A quantitative study of the carbon impurity production mechanisms from an inertial limiter in Tore Supra as determined by visible spectroscopy Page: 3 of 7
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measured by Thomson scattering. The surface temperature of the limiter was determined from
black body radiation measured in the visible. The error bars for these measurements are
+/- 20%. Zeff was determined from continuum bremsstrahlung measurements in the visible.
Two plasma conditions were studied; both were deuterium, ohmic plasmas. The first condition
will be referred to as Series A for which the toroidal magnetic field was 3.8 T, the minor radius
0.72 m, the major radius 2.40 m, the toroidal plasma current 1.54 MA, the safety factor 3.0
and the volume averaged electron density 3.9 * 1019 m-3. For Series B these parameters were
3.80 T, 0.76 m, 2.36 m, 1.69 MA, 3.1 and 3.5 * 1019 m-3, respectively. Identical shots
were repeated for each series to allow data for different wavelengths to be taken for the same
optical view. Leading edge (located 1.5 cm radially away from the LCFS) data was taken for
Series A parameters and data from near the tangency point (a few mm from the LCFS) was
taken for Series B. The limiter was made from a large block of Carbon Lorraine graphite
(5890 PT).
3. Experimental Results and Discussion. During earlier experiments an image of the
C II emission from the entire limiter was measured with an interference filtered, CCD camera
and the visible endoscope. From the 3D Monte-Carlo modelling (BBQ) of this data, it was
determined that chemical sputtering was a significant source of carbon for the leading edge as
well as possibly at the tangency point of the limiter [4]. Hence a search for direct evidence of
chemical sputtering by-products (C2 and CD molecular bands) was undertaken.
CD molecular bands were observed on both the leading edge (Fig. 1) and tangency point of the
limiter for the plasma parameters of Series A and B. However, C2 molecular bands were not
identified in either location. In Fig. 2a and 2b the brightness of the CD molecular band,
integrated over a 15 A region starting at the -4310 A band head, is shown as a function of
time for the leading edge region (Series A) and the tangency point region (Series B),
respectively.
The limiter surface temperature, for the same region as the CD measurements, is also shown as
a function of time. This surface temperature ,was determined from the black body radiation
(measured at -6550 A for Fig. 2a and ~5140 A for Fig. 2b). Since the black body radiation is
too weak to determine the temperature below -1200'C, the pre-shot temperature (measured by
thermocouple gauges within the limiter) was related to the temperature determined from the
black body radiation by the semi-infinite, one dimensional surface temperature scaling law,
which states that the surface temperature varies as the square root of time. From experiments in
which IR camera data is compared to this scaling law, it was determined that the surface
temperature is somewhat over-estimated by this scaling law at the beginning of the shot; this
over-estimation was corrected for. The temperatures measured from the black body radiation
are consistent with IR camera measurements for similar plasmas.
The plasma was moved onto the limiter at t=1.6s. From this time on, the plasma shape
remained constant but the plasma current and density continued increasing until t=3.5s, thus
complicating the calculation for the maximum data points in Fig. 2a (maximum point Te =
20 eV, remaining points Te = 30 eV) and Fig. 2b (60 eV and 80 eV, respectively)
It is shown in Fig. 2a and 2b that the production of methane was significantly reduced for
temperature above 1100'C and negligible for temperatures above 1300'C. Respective
temperature ranges of -350'C to -700'C and 470'C and 770'C corresponded to the maximum
methane flux for the two regions of the limiter. This is consistent with laboratory measurements
[6], in which a surface temperature of around 550'C corresponded to the maximum methane
yield on both the leading edge and tangency point.
In Fig. 3a and 3b the molecular flux (SX/B of 110 diss./photon for highest point and 120
diss./photon for the remaining points [7]) and the C+ particle flux (Te = 30 eV, SX/B = 60
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Tobin, S.J.; Kammash, T. & Hogan, J.T. A quantitative study of the carbon impurity production mechanisms from an inertial limiter in Tore Supra as determined by visible spectroscopy, report, December 1, 1995; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc622672/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.