Physical principles of polarization-spectroscopic measurement of electric and magnetic fields in ionized gases

Abstract

Spectroscopic methods of measuring the parameters of a plasma play a key role in the problem of monitoring the physical processes occuring in a plasma medium. These methods are widely employed for plasma diagnostics in laboratory apparatus (used to solve problems in the area of controlled thermonuclear synthesis, plasma electronics, and plasma technology), and also to investigate plasma in astrophysical systems (for example, in the region of solar flares). One of the advantages of spectroscopic methods is the fact that they have no disturbing effect on the course of the plasma process.Polarization spectroscopy is becoming more and more widely used at the present time to investigate plasmas. All forms of polarization of the radiation of atomic particles are brought about by a disturbance of the isotropy of the plasma medium. The polarization of the radiation may be due, first, to the presence in the plasma of anisotropically distributed electric or magnetic fields, and second, to the anisotropy of the resonant optical or electronic excitation of the atoms. Hence, polarization-spectroscopic methods of diagnostics are employed to determine the parameters of an anisotropic plasma. In the reviews given in [i, 2] a detailed description is given of polarization-spectroscopic methods of measuring plasma parameters, which affect the anisotropy of the processes by which the atoms are excited. Below we consider the main polarization-spectroscopic methods of measuring the parameters of anisotropic electric and magnetic fields in a plasma. The anisotropic electric fields in a plasma may be due to the presence of natural oscillations of the plasma (for example, Langmuir or ion-acoustic oscillations), to the penetration into the plasma from outside of electromagnetic radiation (for example, laser or microwave radiation), and the presence of a space charge (for example, in the cathode layer of a glow discharge). Anisotropic magnetic fields in a plasma in many situations serve for magnetic containment of the plasma.Polarization Spectroscopy of Hydrogen-Like Atoms in an Electric Field. Since atoms of hydrogen (or deuterium) in their excited states have constant dipole moments, their spectra are extremely sensitive to the influence of electric fields. We will first consider the situation when an electric field, oscillating with a frequency m, and having elliptic polarization sft) =so(ezcoscot+~s e~slno~t),acts on a hydrogen atom in a plasma, where s o is the field amplitude, ~ is the degree of ellipticity (0 <_ $ <_ i), and the number s takes two values (s = +i) depending on the direction of rotation of the vector ~(t). Here and henceforth Sx, + ey, and ~z are unit vectors along the x, y, and z axes.In the limiting cases when ~ = 0 and ~ = i, Eq. (i) describes a linearly polarized and a circularly polarized electric field. The modification of the hydrogen radiation spectrum due to the effect of an electric field (i) has been investigated in a large number of publications. In particular, in [3-6] a linearly polarized electric field ...

Subnanosecond ellipticity detector for laser radiation

“…Sensitive terahertz detectors of the type shown here, besides other applications, can be of particular interest for the control of current density profiles which is important for tokamak operation. [4][5][6][7][8] We also note that the CPGE and LPGE are also observed at valence-toconduction band transitions, [9][10][11] at direct intersubband transitions, 1 and in wide band GaN semiconductor heterojunctions. 12 Thus, the applicability of the CPGE/LPGE detection scheme may well be extended into the visible, the near-infrared, and in the midinfrared spectral ranges.…”

Picosecond Polarisation Detector for Infrared and Terahertz Radiation

“…The access to radiation helicity describing the rotation of light polarization vector is obtained applying the circular photogalvanic effect (CPGE) in quantum wells (QWs) [1,2]. Besides other applications, THz ellipsometry can be of particular interest for control of current density profiles which is important for tokamak operation [3,4]. Because the sensitivity of both detector elements varies with the polarization state of radiation, a third detector is applied to measure intensity.…”