Cleaning of inner vacuum surfaces in the Uragan-3M facility by radio-frequency discharges

A method for cleaning vacuum surfaces by a low-temperature (T _e ～ 10 eV) relatively dense (n _e ≈ 10¹² cm^?3) plasma of an RF discharge was developed and successfully applied at the Uragan-3M torsatron. The convenience of the method is that it can be implemented with the same antenna system and RF generators that are used to produce and heat the plasma in the operating mode and does not require retuning the frequencies of the antennas and RF generators. The RF discharge has a high efficiency from the standpoint of cleaning vacuum surfaces. After performing a series of cleanings by the low-temperature RF discharge plasma (about 20000 pulses), (i) the intensity of the CIII impurity line was substantially reduced, (ii) a quasi-steady operating mode with a duration of up to 50 ms, a plasma density of n _e ≈ 10¹² cm^?3, and an electron temperature of up to T _e ～ 1 keV was achieved, and (iii) mass spectrometric analysis of the residual gas in the chamber indicated a significant reduction in the impurity content.     

Nanosecond pulsed electric field generators for the study of subcellular effects

Modeling and experimental studies have shown that pulsed electric fields of nanosecond duration and megavolt per meter amplitude affect subcellular structures but do not lead to the formation of large pores in the outer membrane. This "intracellular electromanipulation" requires the use of pulse generators which provide extremely high power but low energy pulses. In this study, we describe the concept of the required pulsed power sources, their design, operation, and the necessary diagnostics. Two types of pulse generators based on the Blumlein line principle have been developed and are described here. One system is designed to treat a large number of cells in cuvettes holding volumes from 0.1 to 0.8 ml. Pulses of up to 40 kV amplitude, with a duration of 10 ns and a rise time close to 1 ns can be applied to the cuvette. For an electrode gap of 1 mm this voltage corresponds to an average electric field of 40 MV/m. The second system allows for real time observation of individual cells under a microscope. It generates pulses of 10-300 ns duration with a rise time of 3.5 ns and voltage amplitudes up to 1 kV. Connected to a microreactor with an electrode gap of 100 microm, electric fields up to 10 MV/m are applied.     

Pulse design for broadband correlation NMR spectroscopy by multi-rotating frames

We present a method for designing radio-frequency (RF) pulses for broadband or multi-band isotropic mixing at low power, suitable for protein NMR spectroscopy. These mixing pulses are designed analytically, rather than by numerical optimization, by repeatedly constructing new rotating frames of reference. We show how pulse parameters can be chosen frame-by-frame to systematically reduce the effective chemical shift bandwidth, but maintain most of the effective J-coupling strength. The effective Hartmann-Hahn mixing condition is then satisfied in a multi-rotating frame of reference. This design method yields multi-band and broadband mixing pulses at low RF power. In particular, the ratio of RF power to mixing bandwidth for these pulses is lower than for existing mixing pulses, such as DIPSI and FLOPSY. Carbon-carbon TOCSY experiments at low RF power support our theoretical analysis.     

Increase in the discharge duration under the action of ion-cyclotron plasma heating in the T-11M tokamak

Results are presented from experimental and theoretical studies of the heating of a hydrogen plasma with a lithium admixture at the fundamental ion-cyclotron frequency of hydrogen in the T-11M tokamak. It is found experimentally that the action of RF radiation on a hydrogen plasma containing a small amount (less than 4%) of lithium increases the duration of the discharge current pulse. The effect of the increase in the discharge current pulse under the action of RF radiation is simulated numerically.     

Plasma chemistry of the sealed-off slab CO laser active medium pumped by radio-frequency discharge with liquid-nitrogen-cooled electrodes

The long-term time behavior of the output power of a sealed-off cryogenic slab CO laser pumped by a repetitively pulsed RF discharge and operating on the overtone (λ = 2.6–3.5 μm) vibrational?rotational transitions of the CO molecule was studied experimentally. It is shown that adding of an anomalously large amount of oxygen (up to 50% with respect to the CO concentration) to the initial gas mixture CO : He = 1 : 10 leads to a manyfold (by several tens of times) increase in the duration of the laser operating cycle (until lasing failure due to the degradation of the active medium). In this case, the laser life-time without replacement of the active medium reaches 10⁵–10⁶ pulses. Using various diagnostics (including luminescence spectroscopy and IR and UV absorption spectroscopy), regularities in the time-behavior of the concentrations of the main component of the active medium (CO molecules) and the products of plasmachemical reactions (O₃, CO₂) generated in the discharge gap during the laser operating cycle are revealed. Time correlation between the characteristics of the active medium and the laser output power are analyzed. A phenomenological approach to describing the entirety of plasmachemical, purely chemical, gas-dynamic, and diffusion processes determining the behavior of the laser output characteristics throughout the laser operating cycle is offered.     

