Oscillation spectrum of an electron gas with a small density fraction of ions

The problem is solved of the stability of a nonneutral plasma that completely fills a waveguide and consists of magnetized cold electrons and a small density fraction of ions produced by ionization of the atoms of the background gas. The ions are described by an anisotropic distribution function that takes into account the characteristic features of their production in crossed electric and magnetic fields. By solving a set of Vlasov-Poisson equations analytically, a dispersion equation is obtained that is valid over the entire range of allowable electric and magnetic field strengths. The solutions to the dispersion equation for the m = +1 main azimuthal mode are found numerically. The plasma oscillation spectrum consists of the families of Trivelpiece-Gould modes at frequencies equal to the frequencies of oblique Langmuir oscillations Doppler shifted by the electron rotation and also of the families of “modified” ion cyclotron (MIC) modes at frequencies close to the harmonics of the MIC frequency (the frequencies of radial ion oscillations in crossed fields). It is shown that, over a wide range of electric and magnetic field strengths, Trivelpiece-Gould modes have low frequencies and interact with MIC modes. Trivelpiece-Gould modes at frequencies close to the harmonics of the MIC frequency with nonnegative numbers are unstable. The lowest radial Trivelpiece-Gould mode at a frequency close to the zeroth harmonic of the MIC frequency has the fastest growth rate. MIC modes are unstable over a wide range of electric and magnetic field strengths and grow at far slower rates. For a low ion density, a simplified dispersion equation is derived perturbatively that accounts for the nonlocal ion contribution, but, at the same time, has the form of a local dispersion equation for a plasma with a transverse current and anisotropic ions. The solutions to the simplified dispersion equation are obtained analytically. The growth rates of the Trivelpiece-Gould modes and the behavior of the MIC modes agree with those obtained by numerical simulation.     

On the minimal set of plasma parameters to determine the dispersion law of electron whistler waves

The minimal sufficient set of plasma parameters is presented to describe the dispersion properties of electron whistler waves (helicons) in a wide frequency range above the ion cutoff frequency, provided that the wave frequency is significantly lower than the electron plasma frequency. When the gyrofrequency of the lightest ions is much higher than those of heavier ions, it is sufficient to know the relative content of the lightest ions, the highest ion cutoff frequency, the lower hybrid resonance frequency, and the electron gyro- and plasma frequencies. In this case, the frequency of electron whistler waves is determined by the upper root of the biquadratic equation derived, whereas the lower root corresponds to a resonant mode with its refractive index increasing when the frequency tends toward the highest ion gyrofrequency from below. The developed approach is also efficient in plasmas containing a substantial amount of negative ions and/or heavy dust particulates. The accuracy of the approximate solution of the total cold plasma dispersion relation is illustrated graphically.     

Dispersion properties of a magnetoactive plasma-filled waveguide in the vicinity of the upper hybrid frequency. Part I

The dispersion properties of a magnetoactive plasma-filled waveguide in the vicinity of the upper hybrid frequency ω₁ = (ω _p ² + ω _B ² )^1/2 (ω₁ ≤ ω < ω₁ + δ) are studied. In this frequency range, the eigenfrequency spectrum and the structure of the waveguide eigenfields are difficult to analyze, because one of the transverse wavenumbers tends to infinity as the upper hybrid frequency is approached. It is shown that this problem can be solved by transforming the dispersion relation into a form independent of this transverse wavenumber. It is also found that the upper hybrid frequency itself can also be a solution to the dispersion relation. The influence of the properties of the magnetoactive plasma-filled waveguide on such solutions is studied. The structure of the fields for modes with frequencies equal to the upper hybrid frequency and an infinitely large absolute value of one of the transverse wavenumbers is analyzed.     

Dispersion properties of a magnetoactive plasma-filled waveguide in the vicinity of the upper hybrid frequency. Part II

The dispersion properties of a magnetoactive plasma-filled waveguide in the vicinity of the upper hybrid frequency ω₁ = (ω _p ² + ω _B ² )^1/2 (ω₁ ? δ < ω < ω₁) are studied. It is shown that, in this frequency range, the eigenmodes of the plasma-filled waveguide are represented by the families of EH and cyclotron HE modes, the interaction between which is weak everywhere except for the vicinities of certain points in the (ω, k _z ) plane. The equations describing the behavior of the dispersion curves in these vicinities are derived. It is shown that, as a result of the interaction, the high-frequency branches of EH modes acquire the cutoff frequencies corresponding to the high-order propagating HE modes. It is established that the anisotropic HE_+l mode can also interact with cyclotron HE modes. In this case, its dispersion curve enters the lower half-vicinity of the upper hybrid frequency, where it is modified due to the interaction, and then leaves it.     

