Plasma Oscillation Frequency Research Articles

Kinetic theory of acoustic wave in pair-ion plasmas

A theoretical analysis of the ion acoustic wave in pair-ion plasmas is presented using a kinetic model. It is shown that a small concentration of electrons in pair-ion plasmas can break down the quasineutrality in perturbed systems because the electron Debye length can become very large. In the limit 1⪡λDe2k2 (where λDe is the electron Debye length), the damping is reduced and hence the acoustic wave can be easily excited. The real frequency of acoustic wave can be larger than the ion plasma oscillation frequency in this case. But the decreasing number density of electrons can increase the Landau damping significantly in the limit λDe2k2⪡1. The results are discussed in comparison with the recent experimental and theoretical findings. It is pointed out that this investigation can be very useful for further research on pair-ion plasmas.

Detection of terahertz radiation in gated two-dimensional structures governed by dc current

We present theoretical and experimental studies of the direct current effect on the detection of subterahertz and terahertz radiation in gated two-dimensional structures. We developed a theory of the current-driven detection both for resonant case, when the fundamental frequency of plasma oscillation is large compared to inverse scattering time, ${\ensuremath{\omega}}_{0}\ensuremath{\tau}⪢1$, and for the nonresonant case, ${\ensuremath{\omega}}_{0}\ensuremath{\tau}⪡1$, when the plasma oscillations are damped. We predict that, in the nonresonant case, even a very small dc current would increase the detection amplitude up to two orders of magnitude. Physically, this increase is related to an abrupt transition from the linear to saturation region near the knee of the current-voltage characteristic. When the current increases up to the saturation value, the electron concentration near the drain becomes very low and can be strongly affected by a small external field. As a consequence, the two-dimensional channel becomes extremely sensitive to external perturbations. In the resonant case, the detection amplitude has maxima when the radiation frequency is equal to fundamental plasma frequency and its harmonics. We predict that the effective linewidths of the respective resonances would decrease with the increasing current. Physically, this happens because dc current shifts the system towards the plasma wave instability. At some critical current value, the width corresponding to the fundamental frequency would turn to zero, indicating the onset of plasma waves generation. Our experimental measurements performed on $\mathrm{GaAs}$ HEMT confirm the theoretical predictions.

Emission of Electromagnetic Pulses from Laser Wakefields through Linear Mode Conversion

Powerful coherent emission around the plasma oscillation frequency can be produced from a laser wakefield through linear mode conversion. This occurs when the laser pulse is incident obliquely to the density gradient of inhomogeneous plasmas. The emission spectrum and conversion efficiency are obtained analytically, which are in agreement with particle-in-cell simulations. The emission can be tuned to be a radiation source in the terahertz region and with field strengths as large as a few GV/m, suitable for high-field applications. The emission also provides a simple way to measure the wakefield produced for particle acceleration.

Plasma satellites of X-ray lines of ions in a picosecond laser plasma

We present the results of our measurements of the spectra for multicharged ions in a plasma produced by moderately intense (about 1017Wcm−2) picosecond laser pulses. They suggest the existence of intense plasma oscillations with a frequency appreciably lower than the frequency of the laser radiation. The observed spectrum for the plasma satellites of the Lyman Lyα doublet of the hydrogenic F IX ion in a dense plasma was modeled theoretically. The resulting doublet profile was shown to have a complex structure that depends non-trivially both on the plasma density and on the frequency and amplitude of the plasma oscillations. The positions of the satellites and their separations allowed them to be associated with intense electrostatic oscillations with an amplitude of (4–6)×108Vcm−1 and a frequency near (0.7–1)×1015s−1. Assuming the oscillation frequency to be determined by the strength of the magnetic field B generated in the plasma, we obtained an estimate of B that is in reasonable agreement with other measurements and estimates of this quantity. Our theoretical analysis allowed explanation of the emission spectra observed when flat fluoroplastic targets were heated by intense picosecond laser pulses.

Resonant response of a FET to an AC signal: influence of magnetic field, device length, and temperature

