Oblique Electron Research Articles

Quantifying effects of lead shielding in electron beams: a Monte Carlostudy

Lead shielding in contact with the patient's skin is often encountered inradiotherapy with electron beams. The influence of the lead shielding on dosedistributions in the patient cannot fully be assessed using modern treatmentplanning systems. In this work the problem of quantifying the effect of leadshielding on dose distributions is addressed. Monte Carlo dose calculationswere performed in a half-blocked water phantom shielded by lead, using arealistic model for the fluence of an electron linear accelerator. Electronbeam energies of 6-20 MeV and lead thicknesses of 1-7 mm are used for10×10 cm2 and 5×5 cm2 fields. The perturbation of theparticle fluence and dose distributions in water introduced by the leadshielding is quantified. The effect of oblique electron beams on the doseperturbation is shown. A fictitious clinical example, the shielding of an eyein electron beam treatment, is used to demonstrate the usefulness of MonteCarlo based treatment planning algorithms that can incorporate the effects oflead shielding.

Electron magnetohydrodynamic turbulence in a high-beta plasma. II. Single point fluctuation measurements

A magnetic void is created by high electron pressure in a large nonuniform laboratory plasma. A strong instability is observed in regions of high pressure and magnetic field gradients. It is associated with the electron diamagnetic drift through the essentially unmagnetized ions. Its spectrum is broad and peaks near the lower hybrid frequency. The coupled fluctuations in density, electron temperature, plasma potential, and magnetic field are measured with probes and cross-correlated. The temporal correlation extends only over 1–2 oscillations. The fluctuations propagate in the direction of the electron diamagnetic drift but at the lower ion acoustic speed. In the saturated regime of the instability, the fluctuation waveforms are highly nonlinear. Density cavities with δn/n≃−40% are formed with steepened density rise at the trailing edge. The associated high pressure gradient forms a diamagnetic current sheet. Positive density perturbations are smaller (δn/n⩽20%), broader, and produce regions of weak magnetic fields where the electrons become nearly unmagnetized. Amplitude distributions of nonlinear density, magnetic field, and current waveforms are evaluated. The three-dimensional magnetic field fluctuations are analyzed with hodograms. The direction of the average wave vector points essentially across the mean field in the direction of the diamagnetic drift. The magnetic fluctuations can be interpreted as highly oblique electron whistlers, the density fluctuations as sound waves, but both modes are coupled in a high-beta plasma. Fluctuations in the electric and magnetic fields lead to a time-averaged electron drift, i.e., anomalous transport, across the mean field.

An improved technique for breast cancer irradiation including the locoregional lymph nodes

Purpose: To find an irradiation technique for locoregional irradiation of breast cancer patients which, compared with a standard technique, improves the dose distribution to the internal mammary–medial supraclavicular (IM-MS) lymph nodes. The improved technique is intended to minimize the lung dose and reduce the dose to the heart. Methods and Materials: The standard technique consists of an anterior mixed electron/photon IM-MS field. In the improved technique, an oblique electron and an oblique asymmetric photon field are combined to irradiate the IM lymph nodes. To irradiate the MS lymph nodes, a combination of an anterior electron and an anterior asymmetric photon field is used. For both the standard and the improved technique, tangential photon fields are used to irradiate the breast. Three-dimensional (3D) treatment planning was performed for 8 patients with various breast sizes for these two techniques. Dose–volume histograms (DVHs) and normal tissue complication probabilities (NTCPs) were compared for both techniques. The field dimensions and energy of the standard technique were determined at simulation, whereas for the improved technique the fields were designed by CT-based treatment planning. Results: The dose in the breast planning target volume was essentially the same for both techniques. For the improved technique, combined with 3D localization information, an improvement in the IM-MS planning target coverage is seen. The volume within the 95% isodose surface was on average 25% (range, 0–64%) and 74% (range, 43–90%) for the standard and improved technique, respectively. The heart generally receives less dose with the improved technique. However, sometimes a small but acceptable increase in lung dose is found. Conclusion: The improved technique, combined with localization information of the IM-MS lymph nodes, greatly improves the dose distribution in the planning target volume for a large group of patients without significantly increasing the dose to organs at risk.