Phonon dispersion curves and normal coordinate analysis of -poly-L-alanine

The vibrational frequencies of α-helical poly-L -alanine and its N-deuterated analog have been assigned by normal coordinate analyses. The phonon dispersion curves and frequency distribution of α-poly-L -alanine have been calculated by using a model which includes hydrogen bonding. The frequency distribution was used to interpret the inelastic neutron scattering data and to calculate the heat capacity. The low-frequency chain modes involving accordian-like motions of the whole helix have been calculated and their dispersion investigated by means of a simplified model.     

Generation of ion-acoustic and magnetoacoustic waves in an RF helicon discharge

A study is made of the generation of ion-acoustic and magnetoacoustic waves in a discharge excited in an external magnetic field by an electromagnetic wave in the whistler frequency range (ω_LH ? ω ? ω_He, where ω_LH = $\sqrt {\omega _{He} \omega _{Hi} } $ and ω_He and ω_Hi are the electron and ion gyrofrequencies, respectively). The excitation of acoustic waves is attributed to the decay of a high-frequency hybrid mode forming a plasma waveguide into low-frequency acoustic waves and new high-frequency waves that satisfy both the decay conditions and the waveguide dispersion relations. The excitation of acoustic waves is resonant in character because the conditions for the generation of waveguide modes and for the occurrence of the corresponding nonlinear wave processes should be satisfied simultaneously. An unexpected effect is the generation of magnetoacoustic waves by whistlers. A diagnostic technique is proposed that allows one to determine the thermal electron velocity by analyzing decay conditions and dispersion relations for waves in the discharge channel.     

On the dispersion properties of waveguides filled with a magnetized plasma

A study is made of the dispersion properties of waveguides filled with a magnetized plasma. It is shown that the eigenmodes of the waveguides filled with a low-density magnetized plasma fall into two families, which are weakly coupled to one another at all frequencies, in particular, in the cyclotron resonance frequency range. These families differ in transverse wavenumbers and the modes in them have hybrid polarization. Attention is focused on the study of the modes that have predominantly TE polarization at frequencies close to the cutoff frequency. The dependence of the critical frequencies of the TE modes on the plasma frequency, as well as the influence of the plasma on the energy flux and energy density of these modes, is investigated. The effect of mode crowding (the existence of an arbitrarily large number of dispersion curves in a finite frequency range between the cyclotron frequency and the upper hybrid frequency) is examined in detail. The results obtained are used to analyze how the plasma affects the electromagnetic properties of the cavity of the 1-MW 140-GHz continuous-wave gyrotron developed at the Institute of Pulsed and Microwave Technology of the Research Center in Karlsruhe, Germany (Institut für Hochleistungsimpuls-und Mikrowellentechnik Forschungszentrum Karlsruhe) for plasma heating in the W7-X stellarator, which is being constructed in Greifswald, Germany.     

Nonlocal theory of the spectra of modified ion cyclotron oscillations in a charged plasma produced by gas ionization

The problem of the stability of a charged plasma cylinder in crossed longitudinal magnetic and radial electric fields is considered under the assumption that the plasma electrons are magnetized and distributed uniformly in space and that the plasma ions have a low density and move without collisions. By using the Vlasov and Poisson equations for the electric and magnetic fields of arbitrary strengths and by expanding the radial eigenfunctions in Bessel functions, a dispersion relation is obtained for the modified ion cyclotron frequencies. The dispersion relation obtained is solved for the case of hot plasma electrons. At low ion densities, the oscillation spectra are stable and are described by the families of dispersion curves lying closely around the harmonics of the modified cyclotron frequency (including the zeroth harmonic). At higher ion densities, the dispersion curves for the frequencies of the lowest radial oscillation modes in neighboring families can intersect, leading to instability. The maximum instability growth rate can be several tenths of the modified ion cyclotron frequency.     

Analytical solutions for global geodesic acoustic modes in tokamak plasmas

Analytical solutions for global geodesic acoustic modes in the plasma of a tokamak with circular concentric magnetic surfaces are obtained. In the framework of ideal magnetohydrodynamics, an integral equation for eigenvalues (dispersion relation) taking into account toroidal coupling between electrostatic perturbations and electromagnetic perturbations with the poloidal mode number |m| = 2 is derived. In the absence of such coupling, the dispersion relation yields only the standard continuous spectrum. The existence of a global geodesic acoustic mode is analyzed for equilibria with both on-axis and off-axis maxima of the local geodesic acoustic frequency. The analytical results are compared with results of numerical calculations.     

