The thermal EMF (VTC) is generated at the molybdenum/tungsten-rhenium thermojunction. No other thermojunctions change temperature through the pulse. The voltage is transmitted via a 50 Ω coax to a high common-mode rejection instrumentation amplifier with a gain of 248. The differential input RC filter has a 3.0 ms time constant. The signal is put through a unity gain circuit with further filtering (3.0 ms time constant) and then digitized on a D-tAcq ACQ196 unit at 400 kHz. Both the original and improved grounding points are shown (ground taken to be the vacuum vessel potential). With the old grounding method, plasma-induced currents would flow though the stainless steel thermocouple holder, contributing a voltage that was of the same order and sign of VTC, leading to erroneous temperature measurements. In the present design the current path through the stainless steel is eliminated and the molybdenum body is shorted directly to ground. The voltage generated by the plasma-induced current flowing through the molybdenum is much less than VTC.
The 1 cm scale bar is for both the line drawing and inset photograph. The 74% tungsten-26% rhenium ribbon (1) is electrically isolated with mica sheets (2) from the molybdenum probe body (3 and 4). This in turn is swaged into a 6.35 diameter molybdenum tube; forming a customized version of a “self-renewing thermocouple” supplied by NANMAC Corp. Filing the surface of the sensor initiates the thermojunction. The copper collet (7) grounds the molybdenum sensor body to the molybdenum tile with low electrical resistance. Insulators (9 and 17) isolate the stainless steel probe body from ground. The threaded holder (11) allows for a removable coaxial connection.
A cross-calibration procedure is performed on each discharge, which forces the surface thermocouple temperature to agree with the tile thermocouple temperature before the plasma pulse. The time evolution of both signals is shown in the top panel. Using output from the EFIT plasma equilibrium reconstruction code, the magnetic flux surface label (ρ) and magnetic field line inclination angle on each surface thermocouple is recorded (second and third panel). The coordinate ρ is the distance of the magnetic flux surface beyond the last-closed flux surface as it passes the outer equatorial midplane of the plasma. Local heat flux density normal to the surface thermocouple surface is computed from a 1D heat transport model of the surface thermocouple cylinder with temperature-dependent thermal parameters. This quantity and its equivalent heat flux density flowing along magnetic field lines are shown in the bottom panel.
Section 12° of the lower outer divertor module with tiles ramped 2° and sensors. The magnetic field is nearly tangential to the four lowest rows (≲1° for the standard tiles, ∼2–3° for the ramped). The Langmuir probes are the circles in the left-most column of tiles. The surface thermocouples are the left and right-most circles in the center column of tiles. The calorimeters are the two columns of circles running in between the surface thermocouples.
Comparison of raw surface thermocouple voltages for a pulse with plasma contacting the surface thermocouple sensor and one without (top panel). Note that using the tungsten-rhenium as the center conductor results in a decrease in voltage representing an increase in temperature. Vertical axis is inverted for clarity. Both pulses have nearly identical toroidal field (middle panel) and plasma current (bottom panel). There is a small sensitivity to the rate of toroidal field ramp-up and ramp-down, but neither affects the thermal analysis. The heat flux calculation starts after the toroidal field ramp-up induced voltage returns to its starting value and ends before the ramp-down. The sensor does not pick up any voltage contribution from plasma current ramps, as seen by the lack of a response during the case with no plasma contact. This lack of voltage change also indicates that only the surface thermojunction, and none of the others in the circuit, changes its temperature over a plasma pulse. From this comparison we are confident that the voltage response of the sensor is entirely due to changes in its surface temperature.
Comparison of the energy deposited on a surface thermocouple and neighboring calorimeter for all of the L-Mode pulses taken for the 2010 Joint Research Target.1 Calorimeter energy flux is calculated using the temperature difference of a thermally isolated molybdenum slug with an embedded thermocouple. Surface thermocouple energy flux is found by integrating the surface heat flux, which was calculated from the surface temperature evolution. Values typically agree to better than 15%.
Plasma parameters and derived parallel heat flux density profiles obtained from sweeping the “heat flux footprint” over a surface thermocouple and Langmuir probe in close proximity. Horizontal axis is ρ, the magnetic flux surface label, mapped to the outer midplane. The plasma-induced ground current (J gnd) at the outer divertor increases the sheath heat flux coefficient to ∼11, 50% above the nominal floating value of ∼7.5 (dashed line). Using this calculated value of the sheath heat flux transmission coefficient, the heat flux profiles from the Langmuir probe and surface thermocouple are in good agreement across the entire footprint.
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