W. Baker

Machine modification for active MHD control in RFX

Abstract 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.

RFX machine and power supply improvements for RFP advanced studies

Experimental results and theoretical studies call for Reversed Field Experiment (RFX) machine and power supply improvements to allow studies that go beyond those of a conventional Reversed Field Pinch (RFP) with passively stabilized turbulent MHD dynamo. The new paths opened by recent results in RFX and other RFP machines are introduced; then the goals and the design lines of the technical modifications of RFX, mainly addressed to improve the first wall, the plasma magnetic boundaries and to increase the operational flexibility of the toroidal field circuit power supply, are reported.

Recent progress in reversed field pinch research in the RFX experiment

The article presents an overview of recent experimental results obtained on the RFX device. The authors obtained and studied a reversed field pinch plasma with a plasma current of up to 1 MA, negligible radiation losses and low effective charge. The local power and particle balance shows that in standard operation the plasma core is dominated by magnetic turbulence and that the global confinement is mainly provided by the edge region, where a strongly sheared radial electric field is present. With poloidal current drive the amplitude of magnetic fluctuations and the thermal conductivity of the plasma core are reduced, leading to improved confinement. Reduced heat transport is also observed when the width of the n spectrum of magnetic fluctuations is reduced.

Confinement studies on RFX

The results of the first year of operation of the experiment RFX are reported. Profiles of electron density, electron and ion temperature and impurity emission have been measured at plasma current I<0.7 MA. The energy confinement parameters at different density are reported, the best values ( tau E approximately 1ms, beta theta approximately 8%) being obtained operating at higher density. The role of the impurity content in determining the present performance of the experiment is discussed.

High-spatial resolution edge electrostatic probe system for RFX

The RFX reversed field pinch (RFP) is undergoing major modifications of the load assembly and will be equipped with a large number of external coils for active magnetohydrodynamics mode control. The investigation of plasma parameters is then of paramount importance to study the effects on the plasma of both the new passive structure and the action of the external active coils. The study of turbulence is also very important because of its influence on plasma transport. A system of electrostatic probes is needed, fulfilling very challenging requirements in terms of spatial and temporal resolution, which are characteristic features of RFP plasmas. This system should also be integrated with other in-vessel sensors (magnetic and calorimetrical probes) and with other diagnostics placed outside the vessel. A description of the diagnostic system is presented. The global layout of the probes along the first wall is shown: The system allows a two-dimensional investigation of the plasma; both single and triple elect...

INTEGRATION OF THE RFX ELECTROMAGNETIC CIRCUITS

The commissioning of the RFX power supplies and their integration with the machine windings were performed in steps of increasing complexity; the target was to start the experiments with plasma in the final configuration of all systems, tested up to the performances required for first operation. After a brief description of RFX circuits and operation, the final tests on the power supplies, their commissioning with the control, data acquisition and fast protection systems and the integration with the machine windings are presented. Indications on operation reliability following the first experimental phase are finally given.

Installation, Commissioning and Start Up of RFX

The Reverse Field Pinch experiment RFX has been operating since the end of 1991. The paper, after a brief description of the load assembly, presents the most significant aspects of the assembly phase and discusses methods and results of the final tests, carried out before producing the first plasma. Finally, the machine behaviour during the first experimental phase is described.

Construction and assembly status of new components for the modified RFX machine

The modified RFX machine, now under construction, is a new RFP experiment having the possibility of an active control of the radial field at the plasma boundary. The following new components are now under construction or in the assembly phase on site: a thin copper shell, closely fitted to the vacuum vessel, a stainless steel toroidal support structure and 192 saddle coils. The construction phases, the quality control and the acceptance tests for all these new components are described in this paper, together with the assembly status of the modified RFX machine.

Design and construction of a movable in-vessel power electrode on RFX

A movable in-vessel electrode has been designed for the RFX experiment in order to modify the radial electric field. The results of the tests performed on prototypes of the components are reported. The reliability of the structure has been assessed by a finite element structural analysis of the system. The power supply to be used for the electrode operation is described.