The next major step in magnetic fusion studies will be the construction of a burning plasma (BP) experiment where the goals will be to achieve and understand the plasma behavior with the internal heating provided by the fusion-generated alpha-particles (α-particles). Two devices with these physics goals have been proposed, the International Thermonuclear Experimental Reactor (ITER) and the Fusion Ignition Research Experiment (FIRE). Extensive conceptual design work for the instrumentation to try to meet the physics demands has been done for these devices, especially ITER, and the overall requirements are reviewed in this book. This article provides a new look at the measurements specifically important for understanding the physics aspects of the α-particles. An earlier article addressed a similar topic, but two significant events have occurred since then. The first was the completion of physics experiments on JET and TFTR with deuterium-tritium (DT) fueling, and the first chances to study α-physics, and the second is the realization that relatively compact plasmas, making use of advanced tokamak plasma concepts, are the most probable route to burning plasmas and ultimately a fusion reactor. These plasmas require measurements with excellent spatial and temporal resolutions and a large number of diagnostic signals will feed into the control of the plasmas. The BP experiment will provide the science basis for the operation of a fusion reactor. Thus it is imperative that the understanding developed should lead to optimization of the plasma performance and being able to use the α-particles in controlling the plasma. The α-particles will become the dominant heating term in any successful BP experiment, and as such will play a key role in the physics of the plasma. Their generation in the core can be expected to significantly affect the density and pressure profiles associated with the confinement regime of choice. The fusion output in the form of these α-particles has got to be fed back into the control of the heating to affect the pressure gradients; it must also be used to assist the current profile, and possible provide current drive; and it must provide momentum to affect the shear flow with its strong influence on the transport properties. These roles then set the purpose for the α-particle measurements, setting up the requirements for specific diagnostics. Table 1 provides a listing of the diagnostic techniques which are being considered for the α-physics measurements in a BP device. They are discussed in the following sections, but some comments indicate where testing on operating devices is very desirable. Many of the diagnostics necessary for these measurements are not fully developed or will be severely constrained in their operation under BP conditions.
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