Radiation, Hydrodynamics, and Spectral Modeling of Indirect-Drive Fusion Experiments on the OMEGA Laser David S. Conners, Katherine L. Penrose, David H. Cohen (Swarthmore College and Prism Computational Sciences) and Joseph J. MacFarlane (Prism Computational Sciences) What is Inertial Confinement Fusion? Problem: the high temperatures and densities required to overcome the Coulomb barrier and join positively charged nuclei together result in high a pressure that tends to push fusion plasma apart. Energy can be indirectly delivered to the fuel capsule by firing lasers into an enclosure called a hohlraum that contains the capsule; The hohlraum converts the laser energy into thermal X-rays. In order to generate efficient fuel compression, it is important to control and diagnose the way the X-rays in the hohlraum interact with the fuel capsule. Control: Adding dopants (like bromine or germanium) to the outer layers of the fuel capsule (the ablator) influences how the radiation field inside the hohlraum interacts with the ablator. Diagnose: The spectroscopy of NaCl tracer layers buried in the ablator can be used to measure time-dependent conditions in the ablator and thus to characterize the effects of doping on compression. Experimental Design OMEGA Experiments April 2000 Time dependent backlit K α absorption spectroscopy of the NaCl tracer. Hohlraum radiation field parameters set in model to match DANTE measurements. DANTE is an absolutely calibrates ten channel X-ray detector. Goal: To compare spectra of doped and undoped ablator samples. Fusion is the joining together of hydrogen nuclei to form a more massive nucleus, releasing large amounts of energy in the process. A hydrodynamic simulation of the ablation and compression of a solid plastic sample by thermal X- rays. X-rays are incident form the left. The fusing of Deuterium and Tritium (two isotopes of Hydrogen) into Helium. Modeling It takes three steps to create a simulated absorption spectrum for comparison with real data: 1. What is the time-dependent radiation spectrum incident on the ablator sample? 2. What is the hydrodynamic response of the ablator sample to the hohlraum radiation field? BUCKY is a one dimensional hydrodynamics code that takes the radiation field spectrum output from VISRAD and models how the ablator sample’s position, density, and temperature change during the experiment. Bucky simulation of changes over time in the density of a doped plastic ablator exposed to thermal X-rays. 3. How do the spectra generated by the tracer layer in the ablator change over time? SPECT3D is a spectral analysis program written by Joseph MacFarlane of Prism Computational Sciences that can take the hydro-dynamical output from Bucky and simulate a spectrum of the tracer in the ablator sample. Comparing the Model with Data Conclusions Simulated absorption spectrum of chlorine from a salt tracer in undoped plastic ablator. Spectrum was generated at 1000 ps using 841 atomic energy levels of chlorine. Acknowledgments We gratefully acknowledge the support of the Department of Energy through grant DE-FG03- 98DP00250, the Delaware Space Grant Consortium. We would also like to thank Eric Jensen and all the Swarthmore students in the Physics & Astronomy department for their help and support over the last two years. Conservation of momentum causes the fuel to move inward, reaching a core density 20 times that of lead. Note that the fuel is pushed inward, so its own inertia acts to impede its disassembly; hence the term inertial confinement fusion. A spherical plastic fuel capsule filled with hydrogen is bombarded with energy. One solution: The inertial confinement of plasma. This heating causes the outer layer of the plastic to ablate, meaning a rocket-like blow off of material. The thermonuclear burning starts at the core and rapidly spreads throughout the fuel- this is referred to as ignition. Time = 200ps Time = 600psTime = 1000ps VISRAD is a view-factor code written by Joseph MacFarlane of Prism Computational Sciences that allows us to model the radiation field inside the hohlraum and generate the spectrum incident on the ablator. The lasers deliver 7.5kilojoules of energy inside the hohlraum. Notice the “hotspots” where the lasers come in contact with the hohlraum. The ablator sample is the circular area on the far end of the hohlraum. Screen shot from VISRAD showing the lasers entering the hohlraum. A radiation spectrum incident on the ablator at 1000ps It is possible to measure backlit absorption spectra in a hohlraum radiation environment. Doping ablator materials results in a later tracer turn-on time of chlorine K α absorption lines, indicative of the slowing down of the radiation wave by the dopant. Overall, for both the doped and undoped ablator samples, chlorine K α absorption lines are present earlier than was predicted. h x 1s 2p 2s Beryllium-likeHelium-like h x energy We take advantage of the affinity that X- rays have for inner shell electrons. How do we learn things from looking at the chlorine absorption spectra? When the radiation shock wave arrives at the NaCl tracer layer, it provides energy for electrons to make upward transitions resulting in K absorption lines. The appearance of these lines provides us with a local time- dependent diagnostic of the X-ray interaction with the ablator sample. Energy level diagram showing electrons making K α transitions. For more information or to download this poster, please visit Bucky simulation of changes over time in the temperature of a doped plastic ablator exposed to thermal X-rays. undoped plastic1.5% germanium dopant Changes over time in absorption spectra generated by tracer layers in the actual undoped and doped plastic ablators used in the April 2000 experiments. Note the delay in the appearance of K α absorption signals on the undoped and doped sides. Comparison of actual and simulated absorption spectra of salt tracer in undoped plastic ablator at 1000 ps. Future work includes comparison of data with model at all times for both doped and undoped ablator samples. Omega laser target chamber