Toroidal Support Structure Research Articles

Detailed design of the RFX-mod2 machine load assembly

An upgrade of the RFX-mod experiment is presently in the final design phase, aimed at widening the explored operational scenarios both in RFP and Tokamak configuration. The main design driver for this machine upgrade is the enhancement of the ‘shell-plasma proximity’, which is expected to provide a significant improvement in the plasma magnetic confinement. The achievement of this aim implies a major change of the internal components of the machine such as the removal of the present vacuum vessel, transferring the function of vacuum barrier to the duly modified toroidal support structure, and the integration of a new in-vessel support system to sustain the conductive stabilizing shell and the whole first wall. The paper presents an overview of the design choices and the proposed implementations, assessed on the base of engineering analyses and results of experimental tests performed on mock-ups of the new components. The solutions conceived to fulfill vacuum and electrical requirements of the in-vessel components, to guarantee their reliability during normal and abnormal operating conditions events, and interface compatibility with existing components and torus assembly sequence are finally highlighted.

Design concepts of machine upgrades for the RFX-mod experiment

After 10 years of operation since its major modification, an upgrade of the RFX-mod experiment is presently under design. The scientific objective is the improvement of 3D physics studies through a more robust transition to higher confinement regimes in both Reversed Field Pinch (RFP) and Tokamak configuration obtained thanks to an advanced system for the active control of MHD instabilities. The main design driver requirements for this machine upgrade are the removal of the present resistive vacuum vessel and the enhancement of the ‘shell-plasma proximity’, to reduce the deformation of the last close magnetic surface and to improve the self-organized helical plasma regimes. The fulfillment of these requirements implies a major change of the internal components of the machine such as the replacement of the whole first wall, the change of the support system of the stabilizing shell and the modification of the present toroidal support structure to provide the function of vacuum barrier. In combination, other components of the machine will be upgraded, such as magnets and power supply, diagnostic systems and a NBI will be integrated. The paper presents an overview of the engineering design of the new components and highlights the critical aspects of the new torus assembly.

Vacuum boundary modifications of the RFX-mod machine

The results produced by experimental campaigns of RFX-mod shows the need to improve passive MHD control and minimise braking torque on plasma. These improvements require major mechanical changes on the present components of the machine including first wall, vacuum vessel, and toroidal support structure (TSS). The vacuum vessel will be removed and the first wall tiles will be directly supported by the passive stabilising shell, so increasing the poloidal cross section by 28mm radially and bringing the passive stabilising shell as close as possible to the plasma boundary.This paper presents the mechanical design modifications of the torus assembly. Composite rings are designed to support the passive stabilising shell on the TSS. The vacuum boundary is shifted at the TSS by developing different joint solutions compatible with the stringent requirements of the present components: ceramic-metal brazed rings at the two poloidal joints, fully welded solution at the external equatorial joint, and resistive weld plate at the internal equatorial joint. The vacuum vessel portholes for pumping, fuelling, diagnostics, and sensor cable routing are redesigned to be integrated onto the TSS. The design is supported by thermo-mechanical analyses and verifications carried out applying load combinations corresponding to the expected load cases.

Machine modification for active MHD control in RFX

Recent studies on RFP and Tokamak devices call for an active control of the MHD and resistive wall modes to induce plasma mode rotation and to prevent mode phase locking. The results obtained on RFX, where slow rotation of phase locked modes has been induced, support the possibility of extending active MHD mode control through a substantial modification of the device. A new first wall with an integrated system of electric and magnetic transducers has been realised. A close fitting 3 mm thick Cu shell replaces the 65 mm Al shell. A toroidal support structure (TSS) made of stainless steel replaces the shell in supporting all the forces acting on the torus. A system of 192 saddle coils is provided to actively control the MHD modes. This system completely surrounds the toroidal surface and allows the generation of harmonic fields with m=0 and m=1 poloidal wave number and with a toroidal spectrum up to n=24.

Design of a new toroidal shell and support structure for RFX

The physics results obtained from RFP experiments at high current (0.8–1 MA) indicate the need for a better control of the plasma MHD modes, which grow rapidly and lock to the wall during the pulse. The modifications planned for the RFX machine have been designed to guarantee both a passive and an active control of these modes. The present 65 mm thick aluminium shell will be replaced by a 3 mm thick copper shell which is assembled on the vacuum vessel, closer to the plasma edge. In addition 192 saddle coils will be installed to actively control the main mode spectrum of RFX. As a result of these modifications, a new toroidal support structure will be required, which carries out the mechanical functions of the previous shell and supports the new saddle coils. This paper deals with the design of the thin shell and toroidal structure. The solutions which guarantee the required safety level, from a mechanical and thermal point of view, and the technological details of the electrical insulation systems for the various components are highlighted.