FedSat — An Australian research microsatellite

Late in the year 2000 Australia will launch its first satellite in thirty years. FedSat is a research microsatellite with a complement four scientific and engineering payloads. It will be placed in a near-circular sun synchronous 10:30 LT polar orbit at an inclination of 98.7° and altitude 800 km. The primary scientific experiment is a fluxgate magnetometer which will observe field aligned current structures above the auroral zones and other current systems and ultra-low frequency (ULF) plasma waves in the upper ionosphere. A major objective is to study the ionospheric signatures of regular and impulsive ULF waves propagating from the magnetosphere along geomagnetic field lines. An on-board GPS receiver will monitor the vertical and oblique total electron content (TEC) up to the GPS satellites at an altitude of 20,000 km. The communications experiment will test systems of use in future LEO constellations and satellite-ground communications. Research in satellite systems will concentrate on the use of GPS for real time tracking of small satellites while a high performance computer will test the operation of reprogrammable hardware. This paper concentrates on the research to be undertaken using the fluxgate magnetometer and describes the instrumentation.

Oblique electron cyclotron emission for electron distribution studies (invited)

Electron cyclotron emission (ECE) at an oblique angle to the magnetic field provides a means of probing the electron distribution function both in energy and physical space through changes in and constraints on the relativistic electron cyclotron resonance condition. Diagnostics based on this Doppler shifted resonance are able to study a variety of electron distributions through changes in the location of the resonance in physical or energy space accomplished by changes in the viewing angle and frequency, and the magnetic field. For the case of observation across a changing magnetic field, such as across the tokamak midplane, the constraint on the resonance condition for real solutions to the dispersion relation can constrain the physical location of optically thin emission. A new Oblique ECE diagnostic was installed and operated on the PBX-M tokamak for the study of energetic electrons during lower hybrid current drive. It has a view 33° with respect to perpendicular in the tokamak midplane, receives second harmonic X-mode emission, and is constrained to receive single pass emission by SiC viewing dumps on the tokamak walls. Spatial localization of optically thin emission from superthermal electrons (50–100 keV) was obtained by observation of emission upshifted from a thermal cyclotron harmonic. The localized measurements of the electron energy distribution and the superthermal density profile made by this diagnostic demonstrate its potential to study the spatial transport of energetic electrons on fast magnetohydrodynamic time scales or anomalous diffusion time scales. Oblique ECE can also be used to study electron distributions that may have a slight deviation from a Maxwellian by localizing the emission in energy space. In “near-Maxwellian” plasmas, the Doppler broadening of the resonance means that observed oblique ECE from optically thick harmonics will originate from electrons with energies near or slightly above the thermal energy [(1–2)vth for example, depending on the observation angle and spatial gradients]. This is different from observed perpendicular ECE from optically thick harmonics that originate from electrons below the thermal energy. Experiments are presently being performed on the Tore Supra tokamak with two oblique views of ECE in equal and opposite toroidal directions (approximately 25° with respect to perpendicular) using two Michelson interferometers capable of observing second and third harmonic emission in X mode.

Radially localized measurements of superthermal electrons using oblique electron cyclotron emission

It is shown that radial localization of optically thin electron cyclotron emission from superthermal electrons can be imposed by observation of emission upshifted from the thermal cyclotron resonance in the horizontal midplane of a tokamak. A new and unique diagnostic has been proposed and operated to make radially localized measurements of superthermal electrons during lower hybrid current drive on the Princeton Beta Experiment-Modified (PBX-M) tokamak [Bernabei, et al., Phys. Fluids B 5, 2562 (1993)]. The superthermal electron density profile as well as moments of the electron energy distribution as a function of radius are measured during lower hybrid current drive. The time evolution of these measurements after the lower hybrid power is turned off are given and the observed behavior reflects the collisional isotropization of the energy distribution and radial diffusion of the spatial profile.

