Small-scale Plasma Research Articles

Generation of strong magnetic fields by counterpropagating moderate-intensity laser pulses in a low-density plasma

A study is made of the generation of strong quasistatic magnetic fields by counterpropagating moderate-intensity laser pulses of different frequencies in a low-density plasma. Strong magnetic fields are generated by small-scale large-amplitude plasma waves excited at different frequencies by ponderomotive forces in the interaction region of laser pulses. It is shown that magnetic fields are generated most efficiently under resonance conditions such that the frequency difference between laser pulses coincides with the plasma frequency. The spatial distribution of quasistatic magnetic fields is investigated, and the pattern of the contour lines of the electric current is calculated.

Results of radio occultation measurement of polar ionosphere at satellite-to-satellite paths during strong flare solar activity

Features in the amplitude and phase variations of the radio wave propagating through the lower polar ionosphere in radio communication links between CHAMP and GPS satellites are observed. Parameters of sporadic layers and altitude profiles of electron density in the lower polar ionosphere during intensive solar flares are analyzed. It is shown, that the radio occultation technique is effective to find a relationship in a chain of following phenomena: the arrival of a shock wave of solar wind; the protons and electrons precipitation from the radiation belt; the small-scale plasma irregularities excitation in the F region of ionosphere; the intensive sporadic E S layers appearance in the lower ionosphere.

Channeling of high-power radio waves under conditions of strong anomalous absorption in the presence of an averaged electron heating source

Strong anomalous absorption of a high-power radio wave by small-scale plasma inhomogeneities in the Earth’s ionosphere can lead to the formation of self-consistent channels (solitons) in which the wave propagates along the magnetic field, but has a soliton-like intensity distribution across the field. The structure of a cylindrical soliton as a function of the wave intensity at the soliton axis is analyzed. Averaged density perturbations leading to wave focusing were calculated using the model proposed earlier by Vas’kov and Gurevich (Geomagn. Aeron. 16, 1112 (1976)), in which an averaged electron heating source was used. It is shown that, under conditions of strong electron recombination, the radii of individual solitons do not exceed 650 m.

Highly efficient pulsed power supply system with a two-stage LC generator and a step-up transformer for fast capillary discharge soft x-ray laser at shorter wavelength

Highly efficient and compact pulsed power supply system for a capillary discharge soft x-ray laser (SXRL) has been developed. The system consists of a 2.2 microF two-stage LC inversion generator, a 2:54 step-up transformer, a 3 nF water capacitor, and a discharge section with a few tens of centimeter length capillary. Adoption of the pulsed transformer in combination with the LC inversion generator enables us to use only one gap switch in the circuit for charging the water capacitor up to about 0.5 MV. Furthermore, step-up ratio of a water capacitor voltage to a LC inversion generator initial charging voltage is about 40 with energy transfer efficiency of about 50%. It also leads to good reproducibility of a capillary discharge which is necessary for lasing a SXRL stably. For the study of the possibility of lasing a SXRL at shorter wavelength in a small laboratory scale, high-density and high-temperature plasma column suitable for the laser can be generated relatively easily with this system.

Laboratory studies of spectral features of stimulated electromagnetic emission during the ionospheric heating experiments

We present the results of laboratory studies of the formation of a number of spectral components of stimulated electromagnetic emission, which are related to the excitation of small-scale irregularities in the heated ionosphere. In the laboratory experiment, the small-scale irregularity was formed as a result of thermal self-channeling of short-wavelength quasielectrostatic oscillations in a magnetoplasma. Using the method of probing waves, it is experimentally shown that the trapping and waveguide propagation in a small-scale plasma irregularity are exclusively due to Langmuir waves, whereas the upper-hybrid waves with anomalous dispersion are not trapped into the irregularity. It is found that satellites shifted by about 1–2 MHz from the carrier frequency (700 MHz under the experimental conditions) are formed in the Langmuir wave spectrum during the thermal self-channeling. Two mechanisms of generation of spectral satellites have been detected. The first (dynamic) mechanism is observed during the formation of a small-scale irregularity with rapidly increasing longitudinal size. In this case, one low-frequency satellite is excited in the trapped-wave spectrum. The mechanism of the formation of this satellite is apparently related to the Doppler shift of the frequency of the Langmuir waves trapped inside the irregularity. The second (stationary) mechanism is observed in the case of a developed irregularity where its shape is close to cylindrical. In this regime, the trapped-wave spectrum has two symmetric spectral satellites, namely, high- and low-frequency ones. It may be hypothesized that the generation of these satellites is due to scattering of trapped Langmuir waves from drift oscillations of the irregularity.

