PROTO-SPHERA Diagnostics

Report
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.
Outline
-Equilibrium reconstruction diagnostics
-Pinch and electrode diagnostics
-Plasma density and kinetic measurement diagnostics
-Plasma radiation
-MHD and fluctuation diagnostics
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, rin, 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 rin 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
Optical localization of the transition layer
Emission lines considered
Line
Wavelength(A)
Eion(eV)
Ha
6562
13.6
OII
4416
13
CIII
4647
24
OIII
5592
35
OVI
1032
114
Optical localization of the transition layer
The optical reconstruction of the transition layer will be performed by two
kind of detectors:
Fast CCD array
Multianode photomultipliers
-Good spatial resolution
-Good spatial resolution in a
restricted area
-Excellent time resolution
-Good dynamic range
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 CO2 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
CO2 interferometer
Langmuir probes
Thomson scattering
Impurity content
Visible spectrometer
Arrangement of IR camera and CO2 Interferometer
Soft X-ray tomography
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
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 PROTOSPHERA
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.

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