The identification of trimethylsilylated dipeptides with chemical ionization mass spectrometry.

The chemical ionization (CI) and electron impact (EI) mass spectra were compared for over 40 trimethylsilylated (Me₃Si) dipeptides. The dipeptides chosen had all 20 common amino acids represented at amino and carboxyl positions. The CI mass spectra of Me₃Si dipeptides typically contain three ions of high abundance used for dipeptide identification: a sequence-determining ion and two molecular weight-determining ions. The intensity of the molecular weight-determining ions relative to that of the ion that characterizes the N-terminal residue (β-cleavage ion) is greater in the CI mode than in the EI mode. Because the available intensity of the β-cleavage ion is similar in both modes, use of the CI mode will extend the lower limit for Me₃Si dipeptide identification.     

Studies on photosynthetic oxygen-evolving complex by means of Fourier transform infrared spectroscopy: calcium and chloride cofactors

Structural roles of functional Ca²⁺ and Cl⁻ ions in photosynthetic oxygen-evolving complexes (OEC) were studied using low- (640–350 cm⁻¹) and mid- (1800–1200 cm⁻¹) frequency S₂/S₁ Fourier transform infrared (FTIR) difference spectroscopy. Studies using highly active Photosystem (PS) II core particles from spinach enabled the detection of subtle spectral changes. Ca²⁺-depleted and Ca²⁺-reconstituted particles produced very similar mid- and low-frequency spectra. The mid-frequency spectrum was not affected by reconstitution with ⁴⁴Ca isotope. In contrast, Sr²⁺-substituted particles showed unique spectral changes in the low-frequency Mn–O–Mn mode at 606 cm⁻¹ as well as in the mid-frequency carboxylate stretching modes. The mid-frequency spectrum of Cl⁻-depleted OEC exhibited marked changes in the carboxylate stretching modes and the suppression of protein modes compared with that of Cl⁻-reconstituted OEC. However, Cl⁻-depletion did not exert significant effects on the low-frequency spectrum.     

The mechanism of alkylation of DNA by 5-(3-methyl-1-triazeno)imidazole-4-carboxamide (MIC), a metabolite of DIC (NSC-45388). Non-involvement of diazomethane

Comparison of the nuclear magnetic resonance spectra of chemically synthesized methyl-d₁-methanol with the methanol produced in the solvolytic decompostion of 5-(3-methyl-1-triazeno)imidazole-4-carboxamide (MIC) in D₂O under acidic, basic or neutral conditions indicated that no deuterium was exchanged for the hydrogens on the methyl group. Diazomethane can therefore be ruled out as an intermediate in this reaction.The methyl-d₃-guanine isolated after incubation of methyl-d₃-MIC with calfthymus DNA in vitro displayed, on chemical ionization mass spectrometry, a quasimolecular ion (MH⁺) at m/e 169, which was 3 mass units higher than the quasimolecular ion for an undeuterated 7-methylguanine standard. The major fragment ions for 7-methyl-d₃-guanine on electron impact mass spectrometry likewise were situated at positions 3 mass units higher than the fragment ions for 7-methylguanine itself.These data indicate that the methylation of biological macromolecules by MIC must involve the transfer of an intact methyl group.     

Electrospray mass spectrometry of cis-[Ru(NO)Cl(bpy)2] (bpy=2,2-bipyridine): fragmentation from desolvated {[Ru(NO)Cl(bpy)2], Cl} ion pairs by electron transfer and internal Lewis base pathways