Diagnostics of soft X-ray emission from the plasma of a fast capillary discharge

Results are presented from time-resolved measurements of the soft X-ray emission in the 10-to 40-eV spectral range from the plasma of a pulsed capillary discharge in argon at current pulse amplitudes of up to 26 kA and a current rise time of ～10¹² A/s. The experiments were carried out with 0.3-cm-diameter 15.7-cm-long ceramic capillaries filled with argon at a pressure of 0.25–0.5 Torr in the SIGNAL electrophysical facility. The experimental data are interpreted via computer simulations of the magnetohydrodynamics and level-by-level ion kinetics of an argon plasma. The results obtained indicate that soft X-ray laser pulses with a photon energy of 26.4 eV and duration of 1–2 ns are generated ≈33 ns after the beginning of the discharge current pulse.     

Wavelet Domain Radiofrequency Pulse Design Applied to Magnetic Resonance Imaging

A new method for designing radiofrequency (RF) pulses with numerical optimization in the wavelet domain is presented. Numerical optimization may yield solutions that might otherwise have not been discovered with analytic techniques alone. Further, processing in the wavelet domain reduces the number of unknowns through compression properties inherent in wavelet transforms, providing a more tractable optimization problem. This algorithm is demonstrated with simultaneous multi-slice (SMS) spin echo refocusing pulses because reduced peak RF power is necessary for SMS diffusion imaging with high acceleration factors. An iterative, nonlinear, constrained numerical minimization algorithm was developed to generate an optimized RF pulse waveform. Wavelet domain coefficients were modulated while iteratively running a Bloch equation simulator to generate the intermediate slice profile of the net magnetization. The algorithm minimizes the L2-norm of the slice profile with additional terms to penalize rejection band ripple and maximize the net transverse magnetization across each slice. Simulations and human brain imaging were used to demonstrate a new RF pulse design that yields an optimized slice profile and reduced peak energy deposition when applied to a multiband single-shot echo planar diffusion acquisition. This method may be used to optimize factors such as magnitude and phase spectral profiles and peak RF pulse power for multiband simultaneous multi-slice (SMS) acquisitions. Wavelet-based RF pulse optimization provides a useful design method to achieve a pulse waveform with beneficial amplitude reduction while preserving appropriate magnetization response for magnetic resonance imaging.     

Properties of a low-pressure inductive RF discharge I: Experiment

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.     

Effect of the vibrational excitation of CO molecules on the parameters of an RF discharge

A one-dimensional model of an RF discharge in CO-containing gas mixtures is developed. The model takes into account the effect of the degree of vibrational excitation of CO molecules on the structure of the discharge and on its parameters. Experimental data are presented from measurements of the voltage-power characteristics of RF discharges in gas mixtures with different CO contents in the pressure range of 10–100 torr. The model developed is used to calculate the dependence of the root-mean-square discharge voltage on the specific power deposition in an RF discharge under our experimental conditions. The experimental data are compared to the results of numerical simulations. For working gas pressures of about 100 torr, which are typical of the operation of slab CO lasers, the calculated voltage-power characteristics of an RF discharge agree satisfactorily with those obtained experimentally. The theoretical model predicts that the vibrational excitation of CO molecules leads to a redistribution of the RF field in the discharge gap and to an increase in the laser efficiency.     

Properties of a low-pressure inductive RF discharge II: Mathematical simulations

Self-consistent numerical simulations of a low-pressure inductive RF discharge have been carried out. It is shown that, on the one hand, the plasma parameters are determined by the RF power absorbed in the plasma and, on the other, they themselves govern the power absorption. This results in a nonmonotonic dependence of the plasma parameters on the magnetic field, as well as in discharge disruptions, similar to those observed experimentally in such discharges. An inductive RF discharge with a capacitive component is simulated. The experimentally observed characteristic properties of the discharges are explained based on the regular features of the absorption of RF power in the plasma. Traditional inductive plasma sources (both without and with a magnetic field) are considered.     