A theoretical investigation is made of the response of a field-effect transistor (FET) to an incoming electromagnetic radiation in the presence of a perpendicular, weak magnetic field. The influence of an external friction due to electron scattering by impurities and/or phonons, and of the internal friction due to electron-electron scattering, is taken into account. The treatment is valid for a nondegenerate electron gas in which the mean-free path for electron-electron scattering /spl lambda//sub ee/ is much smaller than the device length L and than the mean-free path due to collisions with impurities and/or phonons /spl lambda//sub coll/. These requirements, written as /spl lambda//sub ee//spl Lt/L/spl Lt//spl lambda//sub coll/, are fulfilled for magnetic fields sufficiently weak that Landau quantization is absent and the electron motion is described within the framework of hydrodynamics. It is demonstrated that a high-electron mobility transistor (HEMT), with a short (long) channel, yields a resonant (nonresonant) response to an ac signal induced by the incoming electromagnetic radiation at the plasma oscillation frequencies of the two-dimensional electrons in the device. Keeping the device length and temperature at control, an applied magnetic field can be tuned to achieve the desired effect on the response of the device. It is observed that the lower the temperature, i.e., the higher the mobility, the higher the responsivity of the device. Such response makes the FET a promising device for new types of sources, detectors, mixers, and multipliers. The HEMT-based devices should, in principle, operate at much higher frequencies than the conventional transit time-limited devices, since the plasma waves propagate much faster than electrons.

Analysis of the characteristics of low-frequency modes in a corrugated waveguide filled with plasma

A corrugated waveguide filled with an infinitely thin annular plasma is immersed in a strong longitudinal magnetic field. By means of fluid model and linear field theory，the dispersion relation of the electromagnetic wave propagation along the system is derived. The dispersion characteristics and beamwave interactions of lowfrequency plasma modes are analyzed in two cases of various densities of plasma by numerical computation. It is found that the dispersion relation with out electron beam satisfies exactly the Floquet theorem and the frequency of electromagnetic wave could not be confined by plasma oscillation frequency. The low-frequency plasma modes can even be coupled with TM modes as the plasma density i s increased.

Stable and unstable behavior of inductively coupled electronegative discharges

Plasma instabilities have been observed in low-pressure inductive discharges, in the transition between low density capacitively driven and high density inductively driven discharges when attaching gases such as SF6 and Ar/SF6 mixtures are used. Oscillations of charged particles, plasma potential, and light, with frequencies from a few hertz to tens of kilohertz, are seen for gas pressures between 1 and 100 mTorr and discharge powers in the range of 75–1200 W. The region of instability increases as the plasma becomes more electronegative and the frequency of plasma oscillations increases as the power, pressure, and gas flow rate increase. Time-resolved measurements of electron temperature, and time averaged ion energy distribution at the wall have been made. A volume-averaged (global) model of the instability has been developed, for a discharge containing time varying densities of electrons, positive ions, and negative ions, and time invariant excited states and neutral densities. The particle and energy balance equations are integrated to produce the dynamical behavior. The model agrees well with experimental observations, and also shows a significant influence of the matching network. Values of plasma parameters and of chemical composition are measured to set the values used in the model, as required to obtain the good agreement with experiment. Capacitive coupling plays a crucial role in the instability process. A variable electrostatic (Faraday) shield has been used to control the capacitive coupling from the excitation coil to the plasma. The plasma instability disappears when the shielded area exceeds 65% of the total area of the coil. The model gives a slightly higher value of 85% for instability suppression with the same discharge conditions (Ar/SF6 1:1, 5 mTorr).

Plasma and transit-time mechanisms of the terahertz radiation detection in high-electron-mobility transistors

We develop a device model for a high-electron-mobility transistor (HEMT) affected by the incoming terahertz radiation. The model takes into account the electron plasma oscillations in the HEMT channel, tunnelling of electrons from the channel into the gate layer and electron transit-time effects in this layer. It is shown that the excitation of plasma oscillations accompanied by the delay in the electron propagation across the gate layer and a strong nonlinearity of the tunnelling current can result in significant features of the HEMT high-frequency linear and nonlinear characteristics. We derive a formula for the HEMT gate-to-source/drain admittance. We also calculate the variation of the dc current induced by the terahertz radiation and the HEMT detection responsivity. It is found that the detection responsivity exhibits sharp resonant peaks corresponding to the frequencies of plasma oscillations. The resonant plasma frequencies and the positions of the admittance and detection responsivity peaks depend on the gate length and the lengths of the contact regions (source-to-gate and gate-to-drain spacings) and can be tuned by the gate voltage. The coincidence of the plasma and transit-time resonances can lead to a marked sharpening of the responsivity peaks.

Tunnelling- and barrier-injection transit-time mechanisms of terahertz plasma instability in high-electron mobility transistors

We study theoretically the plasma instability in a heterostructure similar to a high-electron mobility transistor. The instability under consideration is associated with the transit-time effects of the electrons injected from the channel into the gate layer due to tunnelling or thermionic emission. The frequencies of the self-excited plasma oscillations, which fall within the terahertz range, are determined by the electron concentration in the channel and its length. We find the instability criteria expressed via the structural parameters and applied voltage.