The electrospray mass spectrum (ESI-MS) of cis-[Ru(NO)Cl(bpy)₂]Cl₂ (bpy=2,2^′-bipyridine), obtained from 50% CH₃OH/50% H₂O as the mobile solvent, exhibited ruthenium-containing ions derived from a {[Ru^II(NO⁺)Cl(bpy)₂]²⁺, Cl⁻}⁺ ion pair (m/z=514) and [Ru^II(NO⁺)Cl(bpy)₂]²⁺ (m/z=239.5). [Ru^IIICl(bpy)₂]²⁺, from the loss of NO from the 239.5 ion, is detected at m/z=224.5. Only the m/z 514 ion pair is detected when 100% CH₃OH mobile solvent is used, but the presence of even small amounts of water prompted the additional detection of the m/z 239.5 and m/z 224.5 ions under tandem MS-MS conditions. Ruthenium-chloro-containing ions appear as a characteristic collection of eight main, and four lesser, intense ions created from combinations ¹⁰⁴Ru, ¹⁰²Ru, ¹⁰¹Ru, ⁹⁹Ru, ⁹⁸Ru, ⁹⁶Ru, ³⁵Cl and ³⁷Cl isotopes with minor contributions from ¹³C, etc. For convenience of discussion, only the most abundant m/z species are mentioned herein as representative of all the isotopically distributed ions.Four fragmentation channels are detectable from the m/z=514 chloride ion pair: (1) the loss of HCl (main channel; ca. 50% of fragmentation events), (2) the loss of NO (ca. 12% ), (3) the loss of bpy (minor pathway), and (4) the loss of Cl atom (ca. 38% ).Loss of NO from ion m/z 514 yields ion m/z 484, which is the precursor of ions m/z 448 (by loss of HCl), m/z 328 (by loss of bpy) and m/z 292 (by loss of HCl and bpy). Loss of HCl from ion m/z 514 generates ion m/z 478, [Ru^II(NO⁺)Cl(bpyH)(bpy-H)]⁺, deprotonated at the ortho C-H of one bpy ligand. In MS-MS experiments, the m/z 478 ion was established to undergo loss of NO, producing ion m/z 448, rejoining further fragmentation process for ion m/z 448 at this point. Loss of neutral bipyridine from m/z 514 in low yield produces ion m/z 358, which undergoes further loss of NO to form [RuCl₂(bpy)]⁺ ion (m/z=328). MS-MS “neutral loss of 30” spectra confirmed the NO loss events as part of the fragmentation sequence for all four pathways.A fourth species of m/z=479 from the “514” ion is obtained by an internal electron transfer from Cl⁻ of the ion pair, and loss of the resultant neutral Cl atom. The product [Ru^II(NO^·)Cl(bpy)₂]⁺ “479” fragment undergoes facile loss of NO to generate [Ru^IICl(bpy)₂]⁺ (m/z=449). Ion m/z 449 gives rise to ions m/z 413 (loss of HCl) and m/z 257(loss of HCl and bpy). MS-MS experiments confirm the neutral loss of Cl from the m/z 514 ion, and the formation of the m/z 449 ion via m/z 479 and m/z 514 parents. This pathway was not observed in a prior study for the related complex, [Ru(NO)Cl(dpaH)(dpa⁻)]⁺ (dpaH=2,2^′-dipyridylamine), which does not have an external Cl⁻ in an ion pair.     

Atmospheric pressure photoionization mass spectrometry of per-O-methylated oligosaccharides related to D-xylans

Three d-xylan type per-O-methylated trisaccharides with various types of linkages between the d-xylopyranose units were examined by atmospheric pressure photoionization (APPI) mass spectrometry in the positive ion mode. The most interesting feature of a thermospray mass spectrum using the APPI source with UV lamp switched off, is the exclusive production of [M+Na]⁺ adduct ions. [M+Na]⁺ cationized ions are the most abundant species in the case of APPI mass spectrometry. The second ionization process has no analogy in the case of substances studied using APPI to date. This aspect involves the addition of a water molecule to the molecular ion of a per-O-methylated saccharide, giving rise to [M+H₂O]⁺ adduct ions. The [M+H₂O]⁺ species are readily detected at m/z 544, and are clearly visible for all three isomers studied. The MS/MS spectrum of [M+Na]⁺ ions contains a base peak at m/z 375, produced by a Y-type cleavage of the trisaccharide, along with a hydrogen rearrangement on the terminal interglycosidically linkage glycosidic oxygen atom. The [M+H₂O]⁺ species fragment largely give rise to ions at m/z 175, 143 and, as a result, the m/z 111 ion is unique to nonreducing terminal units.     

Parametric turbulence-sustained gas discharges

A brief review is given of papers on the RF production of a plasma whose electrons are heated due to the parametric turbulence driven by an alternating electric pump field and maintain the discharge by ionizing the working gas atoms. Results are summarized from studies of low-frequency parametric turbulence, specifically, ion-acoustic plasma turbulence in a magnetic field, ion-cyclotron turbulence associated with the excitation of ion Bernstein modes, and lower hybrid turbulence in a plasma with ions of one or two species. The turbulence level and the rate of turbulent heating of the electrons and ions are presented, and the results of modeling of these phenomena are described. Attention is focused on experiments in which low-frequency parametric turbulence may be observed.     

