heat the target before compression. Also interaction of the driver with the surrounding plasma can create fast electrons that penetrate and preheat the target.
A widely used parameter to assess the performance of an ICF target is the target gain, G, representing the ratio of the fusion energy output to the driver energy entering the target chamber. Clearly a high gain is desirable for fusion energy and must remain a central focus of any inertial fusion energy program.
The fraction of driver energy that couples to the fusion fuel contained in the target is typically small—a few percent—but the fusion gain can still be substantial. In a National Ignition Facility indirect-drive ignition target driven by ~1MJ of UV laser light into the hohlraum, the shell of fuel implodes with an expected kinetic energy of about 15–20kJ. Roughly half of that energy (7–10kJ) is used to heat up the hot spot and the other half to compress the surrounding shell. If the fusion yield (alpha and neutron energy) is 1MJ (i.e., G = 1), the hot spot energy is amplified 100x by the thermonuclear instability. At 1MJ fusion yield, the alpha particles have deposited 200kJ of energy into the hot spot and surrounding fuel, about 20 times the energy provided by the compression of the hot spot. The thermonuclear burn stays localized near the hot spot and propagates within about 5 times the initial hot spot mass (partial burn). If the burn propagates through the entire DT mass, the gain of a NIF target will exceed ~10 (full burn and 10MJ yield). While a NIF implosion yielding G»1 would elucidate many aspects of the ignition and basic burn physics, a gain of G ≥ 10 is required for demonstrating full burn propagation over the inertial confinement time of the compressed shell (i.e., fuel burn-up fraction compatible with the fuel inertia).
While the target gain can be used to validate the target physics, a new parameter is required for assessing the viability of a fusion energy system. The so-called “Engineering Q” or “QE” is often used as a figure of merit for a power plant. It represents the ratio of the total electrical power produced to the (recirculating) power required to run the plant—i.e., the input to the driver and other auxiliary systems. Clearly QE = 1/f, where f is the recycling power fraction—see Figure A.2. Typically QE ≥ 10 is required for a viable electrical power plant. For a power plant with a driver wall-plug efficiency hD, target gain G, thermal-to-electrical conversion efficiency hth and blanket amplification AB (the total energy released per 14.1 MeV neutron entering the blanket via nuclear reactions with the structural, coolant, and breeding material), the engineering Q is QE = hthhDABG (see Figure A.2). An achievable value of the blanket amplifications and thermal efficiency might be AB ~ 1.1 and hth ~ 0.4 and should be largely independent of the driver. Therefore, the minimum required target gain is inversely proportional to the driver efficiency. For a power plant with a large recirculating power f = 20% (QE = 5), the required target gain is G = 75 for a 15% efficient driver, and G = 160 for a 7% efficient driver.