The divertor RFEA is installed on the Divertor Science Facility (DSF), an assembly designed to manipulate samples and various probe heads into a gap in the MAST lower outer divertor target at R= 0.985 m [12]. The module is protected by a graphite shell on the protruding part of the RFEA which is exposed to plasma, see figure 3.12. The slot in the graphite shell which allows the plasma access to the slit plate has an area of∼30 mm2. The divertor RFEA has a single module facing the direction of plasma flowing towards the divertor targets. A boron nitride cap is included in the module assembly between the RFEA module top and the graphite shell to reduce the likelihood of electrical arcing between the shell and the grids. When fully inserted the DSF RFEA protrudes 25 mm above the target plate. The slit is arranged to intercept ions which would be incident on the target. The point at which the incident ions hit the collector plate is almost level with the top of the divertor target plate as shown in figure 3.13.
It is possible to alter the height of the probe so that it can be flush with the divertor tile for protection, however the probe can only take measurements when it is fully inserted. The positioning of the RFEA in the target plate can be changed from run day to run day. The slit in the front face of the RFEA is at 9◦ to vertical so that the RFEA module and slit entrance are aligned with the average magnetic field for MAST plasmas. This ensures the parallel velocity distribution is sampled by the RFEA. The actual value for the angle at the target can vary over discharges in the range 4−12◦, however this is within the acceptable range of angles∼ ±10◦ that should not affect the measurement of Ti [41].
The use of a protruding probe at the divertor, such as the target RFEA, is limited by the heat flux it can stand. The two limiting factors are the maximum load and the duration this load is incident on the probe head. The use of the divertor probe is possible on MAST because there is a low power incident on the divertor and sweeping of the outer strike point reduces the duration this peak power is incident on the probe. The front face of the RFEA probe shell is exposed to a parallel
3.2. RFEAS IN MAST 47
Figure 3.12: Divertor RFEA probe head attached to the DSF bracket. The graphite head shows the single slit entrance to the probe which is aligned along the average total magnetic field at the divertor target. Only the graphite shell section of the probe head is exposed to the plasma since it is the top most section of the probe which protrudes above the divertor tiles, see figure 3.13.
Figure 3.13: Divertor RFEA in situ at the divertor tile showing the maximum protrusion above the divertor tile which is required for ion temperature measurements. Also shown is a cross-section through the divertor RFEA probe head as positioned in the divertor tile. The magnetic pitch angle at the RFEA can be seen. The magnetic pitch angle line shows the point at which the field line intercepts the collector plate,∼ 2 mm above the divertor tile; this is where theIcol is measured.
power flux density q|| < 6.5 MW/m2 at the divertor for 400 kA discharges; based on infra-red thermography measurements of the target power fluxes, and the local field line angle at the target. The context for this low parallel power density can be understood using equation 3.10 which estimates the effective outboard midplane parallel power density from the power entering the SOL (equation 3.8 where Pohmic
and Pabs
N BI are the ohmic and NBI heating power respectively, Prad is the power lost
by radiation, and ˙W is the change in stored energy as a function of time). The power is split, in this case, 50:50 to the upper:lower divertors and 10:90 to the inner:outer divertors. Equation 3.11 relates the parallel power density to the local field (which is dominated by the toroidal field) and can give the equation for the power flux density at the divertor, equation 3.12.
PSOL =Pohmic+PN BIabs −Prad−W˙ (3.8)
Pin =PSOL×up/downf raction×in/outf raction (3.9)
qomp|| ≈ Pin
2πRomp(Bθ/Bφ)ompλompq
(3.10)
qdiv|| /qomp|| ≈Romp/Rdiv (3.11)
qdiv|| = Pin
2πRdiv(Bθ/Bφ)ompλompq
(3.12) For Romp= 1.4 m, Rdiv= 0.985 m, (Bθ/Bφ)omp≈0.35 andλompq = 8 mm (measured
at the target in a low density plasma, with ¯ne = 1.2 x 1018 m−3, by the infra-red camera and mapped to the midplane using Eich’s formula [69]); a parallel power density at the RFEA of 6.5 MW/m2 then corresponds with a loss power from the plasma of Pheat- Pcorerad ≈250 kW. This is consistent with the expected ohmic heating
and radiated power for 400 kA plasmas in MAST. For higher power discharges the parallel power density is higher,∼25 MW/m2, since values are about Pin∼560 kW
andλompq = 1 cm.
In MAST the change in solenoid flux throughout the shot causes the divertor leg to move across the divertor plates. The natural sweeping of the outer divertor leg makes it possible to get a radial profile of measurements as a function of distance from the separatrix at the target while measurements are taken at the fixed point of the DSF.