Noboru Shimomura

COLLISION-INDUCED INSTABILITY IN A MAGNETOACTIVE PLASMA

The dispersion relations for the waves propagating in a magnetoactive plasma are obtained from the Boltzmann equation with a collision term of the electrons. Two cases in which the wave vector parallel and perpendicular to the uniform magnetic field are studied. Under the assumption that the unperturbed distribution is a spherical shell in velocity space in each case, and two dispersion relations to the zeroth approximation in k are obtained as follows:

Noise Spectroscopy of K Atoms with a Diode Laser

There has been much interest in the influence of the laser noise on the atomic system. So far theoretical works have been performed on calculation of the mean values and the variance of the fluctuated atomic populations, and verified through observation of the fluctuating resonance fluorescence from the atomic vapors. Yabuzaki et al. have observed the excess intensity noise of the resonant light from a diode laser propagated through alkali atomic vapor. They have measured the intensity power spectrum of the light when the laser was tuned near the D2 line of Cs and observed all corresponding hyperfine-splitting resonances in the excited state and Zeeman splitting resonances within a hyperfine level of the ground state. Walser et al. have theoretically studied the intensity fluctuation of laser light propagated through a weakly absorbing medium. The calculated power spectrum of a phase-diffusing field has been found qualitative agreement with the experimental results by Yabuzaki et al. on hyperfine-splitting resonances. Recently, Mitsui has reported a new type of noise spectroscopy with a highly stabilized diode laser. He has investigated the spontaneous noise appeared in optical detection of magnetic resonance of Rb atoms. In this note we report the first observation of hyperfine spectra in the ground state of K and K appearing as the intensity fluctuation at hyperfine splittings of 462MHz and 254MHz of circularly polarized light from a diode laser transmitted through a sample cell. We have also observed the Zeeman hyperfine spectra of K in a static magnetic field. We show that the spectra are strongly dependent of the direction of the static magnetic field. Their splittings as functions of the strength of static magnetic field are compared with the Breit–Rabi formula. The experimental apparatus is quite simple as shown in Fig. 1. The diode laser used was an ordinary Fabry–Perot type with output power of 5mW, operating near the D1 line (769.9 nm) of K atoms. The output of the laser was circularly polarized and applied to a cell containing K vapor at a temperature 90 C, corresponding atomic density to be about 1:15 10 cm 3. The transmitted light was detected by an avalanche photo-diode having frequency response up to 1GHz. The output of the photo-diode was applied to a rf frequency analyzer to obtain the power spectrum of the fluctuation of the transmitted light intensity. A static magnetic field is provided by two pairs of Helmholtz coils mutually perpendicular and about 90 cm in diameter. The strength of the magnetic field is calibrated by using magnetic resonance signals of optically pumped Rb atoms. Figure 2 shows the frequency spectrum of the intensity fluctuation of transmitted light observed when the laser was tuned to the transition from the state F 1⁄4 1 in the ground state to the state F0 1⁄4 1 in the excited state by adjusting the driving current of the laser diode. Sharp resonance signals can be seen at the hyperfine splittings of K and K, 462 and 254MHz, respectively, on the background of broad noise spectrum. When a weak static magnetic field H0 is applied to the K vapor, the hyperfine spectra of K splits into several lines (the effective gyro-magnetic ratio of K is known to be 1⁄4 2 700 kHz/G). Figures 3(a) and 3(b) show the observed Zeeman hyperfine spectra of K for

Transfer of Nutating Spin Polarizations between Rb and Cs Atoms

The transfer of nutating spin polarizations between 85 Rb and 133 Cs atoms is observed for the first time. Maximum transfer occurs when the nutation frequencies are made equal in the rotating frames. The nutating spin polarization of 85 Rb atoms is generated by the periodic excitation of the pumping light and the spin polarization transferred to 133 Cs is observed through the Faraday rotation of a probe beam. The observed time evolutions of the spin polarizations are found to be in good agreement with the results of theoretical calculations based on the Bloch type equation of motion including the effects of optical pumping and spin exchange collisions.

INSTABILITIES OF EXTRAORDINARY WAVE IN A COLLISION-DOMINATED PLASMA.

The instabilities of the extraordinary wave are studied by taking the effect of finite wave number into account. It is assumed that the unperturbed velocity distribution of electrons is a spherical shell in velocity space and the electrons collision frequency ν with neutral particles depends on the magnitude of their velocities v 0 , namely, ν∝ v 0 h . The dispersion relation is obtained for the case in which the refractive index is much larger than unity. The dispersion curves of the wave for \(h{\neweq}0\) show that the occurrence of instabilities may be expected at both sides near the cyclotron frequency and the higher harmonics. The conditions for onset of the collision-induced instability at the cyclotron frequency and the second harmonic are given by h >3 and h >5 respectively in the long wavelength limit. When the wave number and the electron density are chosen suitably, the conditions are modified as h >2.6 and h >3.7.

Collision-Induced Instability at Cyclotron Harmonics

The collision-induced instability at the cyclotron harmonics of the ordinary wave is studied under an assumption ν( V )∝ V h , where ν is the electron collision frequency with neutral particles and V the electron velocity, respectively. When the initial distribution of the electron velocities is a spherical shell in velocity space, it is found that the large values of µ=(Larmor radius)×(wave number) relax the restriction of h which leads to the instability and the condition h =3 is sufficient to generate the instability at the higher cyclotron harmonics if µ is chosen suitably. Furthermore, the growth rate increases with the electron density for given ν and h , and it attains the maximum at \(\omega_{p}{\gtrapprox}\omega_{c}\), where ω p and ω c are the plasma and the cyclotron frequencies, respectively.