Oblique electron transport in the presence of collinear and noncollinear magnetizations

The nonlocal real-space Kubo approach to electron transport in magnetic multilayers is applied to a new geometry, ‘‘oblique transport,’’ whose complexity is traced back to a combination of the layering, of the nonlocal character of the linear response, and of the oblique direction transport in this new geometry. The problem is dealt with by applying a condition on the average current density vector or on the average electric-field vector, depending upon the external driving conditions. Its solution exhibits a characteristic anisotropy and it yields the global oblique conductance and magnetoresistance as simple trigonometric expressions in terms of the in-plane and vertical conductances and magnetoresistances, for arbitrary noncollinear-magnetization configurations.

Standardless X-ray analysis of bulk specimens

The paper gives an overview of the problems of Standardless analysis of bulk specimens by energy dispersive spectrometry (EDS) in scanning electron microscopes (SEM). The interest is concentrated on the present (and future) developments. The influence of the fluorescence excited by the continuum is discussed. Some improvements are proposed for the ionization cross-section. The difficulties due to the Coster-Kronig radiationless transitions are mentioned for the L lines. Emphasis is put on the necessity of an accurate modelling of the detector window, in order to be able to calculate reliably the efficiency of detection, mainly for the ultra-light elements. It is shown that some hypotheses such as the continuity of the fluorescence yield with the atomic number, which are currently accepted for heavier elements, could be wrong in the field of ultra-light elements. The capability of Standardless analysis in special situations is discussed: analysis at oblique electron beam incidence, analysis of specimens with a thin conductive coating, analysis of stratified specimens.

Cyclotron harmonic lines in the thermal magnetic fluctuation spectrum of spiraling electrons in plasmas

Radio frequency (rf) magnetic fluctuations B̃ have been measured with loop antennas in a large pulsed discharge plasma column (ne≲1012 cm−3, kTe≲3 eV, B0≂20 G, Ar, 2×10−4 Torr, 1 m diam×2.5 m length). A 1/f-like noise spectrum is observed in the whistler wave regime (ωce1/2ωci1/2<ω<ωce) both in the Maxwellian afterglow plasma and in the active discharge which contains energetic (45 eV) electrons. Discrete emission lines at the electron cyclotron frequency and its harmonics are found only in the presence of spiraling energetic electrons. These are naturally present in the active discharge but have also been injected as a controlled oblique electron beam into the Maxwellian afterglow plasma. In the latter case up to 15 cyclotron harmonic lines with weak amplitude decay B̃z(ω) are generated in the beam flux tube. From two-point correlation measurements it is shown that the line spectrum is due to ballistic beam modes rather than plasma eigenmodes driven unstable by the beam. The lines evolve from broadband thermal current fluctuations of the beam through a filtering effect. Those fluctuations which rotate synchronously with the ordered cyclotron motion (ω=nωc) constructively interfere (k∥=0) and produce coherent solenoidal rf fields, while others interfere destructively. Axial and azimuthal phase velocity measurements for rf-modulated beams clearly demonstrate the filtering effect. In the present parameter regime (ωp≫ωc) the fluctuations are evanescent and localized near the electron flux tube (rc≳c/ωp). In low density plasmas the fluctuations may couple to propagating electromagnetic waves and be observable externally as in earlier observations by Landauer or Ikegami.

Direct phasing in electron crystallography: determination of layer silicate structures

Direct phase analysis, based on the probabilistic estimate of triple and quartet structure invariant sums, was used to determine the crystal structures of three layer silicates, viz. celadonite, muscovite and phlogopite-biotite, from oblique texture electron diffraction intensity data published earlier by B.B. Zvyagin and his co-workers. In all cases, the resulting potential maps are sufficient to identify most of the atomic positions (including oxygen) found in the original determinations. In addition, most of the crystallographic phase values are identical to the ones found earlier. The major drawback for this technique is due to the paucity of measured intensities, requiring a multisolution approach to this analysis. However, atomic-resolution data enable the choice of the correct map to be made from chemical knowledge of a reasonable layer structure.