Submillimeter spectroscopic study of concentrated electrolyte solutions as high density plasma

The transport property of ions in highly concentrated electrolyte solutions, which are governed by many body Coulomb interaction, are found to be well described by a model that ions behave as high density strongly coupled plasma irrespective of ion species. The frequency of plasma oscillation, which is the long-range collective motion of plasma, is estimated to be order of 1 THz for concentrated electrolyte solutions, and evidence of the plasma oscillation can be searched by submillimeter spectroscopy. Transmission and reflection spectra of the concentrated electrolyte solutions have been measured in the 10 to 50 cm−1 range, and the real and imaginary parts of refractive indices for the solutions are obtained. A close inspection of these spectral data suggests that the plasma oscillation is not excited in the highly concentrated electrolyte solutions. This is probably caused by that the long-range collective motion of ions is disturbed by short-range solute–solvent interaction.

Frequency upshifting of microwave radiation via resonant excitation of plasma oscillations in a thin layer of a time-varying plasma

Interaction of a p-polarized microwave radiation with a thin (compared to the wavelength) plasma layer, whose density slowly (compared to the wave period) grows in time due to ionization by an external source, e.g., laser pulse, is considered. Unlike previous papers on the frequency upshifting of electromagnetic radiation in a time-varying plasma, which treated mainly the cases of unbounded plasma or thick plasma slab, a novel mechanism of frequency upshifting is presented. It is connected with the excitation of free oscillations in the plasma layer at the moment when the plasma density passes the critical value and, thus, plasma resonance in the layer occurs. The frequency of the excited plasma oscillations increases in time following adiabatically the growing plasma frequency, whereas the oscillations themselves gradually radiate. Due to this mechanism, the upshifted electromagnetic radiation with the frequency equal to the maximum value of the plasma frequency in the layer can be effectively generated. Concrete estimations of the effect for a real experimental setup are given.

Plasma oscillations in two-dimensional semiconductor superstructures in the presence of a high electric field

The effect of a high constant electric field on plasma oscillations in the two-dimensional electron gas of a semiconductor with a superstructure was studied taking into account the Umklapp processes. A dispersion relation for the plasma-oscillation frequency ω(k) was derived for high temperatures (≪, where Δ is the conduction miniband width and T is the temperature in energy units). The plasmon frequency in a high electric field is shown to oscillate as the electric field strength and the wave number k vary. For arbitrary values of k, the spectrum is periodic with a period of 2π/d. Numerical estimates show that the oscillations may manifest themselves at electric field strengths higher than 3×103 V/cm.

Electromagnetic instabilities in unmagnetized plasmas

It is shown that local perturbations in nonuniform unmagnetized plasmas can give rise to linearly growing electromagnetic fields on both electron and ion time scales. The plasma vorticity and compressibility couple due to density inhomogeneity, giving rise to instabilities with partially transverse and partially longitudinal characteristics in electron plasmas for omega(pe)</=omega and in electron-ion plasmas for omega(pe)</=omega as well as omega<<omega(pe) (where omega(pe) is the electron plasma oscillation frequency). It is reconfirmed that the pure transverse modes due to the thermoelectric term do not appear in nonuniform unmagnetized electron plasmas. Furthermore, it has been found that the thermal fluctuations in a collisionless inhomogeneous electron plasma happen on a slower time scale of the order of 1/c(s)k (where c(s) is the ion sound speed). It seems that in the presence of a steep density gradient the ion acoustic wave becomes electromagnetic. Since the curl of electric field becomes nonvanishing in the presence of a density gradient, any nonuniform plasma can have magnetic field fluctuations in the limit omega<<omega(pe) as well. It is suggested that in the limit omega<<omega(pe) the ion dynamics becomes important and a pure electron plasma model to study magnetic field instability is not useful. The estimate for the magnitude of slowly and rapidly growing magnetic fields using the electron-ion plasma model in a special range of parameters turns out to be of the order of megagauss, in good agreement with the experimental observations.

プラズマ振動の励起による電子密度の磁気プローブ計測

Electron density in nitrogen glow discharge plasma was determined by measuring frequency of plasma oscillation. Plasma oscillation was excited in the glow discharge by using internal cathode placed close to anode. Signal intensity of plasma oscillation was detected by magnetic probe, i.e. magnetic pick-up coil connected to spectrum analyzer.Spectrum due to the plasma oscillation was clearly observed, and the frequency shifted according to the change of discharge parameters. The electron density measured by the present method was compared with the data of electro-static probe. The present data well agreed with that of the electro-static probe.

Theory of beam-plasma instability in a periodic plasma-filled waveguide.

