Numerical/theoretical study of Laser Light Propagation and Energy Deposition & thermal Transport in laser-produced plasms and Computational study of z-0 pinch Plasma

Abstract

During the last three years, our group has carried out research work in the field of plasma physics and controlled fusion. Our group has published or submitted 9 research papers. During the reported period (01-06-95 to 31-05-98), our activities were concentrated on the following topics: 1. Numerical study of z-θ pinch 2. Study of microturbulence/ energy loss of energetic alpha parties in thermonuclear fusion/ dusty plasmas 3. Thermomagnetic instability 4. Nonlinear waves in nonuniform magnetoplasma A brief description of the research work done during the report is as follows: 1. Numerical study of Z-θ pinch: Z-pinch is one of the earlier approaches to controlled fusion, where the high-power discharge current, applied through a capacitor bank or pulsed power generator, play both the roles of heating and confinement. Plasma is heated by joule heating and confined through the self-generated magnetic field. Earlier low-density pinches driven by low voltages were found highly unstable against the MHD instabilities, namely the sausage (m=0) and kink (m=1) modes. These instabilities disturb the discharges, long before the desired density and temperature could be reached. Therefore, the consideration of Z- pinch though simple and low-cost fusion system was abandoned in favor of more complex configurations. Staged pinch devices have attracted much interest during the las couple of years since they offer the possibility to achieve plasmas of thermonuclear conditions by rather modest means. This staged pinch configuration is found to be much more stable than the ordinary Z-pinch device. To achieve optimum plasma parameters, in the context of controlled fusion or x-ray lasers, the stability of the pinching plasma is vital. Thick gas-puff implosion seems practically suitable to suppress short wavelength perturbations causing hydrodynamical instabilities in staged pinch devices. In this regard the snow-plow effect which we have incorporated in our model, will provide a strong influence on the onset of instabilities. The perturbation whose wavelength is of the order of the sheath thickness will never grow due to the increasing current sheath thickness. Another advantage of the snow-plow effect is the peaking of implosion before the current saturates. This provides stabilization and will help in coupling of the driver energy to the target. In few experiments the switching of the main discharge current to the target have been observed. In such a situation, to ensure efficient coupling of the discharge current and energy of driver to the target, the implosion process of the driver needs to be completed within the time before the current saturates. The proposed spinning scheme of the gas-puff plasma in staged pinch, although minimizes the initial perturbations (causing Rayleigh-Taylor instability), seems doubtful to produce plasma parameters suitable for controlled fusion. Some other measures ae needed to enhance the compression. We have proposed seeding of the target with high-Z impurity to initiate the radiative collapse of the pinching plasma column. This work has been published. The reference are as follows: i. Thermonuclear fusion with a spinning gas-puff staged pinch Plasma phys. Control. Fusion 38, 847 (1996) (Arshad M. Mirza, N.A.D. Khattak, M. Salahuddin and G. Murtaza) ii. Radiative collapse in an impurity seeded spinning gas-puff staged J. Plasma Physics (1998) in press (Arshad M. Mirza, Z. Ahmad, N.A.D. Khattak and G. Murtaza)

2. Study of microturbulence/ energy loss of energetic alpha parties in thermonuclear fusion/ dusty plasmas There has been a great deal of interest in studying he stability of magnetically confined fusion plasmas in the presence of hot energetic alpha particles which are proceeded during the thermonuclear fusion process. The analytical and numerical results for the slowing down of two heavy projectile ions passing through a multi-component dusty plasma are presented. Within the linear dielectric approach, the electrostatic potential and the stopping power of the two projectiles are computed for different values of KD (the normalized effective wave number and R (the separation between the two projectiles) retaining two-ion-correlation effects. The enhancement in the energy loss is observed, and it is compared with that of a single ion projectile case. These results are useful to explain the crystallization of dust grains in astrophysical and laboratory plasmas. We have also studied the linear and nonlinear properties of electrostatic and electromagnetic waves in the presence of high-energy alpha particles in magnetically confined fusion plasmas. It is found that Boltzmannen alphas can introduce new classes of electrostatic and electromagnetic wave spectra. By employing a hybrid approach, in which electrons and ions are treated like fluids whereas hot alphas by means of a kinetic description, a set of nonlinear equations are derived. The stationary solution of the latter are discussed. The work has been accepted/published and the references are given below: • Energy loss of charged projectiles in dusty plasmas Physics of plasmas (1998) accepted for publication (M.H. Nasim, Arshad M. Mirza, M.S. Qaiser, G. Murtaza and P.K. Shukla) • Microturbulence in the preens of energetic alpha particles in fusion plasmas Physics scripta, 57, 261 (1998) (G. Murtaza, Arshad M. Mirza and P.K. Shukla)

