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 Q ≥ 10 is required for a viable electrical power plant. For a power plant with a driver wall-plug efficiency ηD, target gain G, thermal-to-electrical conversion efficiency ηth, 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 = ηth ηthABG (see Figure A.2). An achievable value of the blanket amplifications and thermal efficiency might be AB ~ 1.1 and ηth ~ 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 recirculating power f = 10 percent (QE = 10), the required target gain is G = 150 for a 15 percent efficient driver and G = 320 for a 7 percent efficient driver.

Energy gain does not, of course, guarantee commercial viability. Key challenges remain even after high gain is achieved. These are discussed in detail elsewhere in this report, but they include:

  • Low-cost targets. For example, a target producing a fusion energy, ED, of 200 MJ could make net electricity, Egrid ~ 80 MJ ~ 22 kWh, or about $1 worth of electricity at current prices. The target cost should be some small fraction of this.
  • Repetitive ignition of targets. To produce a gigawatt of electrical power, targets with ED = 200 MJ must be ignited roughly 12 times a second.
  • Reliable target chamber and blanket to extract power and breed tritium. This is a challenge shared with magnetic fusion.

image

FIGURE A.2 Schematic energy flow in an inertial fusion power plant. Note the “Engineering Q” is defined as QE = 1/f. The numbers beside the arrows indicate the proportionality of the energy flows. Tritium breeding (discussed in Chapter 3) is excluded from this diagram for simplicity.



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