A flexible time-dependent, zero-dimensional plasma burn code with radial profiles was developed and employed to study the fractional power operation and the thermal burn control options for an International Tokamak Reactor (INTOR)-sized tokamak reactor. The code includes alpha thermalization and a time-dependent transport loss that can be represented by any one of several currently popular scaling laws for energy confinement time.

Ignition parameters were found to vary widely in density-temperature (n-T) space for the range of scaling laws examined. Critical ignition issues were found to include the extent of confinement time degradation by alpha heating, the effect of auxiliary heating on confinement, and as expected, the ratio of ion to electron transport power loss. Ignition will probably not occur in an INTOR tokamak if all of the alpha power degrades confinement. Applied to a compact tokamak, a simple model showed that ignition would be marginally likely. If only the auxiliary heating degrades confinement, the ignited operating region shows the interesting characteristic of the plasma temperature increasing in response to a decrease in auxiliary power due to the resulting greater decrease in transport losses. If ion confinement is neoclassic (τi/τe large), the ignition criteria are shown to be much more optimistic than for anomalous ion loss, even when the total transport loss is governed by a specific scaling law.

Feedback control of the auxiliary power and ion fuel sources are shown to provide thermal stability for operating points near the low-density, high-temperature portion of the ignition curve. A potential problem will arise if the ignition curve falls below the regions of (n-T) space where the desired reactor net electric power results. Then, net reactor output power occurs in a thermally unstable operating region, and the ignition conditions may need to be “spoiled,” displacing the ignition curve upward in (n-T) space for stable marginally ignited operation. Mechanisms to stabilize this region are investigated, including a “soft beta” limit, auxiliary feedback, impurity radiation, divertor mode variation, varying ion to electron confinement times, and various means of increasing transport power losses. The soft beta limit is unambiguously stabilizing. Confinement degradation by a small fraction (∼15%) of the alpha power would also provide passive thermal stability. Confinement degraded proportionally to plasma temperature, rather than to input power, is shown to marginally provide thermal stability near the ignition curve. In the case of confinement time degraded by auxiliary heating, thermal stability in operating regions well above the ignition curve can be maintained by active feedback of the auxiliary power systems, but very reliable feedback control will be required to avoid thermal runaway.

It is concluded that the problem of thermal control and fractional power operation of a tokamak reactor with ignited plasma is far from trivial. Several possible approaches have been examined for thermally stable operation in the ignited regime. Many of these approaches effectively degrade confinement so that the desired operating point becomes marginally ignited. If ignition is not achieved, thermal stability is achieved in a driven subignited reactor mode. These approaches must be evaluated further and with more refined empirical estimates for the energy confinement scaling in order to reduce the present uncertainty in this area.