3. Thermomagnetic instability There has been a great deal of interest in the problem of magnetic field generation in laser-produced as well as in tokamak plasma. A local dispersion relation for electromagnetic modes in a nonuniform collisional magnetized electron plasma with fixed ion background is derived, taking into account equilibrium magnetic field and pressure gradients, as well as impurity radiation losses. The dispersion relation is than analyzed both analytically as well as numerically. It is found that for a low-β plasma, he principal source for the generation of unstable modes is the impurity radiation losses; whereas for a high- β plasma, the various effects such as the electron streaming, the electron-ion collisions, finite electron thermal conductivity, and Impurity radiation losses are shown to be responsible for unstable perturbations. The results should be useful in the interpretation of nonthermal electromagnetic fluctuations in nonuniform collision dominated magnetoplasma with impurities. In the reported period we have also investigated magnetic-electron-drift-vortex (MEDV) modes in a homogenous magnetized plasma using Braginskii’s fluid model for electrons. It is found that the modes become unstable when the propagation is in both parallel and perpendicular directions with respect to density and temperature gradients. The instability characteristics of the mode are presented by analyzing it analytically as well as numerically. The relevance to wave phenomena in space and laboratory plasmas is pointed out. This work has been published and the references are as follows: • Electromagnetic instability in nonuniform resistive electromagnet hydro-dynamics Phys. Plasmas 3, 731 (1996) (Arshad M. Mirza, G. Murtaza, and P.K. Shukla)

• Magnetic electron drift vortex in magnetized plasma Physica Scripta, 55, 599 (1997) (M. Khizar, Arshad M. Mirza, M. Salahuddin and M.S. Qaiser)

4. Nonlinear waves in nonuniform magnetoplasma We examined the linear and nonlinear properties od drift-ballooning modes in the presence of an equilibrium electric field and stationary charged dust grains. It is found that the presence of these two contribute to the stability of the ballooning mode. Furthermore, the nonlinear coupling between finite amplitude drift-ballooning modes give rise to different types of coherent vortex structures, which can affect the transport properties of an inhomogeneous magnetized plasma. The relevance of the investigation to laboratory and astrophysical plasmas is discussed. We have also derived a set of coupled nonlinear equation for dispersive Alfven waves (DAWs) in nonuniform magnetoplasma with two-ion species by employing a multifluid model. The DAW frequency is assumed to lie between the gyrofrequencies of the light and heavy on impurities. In the linear limit, a local dispersion relation (LDR) is derived and analyzed. The LDR admits a new type of DAW in two-ion plasmas. Furthermore, it is found that stationary solutions of the nonlinear mode coupling equations in two-ion plasmas can be represented in the form of different type of coherent vortex structures. The relevance of our investigation to space and laboratory plasmas is pointed out. We derived the nonlinear mode coupling equations, for low-frequency electromagnetic wave in a nonuniform resistive magnetoplasma which contains equilibrium density gradients and sheared flows. In the linear limit, a local dispersion relation has been derived and various interesting limiting cases discussed. It is found that an equilibrium sheared flow can cause Alfvenic type instability for a uniform density case. On the other hand, the temporal behaviour of the nonlinear system can be described by a set of six mode coupling equations or simply the generalized Lorenz equations which admit chaotic trajectories. The results of our present investigation should be useful to understand the linear and nonlinear features of anomalous transport and saturation level of electromagnetic turbulence in space and laboratory plasmas. The above-mentioned work has been published/submitted and the references are given below: • Drift-ballooning modes in the presence of charged dust impurities in a nonuniform rotating magnetoplasma Phys. Plasmas 5, 167 (1998) (P.K. Shukla, Arshad M. Mirza, G. Murtaza and R.T. Faria, Jr.) • Linear and nonlinear Dispersive Alfven waves in two-ion plasmas Phys. Plasmas 5, (8) (1998) (R.T. Faria, Jr, Arshad M. Mirza, P.K. Shukla, and O.A. Pokhotelov) • Chaos in parallel velocity shear driven electromagnetic turbulence in a nonuniform resistive magnetoplasma Physics of plasmas (1998) submitted (Arshad M. Mirza, Tariq Rafiq, G. Murtaza and P.K. Shukla)