Extraordinary-mode radiation produced by linear-mode conversion of langmuir waves

Linear-mode conversion (LMC) of Langmuir waves to radiation near the plasma frequency at density gradients is important for space and astrophysical phenomena. We study LMC in warm magnetized plasmas using numerical electron fluid simulations when the density gradient is parallel to the ambient magnetic field (B0). We demonstrate that LMC can produce extraordinary- (x-) as well as ordinary- (o-) mode radiation from Langmuir waves, contrary to earlier expectations of o mode only. Equal amounts of o- and x-mode radiation are produced in the unmagnetized limit. The x-mode efficiency decreases as B0 increases, while the o-mode efficiency oscillates due to interference between incoming and reflected Langmuir waves. Both x and o modes should be produced for typical coronal and interplanetary parameters, alleviating the depolarization problem for type III solar radio bursts.     

First direct observation of the decay of neutral charmed particles produced by neutrinos in emulsion

Three examples of the production in emulsion by neutrinos and subsequent decay of neutral short-lived particles are reported. The flight times are ≈ 10^?13s, similar in order of magnitude to those previously determined for charged charmed particles in the same experiment.     

Saturation of the acoustic waves produced by pulsed arcs

Rapid simultaneous measurements of the arc current and voltage and the emitted acoustic wave were made on a small 20 A DC arc in open air, which was subjected to current pulses ranging in amplitude from 0·4 kA to 40 kA and in rise time from 10^?4 s to 10^?7 s. The results show that for a rise time Δt < 10^?5s, the amplitude of the pressure wave generated by the arc no longer varies with Δt but remains constant for a given amplitude of the current pulse. An additional study shows that this saturation is accompanied by a modification in the electrical behaviour of the arc, which suggests that the observed saturation is due to the arc itself and not to absorption of the pressure wave in the surrounding air.     

Stabilization of the ion acoustic instability by high frequency oscillations

The dielectric function for long wavelength, low frequency ion waves in the presence of short wavelength, high frequency electron oscillations is presented. It is shown that high frequency oscillations may stabilize a linearly unstable double-humped ion distribution function.     

First laboratory observation of freely propagating electrostatic waves produced by intense, localized field structures

Freely propagating electrostatic waves (free modes) are observed to be produced by intense, localized field structures in laboratory measurements performed in an unmagnetized plasma in a state of strong Langmuir turbulence. The freely traveling waves conform to the Langmuir dispersion relation and have an average frequency which is approximately 11% higher than the average frequency of the localized field structures. The observations are consistent with the predictions of strong Langmuir turbulence theory.     

A search for the location of the HF excitation of enhanced ion acoustic and langmuir waves with eiscat and the tromsø heater

In an effort to understand the mechanisms which give rise to the enhanced ion acoustic and Langmuir waves in HF modification experiments, we perform measurements with the EISCAT Tromsø heater and the two incoherent scatter radars to locate the enhancement regions in space. Simultaneous measurements of the enhancements were made with both the VHF (224 MHz) and the UHF (933 MHz) radars, the latter being scanned in the magnetic meridian plane between vertical and field aligned through theSpitze angle. The results show that enhanced bottomside and topside ion acoustic and plasma waves at UHF and VHF are a common phenomena in these data. UHF topside enhanced plasma waves were not observed, possibly due to lack of radar sensitivity. At UHF the enhanced ion waves were strongest when the radar was pointed between theSpitze angle and geomagnetic field aligned direction. The topside features we believe give evidence for the production and propagation of Z-mode waves during this experiment. There is no evidence in these data for the decay of Z-mode waves into Bernstein and lower hybrid waves inside the overdense F layer as we believe to have observed previously.     

Thomson-scattering study of the subharmonic decay of ion-acoustic waves driven by the Brillouin instability

Thomson scattering (TS) has been used to investigate the two-ion decay instability of ion acoustic waves generated by stimulated Brillouin scattering in an underdense CH plasma. Two complementary TS diagnostics, spectrally and spatially resolved, demonstrate the occurrence of the subharmonic decay of the primary ion acoustic wave into two secondary waves. The study of the laser intensity dependence shows that the secondary ion acoustic waves are correlated with the SBS reflectivity saturation, at a level of a few percent.     