Experimental study of the processes accompanying argon breakdown in a long discharge tube at a reduced pressure

Results are presented from experimental studies of the breakdown stage of a low-pressure discharge (1 and 5 Torr) in a glass tube the length of which (75 cm) is much larger than its diameter (2.8 cm). Breakdowns occurred under the action of positive voltage pulses with an amplitude of up to 9.4 kV and a characteristic rise time of 2–50 μs. The discharge current in the steady-state mode was 10–120 mA. The electrode voltage, discharge current, and radiation from the discharge gap were detected simultaneously. The dynamic breakdown voltage was measured, the prebreakdown ionization wave was recorded, and its velocity was determined. The dependence of the discharge parameters on the time interval between voltage pulses (the socalled “memory effect”) was analyzed. The memory effect manifests itself in a decrease or an increase in the breakdown voltage and a substantial decrease in its statistical scatter. The time interval between pulses in this case can reach 0.5 s. The effect of illumination of the discharge tube with a light source on the breakdown was studied. It is found that the irradiation of the anode region of the tube by radiation with wavelengths of ≤500 nm substantially reduces the dynamic breakdown voltage. Qualitative explanations of the obtained results are offered.     

Formation of a spark discharge in an inhomogeneous electric field with current limitation by a large ballast Resistance

Results of studies of a spark discharge initiated in argon in a point–plane electrode gap with limitation of the discharge current by a large ballast resistance are presented. It is shown that the current flowing through the plasma channel of such a low-current spark has the form of periodic pulses. It is experimentally demonstrated that, when a low-current spark transforms into a constricted glow discharge, current pulses disappear, the spatial structure of the cathode glow changes abruptly, and a brightly glowing positive plasma column forms in the gap.     

Determination of the electron temperature in microplasma discharges excited on a titanium surface

Results are presented from experimental studies of the emission spectra of microplasma discharges excited on a titanium surface by a pulsed plasma flow. The excited discharges are maintained by current pulses with an amplitude of 200 A and a duration of 20 ms. Analysis of more than 100 spectral lines of titanium atoms and ions in the wavelength range of 350–800 nm shows that the electron temperature of a microplasma discharge is in the range of 0.2–1.3 eV.     

Ionization fraction of the sputtered metal flux in a hollow cathode magnetron

Results from studies of the parameters of a novel type of plasma source—a hollow cathode magnetron—are presented. The magnetron operates at a gas pressure of 5–20 mTorr, the discharge power being in the range of 0.5–4 kW. At discharge powers exceeding 2 kW, a plasma flow with a density of higher than 10¹¹ cm^?3 and length of up to 30 cm forms at the magnetron output. Using a grid quartz crystal microbalance, the ionized copper flux fraction was measured as a function of the gas pressure, discharge power, and distance from the target. At gas pressures of higher than 15 mTorr, the degree of ionization at a distance of 31 cm exceeds 50%.     

Bicuculline application affects discharge pattern and pulse-duration tuning characteristics of bat inferior collicular neurons

This study examines the contribution of GABAergic inhibition to the discharge pattern and pulse duration tuning characteristics of 101 bat inferior collicular neurons by means of bicuculline application to their recording sites. When stimulated with single pulses, 56 (55%) neurons discharged 1 or 2 impulses (phasic responders), 42 (42%) discharged 3–10 impulses (phasic bursters) and 3 (3%) discharged impulses throughout the stimulus duration (tonic responders). Bicuculline application increased the number of impulses and changed the discharge patterns of 66 neurons. Using 50% difference between maximal and minimal responses as a criterion, the duration tuning characteristics of these neurons can be described as band-pass (20, 20%), long-pass (17, 17%), short-pass (33, 32%), and all-pass (31, 31%). Each band-pass neuron discharged maximally to a specific duration (the best duration) which was at least 50% larger than the neuron's responses to a long-duration pulse and a short-duration pulse. In contrast, each long- or short-pass neuron discharged maximally to a range of long or short duration pulses. Bicuculline application changed the duration tuning characteristics of 65 neurons. Possible mechanisms underlying duration tuning characteristics and the behavioral relevance to bat echolocation are discussed. Accepted: 4 November 1998     

Solid state NMR at high magic angle spinning frequencies: Dipolar chemical shift correlation with adiabatic inversion pulse based RF pulse schemes

The efficacy of hetero- and homonuclear dipolar recoupling employing tanh/tan adiabatic inversion pulse based RF pulse schemes has been examined at high magic angle spinning (MAS) frequencies via numerical simulations and experimental measurements. An approach for minimising the recoupling RF power level is presented, taking into consideration the spinning speed, the range of resonance offsets and H₁ inhomogeneities and the available RF field strength. This involves the tailoring of the frequency and amplitude modulation profiles of the inversion pulses. The applicability of tanh/tan pulse based dipolar recoupling schemes to spinning speed regimes where the performance with conventional rectangular pulses may not be satisfactory is demonstrated.     

