancement. The simulation is applied to the analysis of a sample C-Band CCTWT, and comparisons with measured performance of a Ka-Band CCTWT at Communications and Power Industries, Palo Alto, CA, are made.

Assistant Anode in a Cathodic Arc Plasma Source

The performance and characteristics of a cathodic arc plasma source,consisting of a titanium cathode, an anode with and without a tungstenmesh, and a coil producing a focusing magnetic field between the anodeand cathode, are investigated. The high transparency and large area ofthe mesh allow a high plasma flux to penetrate the anode from thecathodic arc. The mesh helps to decrease the arc resistance and theignition voltage of the cathodic arc in the focusing magnetic field, and toincrease the life of the source, which means that the source makesthe cathodic arc easily and greatly stabilized during the operation whena focusing magnetic field exists in the source.