Transport processes in plasma with an admixture of several heavy impurities

A two-temperature magnetohydrodynamic model of an ideal, fully ionized magnetized plasma consisting of electrons and several types of ions is developed for the case in which the mass of ions of the first type is much lower than that of jth-type ions, where j = 2,3,…, m ₁ ? m _j, while the densities of heavy ions are so low that collisions between them can be neglected. The ion component is assigned a common velocity, common temperature, and common density, while its composition can vary in time and space.     

Analysis of the efficiency of lower hybrid current drive in the FT-2 tokamak

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value n _LH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures T _e ). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(N _z ), were calculated. Simulations using the FRTC code with allowance for the P(N _z ) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.     

Generation of optimum pseudorandom signals for respiratory impedance measurements

Spontaneous breathing may impair the reliability of forced oscillatory impedance estimates at low frequencies, especially when the oscillatory power is distributed among many frequency values. Since the amplitude of the external forcing is limited to avoid non-linearities, it is suggested that the total energy of a composite electrical signal driving the loudspeaker be maximized at a given amplitude by finding the optimum phase relationships of the signal components, and that the low-frequency components increase in energy at the expense of the less disturbed high-frequency region. In healthy children and adults and in obstructed patients, the coherences and the coefficients of variation of the respiratory system impedance (Z_rs) at 2 and 3 Hz were studied in the case of three test signals of 2–15 Hz bandwidth. Signals T1 and T2 had a flat power spectrum, whereas the components of T3 decreased sharply between 2 and 5 Hz; T1 was generated by simple random selection of phase angles, while optimization for maximum energy was done for T2 and T3. Optimization alone (T2) increased the reliability of the Z_rs estimates at all frequencies, whereas enhancement of the low-frequency power (T3) resulted in a radical improvement of the estimates at 2 and 3 Hz, without loss in reliability at higher frequencies.     

Tandem mass spectrometric characterization of bile acids and steroid conjugates based on low-energy collision-induced dissociation

We examined the characteristics of several bile acids and some steroid conjugates under low-energy-collision-induced dissociation conditions using a triple quadrupole tandem mass spectrometer. According to conjugation types, we observed characteristic product ions and/or neutral losses in the product ion spectra. Amino acid conjugates afforded specific product ions. For example, glycine-conjugated metabolites routinely produced a product ion at m/z 74, and taurine-conjugated metabolites produced product ions at m/z 124, 107, and 80. When a strong peak appeared at m/z 97, the molecule contained a sulfate group. In contrast to amino acid conjugates, carbohydrate conjugates required a combination of product ions and neutral losses for identification. We could discriminate a glucoside from an acyl galactoside according to the presence or absence of a product ion at m/z 161 and a neutral loss of 180 Da. Discrimination among esters, aliphatic ethers, and phenolic ether types of glucuronides was based upon differences in the intensities of a product ion at m/z 175 and a neutral loss of 176 Da. Furthermore, N-acetylglucosamine conjugates showed a characteristic product ion at m/z 202 and a neutral loss of 203 Da, and the appearance of a product ion at m/z 202 revealed the existence of N-acetylglucosamine conjugated to an aliphatic hydroxyl group without a double bond in the immediate vicinity.Together, the data presented here will help to enable the identification of unknown conjugated cholesterol metabolites by using low-energy collision-induced dissociation.     

Electrical noise from lipid bilayer membranes in the presence of hydrophobic ions

Summary In the presence of the hydrophobic ion dipicrylamine, lipid bilayer membranes exhibit a characteristic type of noise spectrum which is different from other forms of noise described so far. The spectral density of current noise measured at zero voltage increases in proportion to the square of frequency at low frequencies and becomes constant at high frequencies. The observed form of the noise spectrum can be interpreted on the basis of a transport model for hydrophobic ions in which it is assumed that the ions are adsorbed in potential-energy minima at either membrane surface and are able to cross the central energy barrier by thermal activation. Accordingly, current-noise results from random fluctuations in the number of ions jumping over the barrier from right to left and from left to right. On the basis of this model the rate constantk _i for the translocation of the hydrophobic ion across the barrier, as well as the mean surface concentrationN _t of adsorbed ions may be caluculated from the observed spectral intensity of current noise. The values ofk _i obtained in this way closely agree with the results of previous relaxation experiments. A similar, although less quantitative, agreement is also found for the surface concentrationN _t .