Transfer of precessing spin polarizations between different kinds of atoms with same g-factor

In spin-related fundamental researches, there have been considerable interests in the transfer of microscopic orders between magnetic sublevels of different kinds of atoms. As for gaseous atomic systems, collisional transfer of spin polarization from optically pumped atoms A to atoms B is attained through spin exchange collisions. So far many experiments have been reported on the transfer of longitudinal spin polarization created along a static magnetic field. Transfer of coherence, on the other hand,is possible only when the energy matching condition j!A !Bj < is satisfied, where !A and !B are the Larmor frequencies of atoms A and B, and the line width. This is the reason why transfer of spin polarization precessing around the static magnetic field has not been observed except for a few experiments; The coherence transfer has been observed in the experiments where the atoms are subjected to properly chosen rf magnetic fields. Recently, it has been reported that coherence transfer between different kinds of atoms is possible by using the light field which transversely pumps only one kind of atoms under properly chosen experimetal conditions. It has also been pointed out that, as for the system of Rb and K atoms, which happen to have the same gyro-magnetic ratio, the peculier distortion of the magnetic resonance signal line shapes observed in the collisionally pumped atoms can be explained by the spin exchange coupling between the resonant Rb and K atoms. It is suggested that the precessing spin polarization in Rb can be transferred to K atoms or vice versa, since their Zeeman splittings become equal in the hyperfine multiplets jF 1⁄4 1i and jF 1⁄4 2i of the ground state 2S1=2 with the same gyromagnetic ratio 1⁄4 2 700 kHz/G. The experimental apparatus of the present experiment is shematically shown in Fig. 1. In addition to a droplet of natural rubidium and potassium, the sample cell contains 50mbar Ar as a buffer gas. It is subjected to a static magnetic field of H0 1⁄4 0:786G along the z axis and irradiated transversely to the field by a light beam from a GaAlAs laser A (output power = 10mW, beam diameter = 1mm). It is left-handed circularly polarized and tuned to the Rb absorption lines (S1=2, F 1⁄4 1 ! P1=2, F 1⁄4 1 and 2) which are not resolved owing to the Doppler broadening. The intensity of the laser A is modulated by an AOM (acousto-optic-modulator) at the Larmor frequency !A 1⁄4 2 550 (1⁄4 700 0:786) kHz of Rb in a rectangular shape pulse. As mentioned above the precessing spin polarization in Rb can be transferred to K. The precessing spin polarization of Rb and that of K are detected from the Faraday rotations of the beams of probe lasers B ( 1⁄4 794 nm, output power = 3mW, beam diameter = 2mm) and C ( 1⁄4 769 nm, output power = 3mW, beam diameter = 2mm), respectively. The frequency of laser B is chosen to be a few GHz higher than the Rb absorption lines (S1=2, F 1⁄4 1 ! P1=2, F 1⁄4 1 and 2) to prevent the detection beam from destroying the polarization. Similarly, the frequency of laser C is chosen to be a few GHz higher than the K absorption lines (S1=2, F 1⁄4 1 and 2 ! P1=2, F 1⁄4 1 and 2) which are not resolved owing to the Doppler broadening. They are linearly polarized and counter-propagated against the pumping beam,making angles of about 1 to the pumping beam. In each case the transmitted light beam of probe laser passes through the analyzer which makes an angle 45 to the polarization of the incident beam,and then it is detected by a pin-photo diode. The signals are electronically averaged over a few hundred cycles. Figure 2(a) shows the typical examples of signals on the magnetizations M x and M B x of Rb and K, respectively, Fig. 1. Schematic diagram of the experimetal apparatus. AOM, acoustooptic modulator; PD, pin-photo diode; P, polarizer; A, analyzer.

Breakdown of He-Gas by Alternating Electric Field near Ion Cyclotron Frequency

The effect of a magnetic mirror field and ion motion in a high frequency gas discharge are investigated experimentally. The breakdown voltage in the mirror field is about ten percent less than that in uniform field. For both the uniform and the mirror magnetic fields, the minimum voltage of breakdown are observed at ωci/ω1, where ωci and ω are the frequencies of the ion cyclotron and the applied alternating electric field, respectively. These experimental results can be explained by a theory describing energy gain by ions and reduction in the number of electrons by diffusion.

MICROWAVE EMISSION FROM A MAXWELLIAN MAGNETOPLASMA.

The microwave emission from a Maxwellian magnetoplasma in a uniform magnetic field is studied near electron cyclotron harmonic frequencies. Following the dressed test particle approach, the energy lost by one test electron is calculated and, from this, the total emission is obtained by integrating over all electrons weighted by their velocity distribution. The dispersion relation for the electrostatic wave propagating in the direction nearly perpendicular to a magnetic field has, in general, two solutions, comprising a slow and a fast electrostatic wave. It is found on the CMA diagram that the emission by slow electrons whose velocity is nearly the thermal velocity has a peak near lower harmonic side and the shift of the peak from the harmonic scarcely depends on electron density, while the emission by fast superthermal electrons has no peak and the position of the emission by electrons with the specified high velocity moves from the lower harmonic side to the higher side with increase of the density.