Speaker
Description
The divertor, being the main power exhaust of a tokamak, is exposed to high heat
fluxes and therefore must be precisely aligned to prevent leading edges. Since the
transition from carbon to tungsten tiles in ASDEX Upgrade it was found that a specific
assembly in the divertor was misaligned up to 1.5 mm after the experimental
campaigns. This lead to heat spikes on the edges of several tiles and subsequent
melting. To understand the origin of this movement, numeric model was created in
ANSYS Maxwell containing the full 3D coil setup of ASDEX Upgrade, the plasma
current and a segment of the divertor assembly. The plasma current (1.6 MA) was set
to decay within 2.5 ms to 10 ms in order to create a poloidal field change of 50 T/s to
200 T/s while the toroidal field was constant at 2.4 T in the divertor region. The
simulation revealed a parasitic current flowing from the support structure through the
outer tiles. The resulting jxB force can reach up to 2500 N and the torque up to 780
Nm on a single tile depending on the poloidal field change. For carbon tiles, the
maximal force and torque are five times smaller due to the higher electric resistivity. A
current flowing through the conducting support of the assembly through the vessel wall
and back through the assembly causes additional force (max. 5500 N) and torque
(max. 2300 Nm). The other support is isolated by SiN plates. A friction test showed a
static friction coefficient of 0.1 under normal forces larger than 5 kN. A FEM model,
using these results, showed that the friction force at the SiN plates is overcome and
the assembly is moved.
The models and detailed results of the calculations will be presented at the
conference.
