PROTO-SPHERA Diagnostics PROTO-SPHERA WORKSHOP Frascati March 18-19, 2002

The aim of PROTO-SPHERA aim is to test and maintain for several resistive times a spherical configuration where the central conductor is replaced by a current carrying plasma. The size and characteristics of PROTO-SPHERA plasma allows to use diagnostic tecniques similar to those used in tokamak research. For this reason we can use existing hardware for most of required diagnostics.

-Equilibrium reconstruction diagnostics -Pinch and electrode diagnostics -Plasma density and kinetic measurement diagnostics -Plasma radiation -MHD and fluctuation diagnostics Outline

Plasma Configuration The magnetic reconstruction of configurations such as PROTO-SPHERA is challenging since no sensors can be located in the hole of the spherical torus. In this situation is a magnetic reconstruction still possible? In particular can the plasma current be measured? This problem has been addressed by F. Alladio and P. Micozzi Nucl. Fusion 37 (1997) 1459

Equilibrium reconstruction on PROTO-SPHERA -The sensors are located on a sphere -The magnetic signals were generated with a free boundary predictive equilibrium code -They were best-fitted through an iterative equilibrium solution by a parametrization of the sources of the Grad-Shafranov equation.

The results of the analysis can be summarized as follows: A magnetic reconstruction can be performed with sensors placed around the plasma sphere only provided that the location, r in, of a point of the inner magnetic surface can be obtained from independent measurement. In particular an accuracy better than 10% can be obtained if r in is known to within +/- 5 mm.

To localize the transition layer between the pinch and the main plasma we take advantage from the fact that there a large gradient of temperature and/or density is likely to occur. Two different tecniques will be used:  Thomson scattering  Only a few time points (useful for cross check with optical measurements)  Light emission profile measurement of selected lines  Good time resolution

Optical localization of the transition layer MAST Looking at selected lines will help to increase the contrast

Emission lines considered LineWavelength(A)E ion (eV) HaHa OII CIII OIII OVI Optical localization of the transition layer

Fast CCD array -Good spatial resolution Multianode photomultipliers -Good spatial resolution in a restricted area -Excellent time resolution -Good dynamic range Optical localization of the transition layer The optical reconstruction of the transition layer will be performed by two kind of detectors:

The Thomson scattering only requires a suitable arrangement of the spatial channels around the transition layer

Plasma density and kinetic measurements Components from three diagnostics no longer used on FTU can be used on PROTO-SPHERA  Thomson Scattering  Nd Laser 10 pulses 1.5 J/pulse  Two colors CO 2 Interferometer  2 channels in the present configuration  Good time resolution  Far Infrared DCN interferometer  Poor time resolution in the present configuration. It can be used for Faraday rotation measurements.

Pinch and electrode diagnostics  Cathode and Anode temperature  IR thermocamera  Resistivity measurements  Plasma density and temperature  CO 2 interferometer  Langmuir probes  Thomson scattering  Impurity content  Visible spectrometer

Arrangement of IR camera and CO 2 Interferometer

Two photodiodes detectors array are installed on FTU for the tomographic reconstruction of soft X-ray emission. To install on PROTO-SPHERA a similar sistem is much easier Soft X-ray tomography

Plasma radiation The radiation emitted will be measured by: Visible spectrometer UV spectrometer Bolometer

PROTO-SPHERA Diagnostic Lay-Out Top view

Summary A fairly complete set of diagnostics is required to exploit the PROTO-SPHERA physical program. Usual tokamak diagnostic tecniques can be used on PROTO- SPHERA An exception is given by the equilibrium reconstruction, since no magnetic sensors can be placed in the centre of the ST. This problem has been investigated in detail and a satisfactory equilibrium reconstruction can be performed if constraints from independent measurements are used. The extensive use of existing hardware and the favorable layout allows to minimize costs, keeping them at a small fraction (~20%) of PROTO-SPHERA cost.