Quasi-Steady Laser Oscillation in the Recombining Hydrogen Plasma

Abstract

Effects on reflectometer phaseφ and powerP fluctuation signals due to (a) asymmetries in the transmit-plasma-receive antenna geometry (misalignments), and (b) asymmetries in the plasma cut-off layer perturbations (distortions from sinusoidal or Gaussian) are studied using a two-dimensional (2D) physical optics model. Results show the onset of phase runaway with antenna misalignment and/or sawtooth type perturbations when the perturbation amplitude exceeds some critical value. For broadband (Gaussian) turbulence antenna misalignment leads to Doppler shifts in theφ spectrum-provided the reflectometer beam width w and spectral width k w of the turbulence are sufficiently large. Misalignment also generates coherence betweenφ andP at Bragg backscatter frequencies with quadrature (±π/2) phase difference. Sawtooth (asymmetric gradient) perturbations also generate phase-power coherence in quadrature, but at frequencies determined by w and k w , i.e. not Bragg. Cusped or spiky (asymmetric amplitude) perturbations generate asymmetric phase distributions (nonlinear phase offsets) and low-frequency phase-power coherence with 0 or π phase difference. The simulations indicate that a combination of antenna misalignment (or plasma tilt) and cusped reflection layer perturbations can account for a wide range of experimentally reported features in reflectometer signals. † On leave

An experimental study of stationary population inversion in a cold recombining expanding helium plasma jet

“…which is essential in laser operation, and Abel inversion is not applied in figure 3. The axial distance from the nozzle z is (U) 8 cm, (6) 15 cm, (c) 22 cm and ( d ) 29 cm. These distances correspond to the gap positions of the coreless coils.…”

The Behaviour of Population in a Plasma Interacting with an Atomic Gas

“…The quasi-steady state assumption is made for all levels / > /'*. The level /* is determined so as to satisfy the following two conditions: (a) i* ^ 5. and (b) the relaxation time for n, with i ^ /* + 1 is less than one hundredth of an apparent relaxation time for the electron energy estimated from (3), that is, 1.5 ne Te/{-Re-Qe + 1.5 Fe dne/d t) . Integration of the equations was performed by using the Runge-Kutta-Gill method.…”

“…ions in recombining plasmas and laser oscillations based on the population inversions: Hoffman and Bohn [12] observed population inversion between levels with principal quantum numbers j = 3 and 5 of the hydrogen atom during the plasma recombina tion phase in an expansion-flow-system. The laser oscillations were reported to occur in the expanding plasmas for an Arlll line (Campbell et al [2]), and also for the Fx line of the hydrogen atom (Hara et al [3]). n the other hand.…”

“…The quasi-steady state (QSS) assumption for all excited levels has been widely used in most theo retical investigations on population inversion and laser oscillation [2][3][4][5][6], The mechanism generating the population inversion between the lower lying levels in QSS is explained as follows: The upper one of the two levels of interest is populated mainly due to cascade collisional decay from the higher excited levels where the electrons are captured by the threebody recombination, and the lower one is signifi cantly depopulated by the radiative deexcitation.…”

Numerical Investigation of Laser Oscillation in a Recombining Hydrogen Plasma Interacting with Helium Gas in TPD-I

“…In general, there are three different experimental approaches to produce the recombining plasmas: (i) a short-pulsed high power laser beam is injected into a gas to produce a hot plasma, which is then cooled by radiation loss [1], (ii) a dense and hot plasma produced by arc discharge [2] is freely expanded in a vacuum vessel, (iii) a stationary plasma is made to interact with a cool and dense neutral gas [3]. The third one is superior to the two others in the following two points: A stationary population inversion is produced, and the plasma parameters are easily controlled.…”