Controlling the characteristics of a repetitive volume discharge in CFC-12 and its mixtures with argon

The parameters of a repetitive volume discharge in CF₂Cl₂ (CFC-12) and its mixtures with argon at pressures of P(CF₂Cl₂)≤0.4 kPa and P(Ar)≤1.2 kPa are studied. The discharge was ignited in an electrode system consisting of a spherical anode and a plane cathode by applying a dc voltage U_ch≤1 kV to the anode. The electrical and optical characteristics of a volume discharge (such as the current-voltage characteristics; the plasma emission spectra; and the waveforms of the discharge voltage, the discharge current, and the total intensity of plasma emission) are investigated. It is found that, by shunting the discharge gap with a pulsed capacitor with a capacitance of C₀≤3.5 nF, it is possible to control the amplitude and duration of the discharge current pulses, as well as the characteristics of the pulsed plasma emission. The increase in the capacitance C₀ from 20 to 3500 pF leads to a significant increase in the amplitude and duration of the discharge current pulses, whereas the pulse repetition rate decreases from 70 to 3 kHz. The glow discharge exists in the form of a domain with a height of up to 3 cm and diameter of 0.5–3.0 cm. The results obtained can be used to design an untriggered repetitive germicidal lamp emitting in the Cl₂(257/200 nm) and ArCl (175 nm) molecular bands and to develop plasmachemical methods for depositing amorphous fluorocarbon and chlorocarbon films.     

Generation of broadband radio pulses by a new-type reflex triode with a virtual cathode

Results are presented on the development and experimental study of a reflex triode with a new type of virtual cathode. In this device, a discharge excited along a ferroelectric surface is used as a source of electronsand loop antennas are used for emitting radiation. Generation of broadband radio pulses with a central frequency of ～300 MHz and power of ～80 W is achieved.     

Low-current medium-pressure RF discharge with electron photoemission in the electrode sheath and penetration of the sheath electrons into the discharge column

A model is developed for simulating a low-current moderate-pressure RF discharge with allowance for such characteristic discharge properties as the existence of two sheaths near both electrodes throughout the RF field period; the formation of an electron cloud at the sheath boundary that periodically fills the sheath and leaves it, depending on the phase of the applied RF voltage; the production by the sheath electrons of metastable gas particles that interact with the cloud electrons during subsequent field periods, followed by the excitation of metastable states to the emitting levels; the formation of a sheath in a low-current RF discharge due to the overlap of the secondary electron avalanches triggered by electron photoemission from the electrode surface; and the conditions under which the sheath electrons penetrate into the positive column and accumulate there, which makes, thereby making a low-current RF discharge similar to a non-self-sustained discharge. The parameters of the sheath in a low-current RF discharge are determined by the conditions under which the electron photoemission current from the electrode surface in the sheath is self-sustaining and, like the parameters of the positive discharge column, depend on the sort of gas, the gas pressure, the frequency of the applied RF field, and the interelectrode distance. The results of calculating the parameters of the sheath and column of a low-current RF discharge for nitrogen and helium at different pressures, as well as for different field frequencies and interelectrode distances, are presented and are compared with the experimental data.     

Study of the implosion characteristics of quasi-spherical wire arrays on the Angara-5-1 facility at currents of up to 4 MA

Results are presented from experimental studies of the spatial distribution of the density of matter in the central part of the discharge gap and the formation of the temporal profile of the X-ray power in the course of implosion of quasi-spherical wire arrays at discharge currents of up to 4 MA. The spatial distribution of the X-ray intensity in the central part of the discharge gap and the temporal profile of the X-ray power are used as implosion characteristics of quasi-spherical wire arrays. The quasi-spherical arrays were formed by the radial stretching of unstrained wires of initially cylindrical and conical wire arrays under the action of the electrostatic field. The temporal profile of the output X-ray pulse in the photon energy range of 0.1–1 keV is shown to depend on both the geometrical parameters of the quasi-spherical array and the longitudinal distribution of its mass. It is found that a 40% increase in the wire mass due to deposition of an additional mass in the equatorial region of a quasi-spherical array leads to a 15% increase in the average current radius of the pinch and a 30% decrease in the X-ray yield. Experiments with quasi-spherical arrays made of kapron fibers with deposited Al and Bi conducting layers were also carried out. It is demonstrated that application of such arrays makes it possible to control the profile and duration of the generated X-ray pulse by varying the mass, material, and location of the deposited layer. It is found that deposition of an additional mass in the form of a thin Bi stripe on tungsten wires near the cathode end of the array allows one to mitigate the influence of the cathode zipper effect on the pinch compression and formation of the X-ray pulse in tungsten arrays.