PROTO-SPHERA Diagnostics

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PROTO-SPHERA Diagnostics
PROTO-SPHERA WORKSHOP Frascati  March 18-19,
2002  
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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.
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Outline
-Equilibrium reconstruction diagnostics -Pinch
and electrode diagnostics -Plasma density and
kinetic measurement diagnostics -Plasma
radiation -MHD and fluctuation diagnostics
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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
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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.
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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.
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• 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

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Optical localization of the transition layer
MAST
Looking at selected lines will help to increase
the contrast
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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
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Optical localization of the transition layer
The optical reconstruction of the transition
layer will be performed by two kind of detectors
Fast CCD array -Good spatial resolution
Multianode photomultipliers -Good spatial
resolution in a restricted area -Excellent time
resolution -Good dynamic range
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The Thomson scattering only requires a suitable
arrangement of the spatial channels around the
transition layer
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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.

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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

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Arrangement of IR camera and CO2 Interferometer
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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
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Plasma radiation The radiation emitted will be
measured by Visible spectrometer UV
spectrometer Bolometer
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PROTO-SPHERA Diagnostic Lay-Out Top view
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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.