The beam-plasma wave interaction in a periodic plasma-filled waveguide is treated in a mathematically correct manner on the basis of the integral equation (IE) method. It has been shown that the relevant boundary-value problem could be reduced to an IE with a singular kernel for the longitudinal component of the electric field on the waveguide axis. The regularization of the IE was performed by extracting the static part of the kernel. The resulting IE of the second kind with a regular kernel, being rather convenient for a numerical analysis, is treated in a quasistatic approximation as a spectral problem. First-order expressions for eigenfunctions, and an infinite set of dispersion relations linking a wave number and frequency of plasma oscillations which separate radial branches of plasma oscillations from axial ones, have been obtained in the closed analytical form, thus enabling us to avoid the problem with the so-called "dense" spectrum. The solutions of the relevant "cold" dispersion relations establish a periodical dependence of the frequency on the wave number over several periods within the accuracy of order of the neglected terms. In the presence of an electron beam they turn out to be unstable near frequencies providing the resonances of the beam with spatial plasma harmonics. Evaluations of the instability saturation level predict a more efficient beam-plasma wave energy transfer compared with those following from a conventional theoretical analysis based on the formulation of a dispersion relation in terms of an infinite determinant, with following truncation of the latter to the finite sized relation.

Thermalization of an intense, space-charge-dominated electron beam in a long focusing channel.

A continuous electron beam with a correlated emittance will eventually thermalize. Initially, the beam emittance for an intense, high-brightness, space-charge-dominated beam will oscillate, but after a sufficiently long time, it will reach an equilibrium value. The emittance oscillations are due to coherent transverse plasma oscillations in the beam and are a manifestation of periodic energy exchange between potential and kinetic energies. The beam eventually attains an equilibrium emittance, as the beam equipartitions the kinetic and potential energies. This equipartioning is reached as the beam thermalizes due to a form of Landau damping of the radial oscillations at different radial positions within the beam. Slight differences in the transverse plasma oscillation frequency for different radial positions lead to incoherence in the oscillations. In this paper, we calculate the equilibrium time scales required for equipartioning. We show that the equilibrium emittance scalings and magnitude can be predicted by conservation of energy considerations. In addition, we show that, in the space-charge dominated regime, there is a correspondence between the energy-conservation approach and the kinematic approach.

A hybrid semi-Lagrangian Vlasov code. II. Numerical simulation of chirped beatwave experiment

In this paper the results are reported of a set of numerical simulations applicable to the problem of particle acceleration by the beating of very high-frequency laser waves (the ratio of driver frequency to plasma frequency close to ω0/ωp=30), including for the first time beat frequency chirping (i.e., pump frequency linearly decreasing with time) at various rates in order to counteract relativistic electron detuning of the plasma oscillation frequency and the resulting Rosenbluth–Liu amplitude limit. This amplitude limit can indeed be evaded, giving extremely high acceleration gradients of more than 25 GeV/m, for the case of numerical parameters relevant to the UCLA experiment (University of California at Los Angeles). A chirp rate that is too fast leads to a sudden loss of frequency lock and a subsequent decay of the driven electric field, in agreement with the behavior of a similarly driven relativistic Lagrangian Rosenbluth–Liu oscillator. Even at chirp rates somewhat below this limiting value a remarkably localized and extraordinarily intense (but as yet ill-understood) electric field structure is formed, one that is unlikely to be compatible with the general requirement for smooth and coherent acceleration.

Parametric excitation of high frequency plasma oscillations in the ionosphere by a powerful extraordinary radio wave

The generation process of short wavelength (upper-hybrid and electron cyclotron) plasma oscillations resulting from induced scattering of a high power extraordinary radio wave by ions is studied. The threshold field E thr determined by collisional damping of the excited waves is calculated. The minimum value E thr is shown to be ≈ 1 V/m. This value is reached near the reflection level of the extraordinary wave when its frequency is situated between the electron gyroharmonics nω Be . Such a value can be easily exceeded in the heating experiments due to Airy swelling of the pump wave near the reflection level. In the cases nω Be − ω 0 ⪡ ω Be and ω 0 < 2 ω Be the instability quenches because of the absence of plasma oscillations with frequencies close to the pump wave frequency.

Influence of doping on the electronic and magnetic characteristics of the CuO2 plane

The influence of doping on the band structure, the position of the Fermi level, the plasma oscillation frequency, and the magnetic properties of a CuO2 layer is investigated.

Comment on the thermoelectric term

It is shown that the thermoelectric term which appears as a source of fluctuating magnetic field B1 in unmagnetized nonuniform plasmas in the limit ωpe≪ω≪ωpe [where ωpi(ωpe) are ion (electron) plasma oscillation frequencies, respectively] is of smaller order. Therefore, the thermoelectric term does not seem to contribute to magnetic fluctuations linearly. The theory of magnetic field generation based on ion dynamics predicts the same order of magnitude of B1 which is experimentally observed.