444 research outputs found

### Pulse-stimulated radiation from a plasma due to an energy-dependent gyrofrequency

A plasma excited by two short pulses at the electron gyrofrequency which have a time separation Ï„, is considered in the single particle approach. It is shown that the relativistic mass effect can lead to a series of radiation maxima after the second pulse. In the case of a cold plasma in an inhomogeneous magnetic field these maxima arise at multiples of the time Ï„; in the case of a warm plasma in a homogeneous magnetic field at multiples of Ï„/|1 Â± D|, where D is the strength of the second pulse relative to the first one. The shape of the radiation maxima is given by the square of the Fourier transform of the distribution of the inhomogeneities or the initial energies, respectively. The two effects have the tendency to cancel each other. (i) If the plasma is excited by three pulses, the time separation of the second and third pulse being T, radiation maxima occur at times t = KÏ„ + LT, (Â±K, L = 0, 1, 2,... but t > 0) after the third pulse in the case of cold plasma with field inhomogeneities, and at t = (KÏ„ + LT)/|1 Â± D Â± D_2| in the case of a warm plasma. (ii) If collisions are taken into account the dependence on T of the radiation maxima with L = 0 is determined by inelastic collisions only, while the other decay times are determined by all kinds of collisions

### On the Theory of Pulse Stimulated Radiation from Plasma

By including the relativistic mass change in the motion of electrons gyrating in a slightly inhomogeneous field, it is possible to account for the cyclotron echoes observed by Hill and Kaplan

### The Gunn-Peterson effect and the Lyman alpha forest

We show that spatial correlations in a stochastic large scale velocity field
in an otherwise smooth intergalactic medium (homogeneous comoving density)
superposed on the general Hubble flow, may cause a `line-like' structure in QSO
spectra similar to the population of unsaturated Ly-alpha forest lines which
usually are attributed to individual clouds with 10^{11} <= N(HI) <= 5*10^{13}
cm^{-2}. Therefore there is no clear observational distinction between a
diffuse intergalactic medium and discrete intergalactic clouds. It follows that
the HI-density in the diffuse intergalactic medium might be substantially
underestimated if it is determined from the observed intensity distribution
near the apparent continuum in high resolution spectra of QSOs. Our tentative
estimate implies a diffuse neutral hydrogen opacity tau_{GP} = 0.3 at z = 3 and
a current baryon density Omega_{IGM} = 0.08$, assuming a Hubble constant H = 70
km s^{-1} Mpc^{-1}.Comment: 9 pages, 3 Postscript figures, a MNRAS Letters submissio

### Pulse Stimulated Radiation from a Plasma Involving the Second Harmonic of the Gyrofrequency

If a plasma is excited by two short pulses at the gyrofrequency and its second harmonic, respectively, with a time separation Ï„, a radiation maximum is predicted to occur at the gyrofrequency a time Ï„ after the second pulse

### Pulse-Stimulated Radiation from a Plasma at Harmonics of the Gyrofrequency

A magneto-plasma excited by two short microwave pulses at the electron gyrofrequency or a multiple thereof, which have a time separation Ï„, is considered in the single particle approach. Due to non-linearities there arise after the second pulse, radiation maxima at the gyrofrequency and its harmonics. In a situation where the time separation of these maxima is Ï„* (in the simplest case Ï„* = Ï„) at the fundamental, it is Ï„*/n at the n-th harmonic.
As the resonances at the harmonics are a finite Larmor radius effect, excitation at these frequencies requires a non-zero initial energy. As an example, the case is studied where the first pulse is at the gyrofrequency and the second at the second harmonic. In this case radiation maxima at 2Ï‰_c occur at times t_â„“ = â„“Ï„ (â„“ = 1, 2, 3...) after the second pulse, at the fundamental, maxima arise at times t_â„“ = (2â„“ - 1)Ï„. If there is no non-linearity involved other than the non-linear driving force, during the second pulse there arises only one radiation maximum at t = Ï„

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