On collisionless damping of ion acoustic waves

Exact theoretical treatments show that the damping of ion acoustic waves in collisionless plasmas does not vanish when the derivative of the undisturbed distribution function at the phase velocity equals zero.     

Increase of the backward Raman reflectivity caused by the langmuir decay instability in an inhomogeneous plasma: the loss of gradient stabilization

We investigate the nonlinear evolution of the backward stimulated Raman scattering (BSRS) in the regime where the nonlinear saturation mechanism is the Langmuir decay instability resulting from the coupling of the BSRS-generated Langmuir wave with the ion acoustic waves. We present numerical results obtained with a fluid-type code in one- and two-dimensional spatial dimensions, in the case of an inhomogeneous plasma. The plasma density is under quarter-critical and depends linearly on the longitudinal spatial coordinate, in the regime where the Rosenbluth gain factor for the amplitude, denoted as G(Ros), is in the range pi/2< or =G_(Ros)< or =6. We observe that the Langmuir decay instability is able to suppress the gradient stabilization and restore the absolute nature of BSRS, thus leading to a significantly increased BSRS reflectivity.     

Effect of ionic temperature on the modulational instability of ion acoustic waves in the presence of a magnetic field

The modulational instability of ion acoustic waves is studied in the presence of a dc magnetic field, taking the ion temperature into account. It is well known that the instability sets in for wave numbers exceeding 1.47 k_D when there is no magnetic field and the ion temperature is negligible. The instability behaviour, however, changes drastically when either the magnetic field is switched on or the ion temperature becomes important or both. In general three different regions emerge wherein the waves becomes modulationally unstable. The relative sizes of these regions change as the magnetic field, the angle of propagation and the ion temperature are varied.     

First observation of 54Zn and its decay by two-proton emission

The nucleus 54Zn has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasifragmentation of a 58Ni beam at 74.5 MeV/nucleon in a (nat)Ni target. The fragments were analyzed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight 54Zn implantation events were observed. From the correlated decay events, the half-life of 54Zn is determined to be 3.2(+1.8)(-0.8) ms. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial half-life are compared to model predictions and allow for a test of these two-proton decay models.     

Characteristic properties of ion acoustic harmonics produced by stimulated brillouin scattering

The theory of ion acoustic harmonic production with stimulated Brillouin scattering is extended up to the third harmonic. Stationary spatial solutions are given for normalized ion acoustic wave amplitudes and characteristic properties derived, which may serve experimentalists for future detailed investigations.     

Electron current driven ion acoustic standing wave instability

By means of a positively biased grid an electron current driven instability is excited in a single ended Q-machine. The instability shows the behaviour of a half standing ion acoustic wave with nodes at the hot plate and the grid. For high grid biases an increased plasma temperature is found.     

Radiation of ion acoustic waves in a dispersive positiveion-negative ion plasma

Experimental results that illustrate some properties of the radiation of a longitudinal ion acoustic wave launched from a solid metal disk antenna inserted in a dispersive positive-ion-negative-ion plasma are presented. The negative ions replace the free electrons in the plasma and increase the electron Debye length, hence increasing the dispersion of the plasma. It is observed that the radiation of waves in a dispersive media is significantly more complicated than in a nondispersive media. It is not possible to draw one universal radiation pattern for the radiation of the waves in this case, since so many frequency components are present in the wave and they change as the wave evolves     

First observation of the decay Ds+-->pn

Using e+e--->Ds*-Ds+ data collected near the peak Ds production energy, Ecm=4170 MeV, with the CLEO-c detector, we present the first observation of the decay Ds+-->pn. We measure a branching fraction B(Ds+-->pn)=(1.30+/-0.36(-0.16)+0.12)x10(-3). This is the first observation of a charmed meson decaying into a baryon-antibaryon final state.     

Trapped electron in ion acoustic waves and solitons

Experimental evidence of trapped electrons in ions acoustic nonlinear waves and solitons propagating in a stable plasma is reported. Agreement between theory and experiment was found for the dependency of the energy of the trapped electrons versus wave amplitude.