Generation of quasistatic magnetic fields in the interaction of counterpropagating laser pulses in a low-density plasma

The effect of generation of quasistatic magnetic fields in the interaction of counterpropagating moderate-intensity laser pulses in a low-density plasma is considered. It is shown that the onset of strong magnetic fields is attributed to small-scale large-amplitude plasma waves excited in the region of interaction of laser pulses under the action of the averaged ponderomotive forces. The spatial distribution of quasistatic magnetic fields is investigated, as well as the structure of the magnetic-field and current lines.

Sporadic structures and small-scale irregularity in the nighttime polar ionosphere in the period of high solar activity according to the data of radio occultation measurements on satellite-to-satellite paths

Results of the analysis of 327 sessions of radio occultation on satellite-to-satellite paths are presented. The data are taken in the nighttime polar ionosphere in the regions with latitudes of 67°–88°, and in the period of high solar activity from October 26, 2003 to November 9, 2003. Typical ionospheric changes in the amplitude and phase of decimeter radio waves on paths GPS satellites-CHAMP satellite are presented. It is demonstrated that these data make it possible to determine characteristics of the sporadic Es structures in the lower ionosphere at heights of 75–120 km. Histograms of distribution of the lower and upper boundaries, thickness, and intensity of the Es structures are presented. Dispersion and spectra of amplitude fluctuations of decimeter radio waves caused by small-scale irregularity of the ionospheric plasma are analyzed. The relation of the polar Es structures and intensity of small-scale plasma irregularity to various manifestations of solar activity is discussed. The efficiency of monitoring the ionospheric disturbances caused by shock waves of the solar wind by the radio occultation method on satellite-to-satellite paths is demonstrated.

Measurement of electron density in atmospheric pressure small-scale plasmas using CO2-laser heterodyne interferometry

CO2-laser heterodyne interferometry was applied to measure electron density ne in three different types of high pressure (including atmospheric pressure) small-scale plasma sources: a short hollow cathode (HC) discharge tube, a pulsed dc plasma jet and a micro-HC plasma jet. The interfering contribution of the gas density reduction due to Joule heating of the measured phase shift was separated from the electron component based on these different temporal dependences. The typical values of ne measured in the short HC discharge tube with helium gas were on the order of 1013 cm−3 at a discharge current that varied from 25 to 225 mA and the pressure from a few tens to hundreds of Torr; the values measured in argon gas were further increased by a factor of five to six. For the dc He plasma jet ejected into open air, the radial profile of ne on the order of 1014 cm−3 presented a hollowed distribution based on the tubular cathode structure. The micro-HC structure allowed us to evaluate ne in both the parallel and the perpendicular directions with respect to the plasma jet axis, and the derived values of ne from both directions were consistent. Thus, we verified that this diagnostic technique can be applied to measure ne in various sub-millimeter scale plasmas operated at atmospheric pressure in pulsed operation modes with a sensitivity of about 1013 cm−3 (at an optical length of 1 mm) and a spatial resolution better than 100 µm.

Method for super Fresnel resolution in electromagnetic diagnostics of inhomogeneous plasma

The method of electromagnetic diagnostics is suggested, which promises to perform super Fresnel resolution of plasma inhomogeneities, that is resolution, distinguishing details smaller than Fresnel radius. To realize super Fresnel resolution it is suggested to represent the wave field of the source in the form of double weighted Fourier transform (DWFT), deals with Fourier transform simultaneously in coordinates of the sources and in coordinates of receivers. Important property of DWFT is that DWFT transfers into geometrical optics (GO) approximation for smooth inhomogeneous media and becomes equivalent to the Rytov or to small angle Born approximation in the case of weak inhomogeneities. As a result, inverse DWFT allows obtaining linear integral of plasma density both for large scale inhomogeneities, as in GO approximation, and also for inhomogeneities, whose transverse sizes are small as compared with Fresnel radius. DWFT embraces also the results of the phase screen method and allows to take into account phenomenon of micro-multirayness and to describe strong amplitude fluctuations. Using inverse DWFT algorithm, the authors study resolution of systems consisting of discrete sources and receivers. Both analytical estimates and the results of numerical modeling evidence opportunity to observe small scale plasma inhomogeneities with super Fresnel resolution.

