B.3.1 Classification and present status of open magnetic field configurations
It was shown in Section B.2 that in order to achieve large specific power it is necessary to use to the largest possible extent fusion in the form of direct propulsion, with the possibility of direct electricity conversion. This is not easy to achieve in equilibrium configurations, such as conventional tokamaks, where plasma does not escape from the reaction chamber, but could be achieved by open magnetic field (OMF) configurations.
The topology of OMF configurations may vary: mirrors topology is cylindrical, as in Figure 8.9, but field-reversed configurations and spheromaks transitioning to a torus in the confinement region may be viable. Nevertheless, the common feature of open magnetic field configurations is that the magnetic field lines escape from the plasma confinement zone without intercepting any wall, and such a feature enables using fusion plasma both for direct propulsion and direct conversion. Note that such a feature may also be common to other systems such as the very low aspect ratio (spherical) tokamak, not considered here but already proposed for propulsion applications.
The best plasma performance achieved so far has been obtained in closed magnetic field configurations (specifically, in tokamaks). However, for propulsion, open magnetic field configurations have intrinsic advantages:
— easy steady-state operation;
— natural particle exhaust;
— high p (= thermal pressure/magnetic pressure);
— direct conversion of fusion power into thrust.
In the following, we consider three main classes of OMF configurations:
— open-ended systems, such as mirrors;
— closed-field line systems, such as field-reversed configurations (FRCs) and spheromaks;
— levitated dipoles.
The analysis below addresses the potential of these configurations to achieve high-,3 values, which is mandatory for the use of advanced fuels, and good confinement properties (i.e., large nr values and reasonable fusion gain) under conditions typical of sustained thrust. To fully assess the potential of a configuration requires a good theoretical understanding of the underlying physical processes. Unfortunately, this is not possible in all configurations. In some limiting cases the answer provided by the experimental evidence obtained so far may be enough to draw a conclusion about extrapolating the results to a range of parameters relevant to a burning plasma. This is the case of ideal MHD stability, where the stability of a given magnetic configuration depends only on the shape of the magnetic fields and on fl. However, weaker MHD modes are heavily affected by kinetic effects related, for instance, to finite-particle orbit width. In some cases even the application of the ideal MHD model is questionable, due to the large orbit size in some of the configurations examined below. In order to understand the gap between the configuration proposed and existing devices we consider in the following three dimensionless parameters
— the ratio fl between plasma pressure and magnetic pressure;
— the collisionality parameter (usually indicated by k*) defined as the ratio between the typical scalelength along the magnetic field and the mean free path of Coulomb-driven collision;
— the normalized Larmor radius p* defined as the ratio between the ion Larmor radius and the typical scalelength transversal to the magnetic field.
It can indeed be easily shown (see, e.g., [Kadomtsev, 1975]) that plasma physics equations (i.e., the Boltzmann plus Maxwell equations) can be cast in dimensionless form and, if the Debye length does not play any role in the processes underlying stability and transport (which is always the case), full similarity between plasma behaviors is assured by identical values of the three dimensionless parameters defined above. For comparison, present tokamak experiments have achieved values of fl and k* similar to those of interest for ITER and the extrapolation in p* is about a factor of 3.
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