Instability of elliptical vortex cores in electron clouds

We investigate the stability of a magnetized electron cloud equilibrium with an elliptical E × B vortex core to the excitation of high frequency electrostatic waves. It is demonstrated that the finite ellipticity of the flow drives the instability of oblique electron plasma oscillations. When the flow ellipticity is small, certain matching conditions between the near rotation frequency of the vortex and the oscillation frequency of the plasma wave are needed. For larger ellipticities, the instability domain is shown to be much broader. Equilibria with elliptic vortex cores are therefore likely to be found with considerable electrostatic noise which will contribute to the transport of the electron fluid across field lines.

Oblique broadband electron plasma waves above the plasma frequency in the electron foreshock: Linear theory and spacecraft observations

The linear dispersion relation of the beam‐plasma instability and the ISEE 1 wave observations in the electron foreshock are examined in order to study the existence of broadband emissions polarized in an oblique direction with respect to the direction of the electron beams reflected from the bow shock. The present work focuses on broadband emissions observed above the electron plasma frequency ωpe, in the frequency range 1 − 1.5 ωpe and above. The linear dispersion relation of waves is investigated numerically and the dependence of the unstable frequency bandwidth on the beam to plasma temperature ratio, Tb/Tp, and density ratio, Nb/Np, is studied. It is shown that electron plasma waves can initially grow in an oblique direction up to 2.5 ωpe and above, when the beam temperature is low, Tb/Tp below 10−2, and its density close to or above Nb/Np = 0.01. The unstable frequency bandwidth over which emissions can grow is shown to be highly controlled by the density ratio Nb/Np. The statistical study of the directivity of broadband emissions observed by ISEE 1 supports the existence of oblique emissions in the frequency range 1–1.5 ωpe, whereas the emissions with frequencies close to ωpe propagate mainly in the parallel direction.

HF radar observations of E region plasma irregularities produced by oblique electron streaming

Data obtained with the Applied Physics Laboratory HF radar located in Goose Bay, Labrador, have been used to determine the characteristic features of high‐latitude ionospheric irregularities at decameter wavelengths. In this paper, we describe a set of four events exhibiting particular characteristics. These observations took place in the postmidnight sector at E region altitudes. The scanning capabilities of the radar indicated that arclike regions of irregularities were moving approximately along L contours with a drift velocity of the order of 200 m/s or less. For periods of a few minutes to a few tens of minutes, localized regions of irregularities exhibiting high Doppler velocities (350 to 650 m/s) and large signal to noise ratios appeared within the radar arcs. Among the high Doppler velocity signals, two distinct types have been identified. Both types can be present simultaneously. One type is distributed between 320 and 550 m/s and has an average value of 445 m/s, while the other is distributed between 500 and 650 m/s and has an average value of 580 m/s. If one assumes the lower of the high‐velocity signals to be the ion acoustic velocity Cs, the higher velocity can be interpreted as electrostatic ion cyclotron (EIC) waves produced by NO+ ions. These EIC waves follow perfectly the dispersion relation established from the fluid approximation ωr = (Ωi² + k² Cs²)½. The radar echoes with low Doppler velocities are associated with irregularities produced by the gradient drift instability which presumably was operative at the top of the E layer. We suggest that magnetic field‐aligned drifts combined with the subcritical perpendicular electron drifts are responsible for the production of both the EIC waves and the ion acoustic waves that were observed.

Calculation of intensity distribution in the case of oblique texture electron diffraction

A new method is described to determine accurately the intensities of reflexions in the case of oblique texture electron diffraction. The method is based on a comparison between experimental values of intensities and those calculated with a formalism that takes into account the orientation function of the particles. It allows the problems that arise from frequent overlapping of reflexions to be surmounted. A concrete example of the application of this method is given the structure refinement of a mica with muscovite–phengite composition.

