Scales Work as Part of Thermonuclear Ignition Target Assembly
- Friday, 01 June 2007
Next, the Plasma Electrode Pockels Cell (PEPC) uses electrically induced changes in the refractive index of a potassium dihydrogen phosphate (KDP) electro-optic crystal, and contains the laser beams in the main amplifier cavity, causing them to pass four times through the main amplifiers. The PEPC traps the laser between two mirrors as it makes four one-way passes through the main cavity amplifiers before being switched out. The PEPC allows light to pass through or reflect off a combined polarizer. The pulses then travel through two stages of spatial filters, which focus the beams through pinholes to remove high-frequency intensity variations; the precise characteristics of the laser beams are maintained en route to the target chamber.
NIF’s neodymium glass laser generates light at a wavelength of about 1.053 micrometers in the infrared region. Because inertial fusion targets perform more efficiently when driven by ultraviolet radiation, Final Optics Assemblies (FOA) convert the infrared light to ultraviolet, using a system of two nonlinear KDP crystal plates.
The mechanical interface between the FOA and the target is the target chamber, a 1,000,000-pound sphere equipped with stationary shutter-like louvers and a 16"-thick neutron-shielding concrete shell that provides a vacuum environment for the target and mounting and alignment points — 72 for the FOA and 48 for indirect-drive experiments for the 192 laser beams that focus on the target. X-ray spectrometers, microscopes, and cameras are mounted around the equator and at the poles of the target chamber. The targets are positioned, three at a time, in a 1-cm canister, the hohlraum. Laser beams pass in groups of four through the FOA’s frequency- conversion crystals, then through wedge-shaped lenses that focus the ultraviolet beams at the target. Although coming in pulses lasting 3 to 10 nanoseconds, the laser system is equal to 1,000 times the electric generating power of the United States. Delivering 1.8 million joules of ultraviolet laser energy and 500 terawatts of power to the target, the lasers enter the hohlraum from top and bottom, generating x-rays that heat the capsule to Sun-like temperatures, blowing off (“ablating”) the target’s shells. Simultaneously, the x-rays create massive pressures that compress and fuse the atoms of the DT fuel, resulting in a burst of energy. The shortness and small size of the burst is many times smaller than a conventional nuclear explosion.
The new target design has allowed NIF to create thermonuclear ignitions mimicking conditions found in the Sun or an exploding nuclear event. The primary mission of NIF will be to attain fusion ignition in the laboratory, exploring fusion’s potential as a clean, long-term energy source. Such experiments bypass the need for actual nuclear weapons testing while continuing research into the nuclear, fusion, and high-energy density sciences. High-energy density regimes achieved by NIF experiments could also lead to fresh insight into the origin of the universe: though roughly 10% uncompleted, NIF laser beams are already in use to simulate supersonic plasma jets occurring in some galaxies and black holes.