Physics of Absorption and generation of Electromagnetic Radiation

The chapter is divided into two parts. In the first part, the chapter discusses the theory of propagation of electromagnetic waves in different media with the help of Maxwell’s equations of electromagnetic fields. The electromagnetic waves with low frequency are suitable for the communication in sea water and are illustrated with numerical examples. The underwater communication have been used for the oil (gas) field monitoring, underwater vehicles, coastline protection, oceanographic data collection, etc. The mathematical expression of penetration depth of electromagnetic waves is derived. The significance of penetration depth (skin depth) and loss angle are clarified with numerical examples. The interaction of electromagnetic waves with human tissue is also discussed. When an electric field is applied to a dielectric, the material takes a finite amount of time to polarize. The imaginary part of the permittivity is corresponds to the absorption length of radiation inside biological tissue. In the second part of the chapter, it has been shown that a high frequency wave can be generated through plasma under the presence of electron beam. The electron beam affects the oscillations of plasma and triggers the instability called as electron beam instability. In this section, we use magnetohydrodynamics theory to obtain the modified dispersion relation under the presence of electron beam with the help of the Poisson’s equation. The high frequency instability in plasma grow with the magnetic field, wave length, collision frequency and the beam density. The growth rate linearly increases with collision frequency of electrons but it is decreases with the drift velocity of electrons. The real frequency of the instability increases with magnetic field, azimuthal wave number and beam density. The real frequency is almost independent with the collision frequency of the electrons.

DISTRIBUTION OF THE BEAM DENSITY AT THE TARGET OF SUBCRITICAL FACILITY “NEUTRON SOURCE”

NSC KIPT subcritical facility “Neutron Source” uses rectangular tungsten or uranium target of 6464 mm top cross-section. To generate maximum neutron flux, prevent overheating of the target and reduce thermal stress during the facility power operation one should provide uniform electron beam distribution at the target top surface. During the facility design three different possibilities of electron beam density redistribution above the target surface were considered. These were the fast beam scanning with two dimensional scanning magnets; the method of uniform beam distribution formation with linear focusing elements (dipole and quadrupole magnets) and nonlinear focusing elements (octupole magnets), when final required rectangular beam shape with homogeneous beam density is formed at target; and combined method, when one forms the small rectangular beam with homogeneous beam density distribution and scan it over the target surface with scanning magnets. In the paper the all three methods are considered and discussed taking into account the layout of the transportation channel of NSC KIPT subcritical facility “Neutron Source”. For the first stage of the facility start-up and pilot operation the fast scanning method was chosen, realised and tested. The results of the beam distribution measurements over the surface of the target during the facility adjustment and start up are presented.

Living up to the Hype(-rion)! – observations on ion microprobe geochronology using a high-brightness oxygen plasma source

<p>The recent introduction of a high-brightness RF plasma oxygen ion source (Hyperion H201, Oregon Physics) to large geometry secondary ion mass spectrometers (e.g. CAMECA IMS1280/1300) has increased the range of available primary beam options compared to the several decades old technology of the duoplasmatron it replaces. Notably, the new source provides considerably higher beam density (ca. 10x and 3x for O<sup>-</sup> and O<sub>2</sub><sup>-</sup> respectively), which in principle allows for higher spatial resolution and/or shorter analysis times, coupled with unprecedented long-term beam stability.</p><p>Incorporating the RF plasma into both conventional spot analysis and ion-imaging geochronology routines at the NordSIMS facility has, however, revealed that the source upgrade has consequences for data-acquisition and data reduction strategies, which need to be modified in order to avoid degradation in precision. The most significant difference using the new source for spot analyses is the significant change in aspect ratio (width/depth) of the analysed volume. During a comparable length analysis, a three times brighter O<sub>2</sub><sup>-</sup> primary beam (still favoured for U-Th-Pb geochronology) will sputter a three times deeper crater that is half the width of a comparable intensity duoplasmatron beam, an effective aspect ratio change of six times, introducing “down-hole” inter-element and, to a lesser degree, isotope fractionation effects that SIMS has largely been free of. Depending on the target matrix, this can have a marked effect on the within-run ratio evolution during an analysis, particularly the inter-element ratios Pb/U and UO<sub>n</sub>/U required for full U-Pb geochronology, with standard error of the mean values several times higher than counting statistics, compared to analyses with the lower beam density of the duoplasmatron where s.e. mean commonly closely approaches Poisson counting statistics during a ca. 10 minute analysis. In line with previous observations [1], some improvements can be made by using a Pb/UO vs. UO<sub>2</sub>/UO calibration scheme instead of Pb/U vs. UO<sub>n</sub>/U, but clearly this is not the complete answer. Shortening analyses via fewer cycles in a peak-hopping routine also means smaller √n, affecting s.e. mean; lower integration times can be introduced to permit more cycles, but magnet settling times between peak jumps cannot be reduced in proportion, so the duty cycle is less efficient.</p><p>Strategies developed to mitigate this degradation and take full advantage of the new RF source include: 1) rastering of critically focused primary beams to retain high aspect ratio (at the expense of improved spatial resolution); 2) use of a defocused aperture-projected (Köhler-mode) primary beam (effectively lower beam density); 3) modelling of within-run ratio evolution based on standard analyses in a manner similar to that employed by laser ablation methods [2]; and/or 4) introduction of multicollection capabilities [3] to increase duty cycle efficiency in a shorter analysis. Ultimately, the choice of which method(s) to use will depend upon the goal of a specific project.</p><p>References: [1] Jeon, H. & Whitehouse, M.J.., Geostds & Geoanal. Res. 2014, 39, 443-452]; [2] Paton, C. et al., Geochem. Geophys. Geosyst., 2010, 11, Q0AA06]; [3] Li et al., J. Anal. At. Spectrom., 2015, 30, 979-985</p>