Prior art tokamak fusion reactor (TFR) concepts were directed to large machines with blanket and shield elements positioned in between the plasma fusion region of the TFR and the large superconducting toroidal field (TF) coils.  In U.S. Pat.  No.  4,367,193 and U.S. Pat.  No.  4,363,775, there is disclosed a small machine with the blanket means positioned external to the normally conducting TF coil assembly.  It is known in the art that such blankets can advantageously use the neutrons generated in the fusion plasma to breed new fuel to produce thermal energy, and to create additional energetic reactions.  This invention is directed to those TFRs utilizing external blankets (XBTFR) such as those disclosed in the commonly assigned U.S. Patent applications referred to above. 
In the case where a TFR uses the deuterium tritium (d,t) reaction, approximately 80% of the energy output is in the form of the kinetic energy of fast neutrons.  In the small machine referred to in the above-mentioned U.S. patents, the TF coil is exposed to the neutron flux.  The neutron radiation heat loads precludes the use of superconducting materials for the TF coils in this small machine design. 
Inasmuch in the small TFR design, the TF coil surrounds the plasma region, the neutrons created as a result of the fusion reactions must pass through it.  In this regard, many prior art TF coil assembly designs will absorb a considerable fraction of the neutrons created by the fusion reactors and that those do emerge without being absorbed in the TF coils will have lost much of their kinetic energy in the TF coils. 
While it is a feature of the TFR design disclosed in U.S. Pat.  No.  4,367,193 and U.S. Pat.  No.  4,363,775 to remove the energy deposited in the TF coils and recover it as useful heat, energetic neutrons are far too valuable for breeding fuel for fusion and fission reactors and for generating high temperature heat in the blanket to be used merely as a source of heat in the TF coils. 
In addition, there is a need for access to the fusion region of a XBTFR in order to attach vacuum purge lines, cooling inlet and outlet lines, fuel feed lines, electrical power connections and diagnostic equipment. 
Applicants have found that one of the consequences of the TFR geometry is that the current density and mechanical stresses imposed on the TF coils are much greater in the region of the inner part of the TF coil, the region nearest the center or the main axis of the machine.  Another consequence of the XBTFR geometry is that most of the neutrons generated in the fusion plasma exit through the outer part of the TF coil or the region farthest from the center or main axis of the TFR. 
Co-pending and commonly assigned Ser.  No.  340,237, filed Jan. 15, 1982 entitled "Composite Coils for Tokomak Reactors and the Method of Using Same" address these problems by the use of a coil where inner section is composed of copper or copper alloy which has both a high electrical conductivity and a high tensile strength to withstand the forces accompanying the strong magnetic fields and an outer section whose outer section is composed of Al or Al alloy which also has a high electrical conductivity but which has a low co-efficient neutron absorbtion. 
While the present invention can make use of the teachings of Ser.  No.  340,237 in the design of the TF coils, the present invention is directed to a structure for achieving the segmenting of the TF coils.