PMSE and E-region plasma instability: In situ observations

From 29 June to July 6, 2003, the ROMA-Svalrak (Rocketborne Observations of the Middle Atmosphere at the Svalrak facilities) sounding rocket campaign took place at Ny-Ålesund (Spitsbergen, geographical coord. 79 ∘ N , 12 ∘ E , geomagnetic coord. 76 ∘ N , 110 ∘ E ). Three sounding rockets were launched to study neutral air turbulence and small scale plasma dynamics around polar mesosphere summer echoes (PMSE). During all three flights both PMSE and plasma instability events were observed. It is known that small-scale field aligned structures in the E-region plasma density can be created by unstable electromagnetic waves. The mechanism responsible for creating the structures causing radar echoes (PMSE) is believed to be neutral air turbulence in the presence of heavy charged particles. E-region plasma irregularities recorded during the last rocket flight (labeled RO-MI-03) were observed only during the upleg of the trajectory but not during the downleg. Also, on the upleg there was no clear spatial separation between PMSE and the plasma instability regions. In the current paper we consider this transition region in detail.

Coronal Magnetism: Difficulties and Prospects

Prospects for advances in understanding the properties of the coronal magnetic field are discussed. A new generation of ground-based instrumentation presents possibilities of improved direct measurements of the field (the Advanced Technology Solar Telescope: ATST) and its inference from radio observations (the Frequency Agile Solar Radiotelescope: FASR). The latter in particular promises major advances in determining the structure of the strong magnetic fields present in active regions. Interpreting observations of coronal oscillations using MHD wave models to infer a magnetic field strength has become popular. While limb observations yield field strengths compatible with those obtained from infrared spectroscopy, disc observations yield values that seem on the low side, suggesting the need for a programme of forward modelling with realistic global magnetic fields. Global magnetic field models can now provide information on the field in the corona, and towards the Earth through the solar wind. Major challenges for such modelling are the incorporation of small-scale plasma effects.

VLF transmitter signals as a possible tool for detection of seismic effects on the ionosphere

An overall consistent scheme is presented of using VLF transmitter signal spectral broadening observed on a satellite as a detection means of seismic activity. This includes the mechanisms for formation of small-scale plasma density irregularities, and generation of quasi-electrostatic lower hybrid resonance waves due to the scattering of transmitter signal from small-scale plasma irregularities. Both points are discussed in detail on quantitative level.

Collective backscattering of gyrotron radiation by small-scale plasma density fluctuations in large helical device

A version of the collective backscattering diagnostic using gyrotron radiation for small-scale turbulence is described. The diagnostic is used to measure small-scale (k(s) approximately 34 cm(-1)) plasma density fluctuations in large helical device experiments on the electron cyclotron heating of plasma with the use of 200 kW 82.7 GHz heating gyrotron. A good signal to noise ratio during plasma production phase was obtained, while contamination of stray light increased during plasma build-up phase. The effect of the stray radiation was investigated. The available quasioptical system of the heating system was utilized for this purpose.

Study of the impact of small-scale plasma modulations and the size of the plasma on seeded soft x-ray laser homogeneity.

In the present work, we have used the 2D AMR hydrodynamic code with radiation transport ARWEN to study the plasma homogeneity alteration on small scale (1-10 μm) that may arise from initial target defects or driving laser intensity modulations. These simulations show that target rugosity imprints strongly the plasma creating a complicated pattern of electronic density. In addition to this, we have done several simulations of plasmas of different size (from 20 μm to 2 mm height) to study the different stages of a seeded soft x-ray laser amplifier. We have observed that 2D hydrodinamic effects lower the gain in small plasmas. As the main pulse, which creates the population inversion, interacts with a fully developed plasma, we have implemented a ray-tracing subroutine for the laser energy deposition in order to treat correctly the refraction effects of the short pulse. Simulations show that these effects are not negligible in rugous targets. Finally, simulations of the behaviour of the seeded beam in the amplifying plasma have been done with the 3D ray-tracing (amplification in non-homogeneous media) code SHADOX, as a post-processor of ARWEN. Prior ray-tracing, gain calculation is done on the output of ARWEN using a simple 3-level atomic model.

Experimental Study of the Dynamics of Large- and Small-Scale Structures in the Plasma Column of Wire Array $Z$-Pinches

The dynamics of large- and small-scale plasma structures is investigated in the precursor of 1-MA wire array Z-pinches by laser probing diagnostics. It is found that plasma streams from the wires induce density perturbations in the precursor. Small-scale perturbations and large-scale cells arise in the nonlinear stage before implosion. The spatial and temporal scales of the observed structures are in agreement with the theoretical investigation for current-driven excitation of electromagnetic flute modes.

