When a star goes supernova the explosion it creates travels outward into interstellar space, picking up material as it goes. This outward expanding mass of swept up gas is known as a supernova remnant (SNR). After 10,000 – 15,000 years certain SNRs, such as the Cygnus loop and Vela SNR, exhibit a large amount of structure, and it has been theorized that these structured SNRs can play a role in star formation. Because these particular supernova remnants cool primarily by giving off electromagnetic radiation, they are known as radiative supernova remnants. In 19831, Dr. Ethan Vishniac proposed a mechanism by which the structure seen on these radiative SNRs arise, an overstability he called the pressure driven thin shell overstability (PDTSO). Over the next decade, Vishniac and Ryu developed2, 3 this theory for more realistic SNRs and predicted growth and decay rates for the overstability that depended on the thickness of the SNR and the wavelength of the perturbation on it. The PDTSO is now known as the Vishniac overstability, and there have been several attempts to try and verify the predictions of Vishniac and Ryu, both through computer simulation and laboratory experiment.
Since it is impossible (and undesirable) to create a supernova in the lab, laboratory experiments have looked at explosions created through other means. The types of SNR that we are interested in are usually 10,000 years old or older and many light years in diameter, meaning the original explosion of the star was short in time and contained in a small space compared to the current age and size of the SNR. Lasers are a common means of creating explosions. Lasers are capable of delivering a large amount of energy (though not nearly as much as is released in a supernova) into a small volume in a short amount of time. These type of “point” explosions (explosions that are contained in a small amount of space and short in time) in an atmosphere result in blast waves. A blast wave is also known as a decaying shock wave, because it is a shock wave that travels out from the origin of the explosion sweeping up material and getting weaker as the energy is spread over a larger and larger volume.
The size and time scales of laser experiments are much different from those in a supernova remnant. Scaling laws are used in order to ensure that the physics being studied in the laser experiment is similar to that of the supernova remnant. Scaling laws for going from SNRs to laser experiments were developed by Ryutov et al. in 19994. While these scaling laws are not perfect for our particular experiment (they assume no radiation), they do allow us to ensure our experiment is as similar to the astrophysical case as is possible.
In our experiments high energy laser pulses (up to 1000 J) illuminate a solid target inside a target chamber that is filled with .01 atmospheres of gas. When the laser hits the solid target it results in an explosion that travels through the gas. Often we place regularly spaced wire arrays in the path of the blast wave to induce perturbations of the blast wave surface. We then use a second probe laser that is delayed relative to the first to look at the evolution of the blast wave at various times relative to the initial blast (from 10 nanoseconds to 10 microseconds later). We can compare the evolution of these perturbations to the theoretical predictions of Vishniac and Ryu. By changing the spacing of the wire array and the type of gas in the target chamber, we can examine various parts of the theory and determine of the predictions of Vishniac and Ryu and correct. So far our experiments are in qualitative agreement with the theory but there are still several aspects to look at.
1 E. T. Vishniac, Astrophysical Journal 274, 152 (1983).
2 D. Ryu and E. T. Vishniac, Astrophysical Journal 313, 820 (1987).
3 D. Ryu and E. T. Vishniac, Astrophysical Journal 368, 411 (1991).
4 D. Ryutov, R. P. Drake, J. Kane, et al., Astrophysical Journal 518, 821 (1999).