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OMEGA Validating Direct-Drive Ignition Target Designs

September 11, 2006

Under the right conditions, the fusion energy released by imploding a spherical capsule filled with a relatively thin layer of DT ice is predicted to be greater than the energy of the laser pulses used to drive the implosion. The gain of the target implosion is defined to be unity when the energy released from the DT fusion is the same as the energy of the laser light on the target. High target gains are therefore required to produce net energy because of the relatively low energy efficiency of large laser systems.

Directly illuminating the capsule is the most efficient way to couple the laser energy into the capsule and is expected to produce gains in excess of 30 on megajoule-class lasers such as the National Ignition Facility (NIF), which is under construction in Livermore, CA. High-gain, direct-drive target designs are being validated with scaled experiments on the symmetric 60-beam, 30-kJ OMEGA laser at the Laboratory for Laser Energetics (LLE) at the University of Rochester. These experiments utilize power- and energy-scaled cryogenic targets similar to those being developed for the NIF.

In direct-drive inertial confinement fusion (ICF) , a spherical capsule containing a relatively thin, uniform layer of frozen thermonuclear fuel (DT) is imploded by symmetrically illuminating the capsule surface with high-power laser beams. The laser energy ablates mass from the capsule and accelerates the fuel shell inward via the rocket effect. The frozen fuel layer is compressed to several hundred times liquid density as it implodes and the central, gas-filled cavity heats rapidly. If the temperature and areal density of this central "hot spot" reaches approximately 10-12 keV and 300-400 mg/cm2, respectively, a thermonuclear burn wave initiates because of the rapid deposition of energy from the DT alphas and propagates through the high-density fuel surrounding the hot spot. This process can also be initiated using x-ray or indirect drive.

With x-ray drive, the capsule is mounted in the center of a cylindrical high-Z (e.g., gold) can called a hohlraum. The laser beams illuminate the wall of the can (instead of the capsule surface) filling it with a uniform x-ray radiation field. The radiation field is then absorbed by the capsule, ablating mass and accelerating the fuel shell inward. The capsule physics during the implosion process is independent of the drive type.

The National Ignition Facility (NIF), a 192-beam, 1.8-MJ, UV laser under construction at Lawrence Livermore National Laboratory, was designed to study and use igniting plasmas for stockpile stewardship. A series of experiments on the 60-beam, 30-kJ, UV OMEGA laser at the University of Rochester's Laboratory for Laser Energetics is under way to validate the capsule physics leading to ignition. These experiments utilize a target that is energetically scaled from the ignition designs for the NIF (the energy scales as the cube of the capsule radius, the power as the square of the radius, and the implosion time as the radius). The targets are thin spherical shells of plastic filled with approximately 1000 atm of DT. When cooled to the DT triple point, a layer of frozen fuel is formed on the inside of the CH shell and the gas cavity contains a dense DT vapor.

The current ignition-scaled experiments on OMEGA use deuterium fuel. The implosion dynamics are independent of the fuel choice and the deuterium fuel is considerably easier to handle compared to DT. At LLE, targets are permeation filled with 1000 atm of deuterium and cooled to near the triple point (18.7 K) in the Fill/Transfer Station (the process takes about four days). The capsules are then loaded into moving cryostat transfer carts. These carts contain all the necessary systems to create and manipulate an ice layer inside the plastic capsules. The capsules are located at the center of a 1-in. layering sphere with four viewing ports, a keyhole for the target access, and a small port for an infrared (IR) laser to deliver up to 150 mW of 3.16 micron light to the inside surface of the layering sphere (effectively, the layering sphere is an integrating sphere for the IR light). The IR power is preferentially coupled into the deuterium molecules and low-pressure (hundred's of millitorr) helium is used to conduct heat from the capsule to the layering sphere (maintained at ~16 K). The settings for the exchange-gas pressure, the temperature of the layering sphere, and the IR laser power control the layering rate of the ice and the ultimate smoothness of the inner surface (in other words, these settings control the shape and local gradients of the isothermal "surface" around the capsule). The goal is to achieve a very smooth inner surface to minimize perturbations that grow because of the Rayleigh-Taylor (RT) instability during the deceleration of the fuel shell as the pressure in the hot spot reaches tens of gigabar. Perturbation growth of the colder fuel into the hot spot can prevent the temperature from reaching the threshold for burn initiation.

Experiments to date show good agreement with scaling models. For further information, contact Thomas C. Sangster (csan@lle.rochester.edu) or John M. Soures (jsou@lle.rochester.edu) or view their paper at http://fed.ans.org/News/0606.pdf.