Electron acceleration by high current-density relativistic electron bunch in plasmas

Electron acceleration by a short high-current relativistic electron bunch (EB) in plasmas at three characteristic densities is studied by particle-in-cell simulation. It is found that if the EB is appropriately matched to the background plasma, the blowout space-charge field of the EB can accelerate the trailing bunch electrons at very high energy gain rate. This high energy gain, as well as the large-amplitude wakefield, the turbulent small-scale electron plasma waves, and the formation of large current peaks, are studied. The evolution of the EB, its blowout field, and other related parameters are shown to be self-similar.

Laboratory Study of Nonlinear Trapping of Magnetized Langmuir Waves Inside a Density Depletion

The formation of a small-scale plasma density depletion region extended along the ambient magnetic field and caused by the nonlinear interaction of the upper-hybrid plasma waves with a magnetoplasma has been observed under laboratory conditions modeling the ionospheric heating experiments. Plasma waves are trapped inside the depletion due to their specific dispersion properties. The threshold of the nonlinear wave trapping significantly increases in the vicinity of the harmonics of the electron gyrofrequency.

Properties of a planar wire arrays Z-pinch source and comparisons with cylindrical arrays

Planar wire array plasmas created by a 1 MA current discharge provide a novel method to investigate dense plasma dynamics, heating mechanism and radiation properties. This is a strongly inhomogeneous on small scale plasma, which has high resistivity with density and magnetic field dependence. The planar array dynamics leads to generation of a plasma with a chain of hot spots in a dense, high-opacity column. The single planar array consists of a number of wires with inter-wire separation of ≤1 mm in a linear row. It has been shown that a strongly inhomogeneous on small scale planar wire array plasma can radiate much more energy than the kinetic energy of imploding plasma. The results of recent experiments on scaling of radiation yields and powers with array masses, materials, inter-wire gaps and array width at 1 MA, 1.5 TW power Z-pinch Zebra generator at University of Nevada at Reno are presented. Radiation properties of planar arrays were compared with low-number cylindrical arrays and compact cylindrical array loads. Data on the generation of the hot spots during implosion of planar array plasma and its impact on the radiation pulse are reported.

The annular glow discharge: a small-scale plasma for solution analysis

A recent trend in plasma spectrochemistry, and in analytical chemistry in general, has been towards miniaturization. Miniaturized plasma sources often have properties leading to low cost, potential portability, and simplified integration with separation instrumentation such as chromatographs. However, these discharges have not often performed well with solution samples. To address this shortcoming, we have developed a new, small-scale (5 × 2 mm) plasma, maintained in atmospheric-pressure helium between a tubular cathode and a rod-shaped anode (both made of steel). The discharge extends between the two electrodes and an annular glow is visible within the cathode tube. An aerosol is introduced into the plasma through the cathode, and atomic emission is observed in the near-cathode region. In this study, we observe the effects of solvent addition on the plasma in terms of its electrical and spectroscopic characteristics and how they change with solution flow rate. Given that even a robust source such as the inductively coupled plasma is different under wet and dry conditions, it is not surprising that the discharge introduced here is affected by the presence of an aerosol. However, the discharge is stable even with significant solvent loading. In this preliminary investigation, flow injection has been used to establish detection limits for several metals between 7 ppb (0.7 ng absolute) for Cd and 111 ppb (11 ng absolute) for Cu.

Laboratory modeling of nonlinear trapping of Langmuir waves inside a small-scale magnetoplasma irregularity

Laboratory studies modeling the small-scale plasma filamentation under the conditions relevant to the ionospheric heating experiments are presented. Filamentation is caused by nonlinear interaction of the short-wavelength quasi-electrostatic waves of the Z-mode with the plasma. This interaction occurs due to thermal plasma nonlinearity, i.e. due to local plasma heating and consequent thermodiffusion, which results in the formation of a narrow magnetic-field-aligned density depletion. The quasi-electrostatic plasma waves, in their turn, are trapped inside the depletion due to their specific dispersion properties. Such nonlinear wave trapping have been observed only at the critical plasma density ( ω = ω p). The trapped eigenmode has been associated with magnetized Langmuir waves, whose dispersion properties are analyzed using the kinetically modified plasma dispersion relation. Threshold of the nonlinear wave trapping exhibits significant growth in the vicinity of harmonics of the electron gyrofrequency.