Use of diffraction and Mössbauer methods for the structural and crystallochemical characterization of nontronites

The aim of this work is the study of the order-disorder distribution of the Fe3+ cations in a dioctahedral smectite: the Garfield nontronite. The structural characteristics have been determined by selected-area electron diffraction, oblique texture electron diffraction and X-ray diffraction. These methods lead to the conclusion that the octahedral vacancies are in trans positions and that part of the Fe3 + ions is occupying the tetrahedral sites. The Mössbauer spectra of the nontronite were decomposed into four doublets. According to the determined structural characteristics, these doublets are interpreted in terms of trans vacant octahedral positions.

Oblique Electron Thermal Waves in a Magnetized Plasma

Propagations of an oblique electron thermal mode under the electron plasma frequency without boundary effects are investigated experimentally and theoretically in a magnetized plasma. The phase, ray, and group velocity surfaces of the electron thermal mode are obtained in a polar coordinate. The experimental observation of the electron mode radiated from a point source is found to be in fair accord with the theoretical wave fronts which are obtained from the ray velocity surface. Wave fronts and ray trajectories of an oblique electron thermal mode radiated from a point source are numerically obtained in an inhomogeneous magnetized plasma with a use of an electrostatic kinetic theory. The spatial numerical results are indicated mainly below the electron plasma and cyclotron frequencies. Reflections of the mode in the plasma density lower than the electron plasma frequency are made clear numerically.

Filamentary Collapse in Electron-Beam Plasmas

Observations of a form of three-dimensional turbulence, which occurs in beam-plasma interactions are described. Large-amplitude, beam-driven instabilities in both the oblique electron plasma and electron-cyclotron modes are shown to collapse into thin, magnetic-field--aligned filaments, and to exhibit a variety of modulation time scales characteristic of both electron and ion modes in the system. On the largest time scales, associated density depletions are observed.

Amplification of electrostatic noise in cyclotron resonance with an adiabatic auroral beam

The adiabatic evolution of the auroral beam in the convergent geomagnetic field gives rise to a region of ∂F/∂ν⊥ >0 in the electron distribution function. The altitude dependence of this source of free energy is examined, and its ability to drive electrostatic waves toward instability is considered. General linear properties of the possible instabilities are discussed, and a detailed numerical calculation based on a collisionless height dependent model of an auroral arc (Maggs and Lotko, this issue) is described. Of the three wave modes considered, the upper hybrid mode is optimally amplified at altitudes below about 1000 km. Oblique electron Bernstein modes may be weakly unstable at the lowest altitudes, and the whistler mode appears to be stable at all altitudes. Wave amplification is limited by a finite resonance time which is determined by refraction of parallel wavenumbers by the topside ionospheric density gradient. Slow beams originating at relatively high altitudes and nearly perpendicularly propagating waves with frequencies away from cyclotron harmonics are favored by this instability mechanism.

Reflection of oblique electron thermal modes in an inhomogeneous magnetized plasma

In an inhomogeneous magnetoplasma, reflection of an oblique electron thermal mode radiated from a local source is investigated experimentally and theoretically near the electron plasma frequency layer. The experimental observation of reflection in the plasma density region lower than the ƒp layer is found to be in qualitative accord with the theoretical reflection, which is obtained from a kinetic theory in an inhomogeneous magnetoplasma. The reflection of the thermal mode is also compared with that of an electromagnetic mode at the ƒp layer.

Oblique ion lens for use with imaging mass spectrometer systems

Points out the possibility of using a lens system involving electric and magnetic fields which has been used successfully to image electrons in an oblique electron multiplier array device (Johnson and Hallam 1973). The oblique nature of this crossed field device is useful in eliminating difficulties with irradiation of the surface to be investigated with charged particles or photons.