IOP PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY Plasma Sources Sci. Technol. 18 (2009) 045012 (6pp) doi:10.1088/0963-0252/18/4/045013 Compression of a laser initiated hollow gas embedded z-pinch F Veloso 1 , H Chuaqui, N Correa, M Favre and E Wyndham Pontificia Universidad Cat´ olica de Chile, Departamento de F´ ısica, Casilla 306, Santiago 22, Chile E-mail: fveloso@uc.cl Received 20 March 2009, in final form 24 June 2009 Published 18 August 2009 Online at stacks.iop.org/PSST/18/045013 Abstract Experimental evidence on the compression of a laser initiated hollow gas embedded z-pinch discharge is presented. The hollow z-pinch is initiated by a laser-produced annular plasma formed on the cathode surface of the pulsed power generator in an ambient atmosphere of 0.3 atm of hydrogen gas. Two different initial radii of R 0 = 2 mm and R 0 = 3 mm of the z-pinches were investigated using schlieren imaging and Mach–Zehnder interferometry. The z-pinches presented hollow electron density profiles with peak values on the order of 10 18 cm −3 and a time average Bennett temperature of 150eV. Imploding radial velocity of 5 km s −1 is measured. The hollow profile and stability of the plasma column are maintained during the complete observation time. This observation time is approximately ten Alfv´ en transit time and it is longer than the current peak time. 1. Introduction It is well known that the z-pinch is the simplest experimental plasma confinement configuration and it is highly susceptible to magnetohydrodynamics (MHD) instabilities which cause the disruption of the plasma column at early times. However, it has been proved (both theoretically and experimentally) that the presence of non-ideal effects [1] (such as resistivity, viscosity, amongst others) and a suitable choice of plasma profiles [2] are an important contribution to the stabilization of the plasma column. In particular, the stability criteria obtained by Kadomtsev [2, 3] predict an enhanced stability to the MHD m = 0 sausage mode for a z-pinch with finite pressure on its boundary. This prediction was verified on z-pinches surrounded by neutral gas; which are called gas embedded z-pinches. The main features of the gas embedded z-pinch are the absence of MHD m = 0 sausage mode (the MHD m = 1 kink mode is always present) and the continuous expansion of the plasma column [4–7]. It was observed that the plasma expands radially and the line density increases in time, possibly due to ionization of the neutral background gas in the plasma edge at a higher rate than the recombination rate of the plasma particles. The only report of compression of a gas embedded 1 Present address: Departamento de Plasmas Termonucleares, Comisi´ on Chilena de Energ´ ıa Nuclear, Casilla 188-D, Santiago, Chile Center for Research and Applications in Plasma Physics and Pulsed Power, P 4 , Casilla 188-D, Santiago, Chile z-pinch was achieved by combining two concentric discharges showing expansion when they are formed separately [8, 9]. On the other hand, annular elongated plasma configura- tions (or hollow plasma cylinders) have attracted the attention of many plasma physicists because of their applicability in different areas. One of these applications is the production of hollow plasma channels to be used as plasma waveguides for high intensity laser pulses. These plasma channels have usually radii of tens of micrometres, and a measurable lower plasma density on-axis than off-axis [10, 11]. A different application of hollow plasma configurations is the possibil- ity of producing positron bunches depending on the diffu- sion produced by the density gradients on the inner region of the annular plasma [12]. In z-pinches, a theoretical work has recently found the existence of self-similar solutions to the annular z-pinch, even when resistivity is included in the model [13]. Experimentally, the hollow profile can be achieved with hollow gas-puffs or wire array discharges. Nevertheless, on these kinds of pinches, the annular shape and the compres- sion of the plasma are not uniform along the z-axis (see for example, [14, 15]). This paper describes an experiment which combines both gas embedded and hollow plasmas, the laser initiated hollow gas embedded z-pinch. In the gas embedded z-pinch, the plasma is formed from the ionization of the background gas in such a way to produce a cylindrical plasma column. This preferential geometry is achieved by inducing a lower impedance zone on the electrodes 0963-0252/09/045012+06$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK