Optical design study of an infrared visible viewing system for Wendelstein 7-X divertor observation and control

Abstract

For the Wendelstein 7-X stellarator, which will allow quasicontinuous operation (tau < or = 30 min) with 10 MW of electron cyclotron radiation heating power, a conceptual design study for an IR/visible viewing system (IVVS) has been elaborated. Ten such systems, as part of the machine protection system, will be required for real time monitoring of all ten discrete, water cooled divertor modules with high spatial (< 10 mm) resolution, in order to prevent local overheating of the target tiles, which could easily lead to their destruction. On the physics side, the systems will be used for divertor symmetry investigations by studying the power load distribution on all targets modules and by observing the island divertor plasmas in the light of H alpha, C II, and C III using the visible imaging section of the systems. The optics of the system can be divided into three parts: a mirror based optical head, creating an intermediate image, a Cassegrain telescope system, and individual lens based imaging optics adapted to the various detectors for IR (3-5 microm and 8-14 microm) and visible observations, with their optical light paths being separated by in-vacuum dichroic beam splitters.

Construction of Wendelstein 7-X - Engineering a steady state stellarator

“…One of the most critical diagnostic items for steady state operation of W7-X will be the combined infrared/visible divertor observation endoscopes [29,30] which need to ensure that none of the targets of the 10 discrete divertor modules are ever exposed to excess heat loads which could easily destroy them. These systems are designed such that the entrance pupil of their optics is lying inside an ~5 mm observation pinhole inside a water cooled stainless steel plate facing the plasma.…”

Diagnostics design for steady-state operation of the Wendelstein 7-X stellarator

“…Earlier work has demonstrated the feasibility of this aim. 7 In some cases, the total integrated ECRH stray radiation power load ͑as well as the thermal power load from plasma radiation͒ on sensitive diagnostics components, such as windows or detectors, can be significantly reduced by observation through pinholes, like in the case of the toroidal video observation, 6 the IR/visible endoscopes for two-dimensional divertor temperature control and plasma symmetry investigations, 8 or the bolometer cameras. 5,9 In the last case, the detectors are additionally shielded by a cooled wire grid right in front of the detectors.…”

“…Of the ten long pulse compatible IR/visible endoscope systems 8 for divertor temperature control and plasma symmetry investigations for all ten W7-X divertor modules, initially, i.e., during the uncooled divertor operation phase, only two systems will be installed, one looking at an upper and one at a lower divertor. The other eight systems will be replaced during the initial, short pulse operation phase of W7-X by a rather simplified version with the microbolometer IR and video/H ␣ cameras being mounted at the plasma facing end of the port tubes looking through windows equipped with uncooled shutters.…”

Diagnostics development for quasi-steady-state operation of the Wendelstein 7-X stellarator (invited)

“…3. Mock-up Rogowski coil section (centre) consists of 10 individually 3D shaped rods (top left) with tightly wound wire placed into thin SS tubes for ECRH protection with 0.7 mm holes for outgassing and copper rails clamped between rods (bottom right) to improve cooling endoscope systems 13 looking at all 10 discrete divertor modules will be the systems which need to ensure the integrity of the actively cooled W7-X divertor, to be installed in 2018. Being a machine safety control system, the shutter of these systems needs to remain open throughout the entire discharge.…”

“…They are viewing the plasma only through a small pinhole which at the same time forms the entrance pupil of the observation optics. 13 In cases where large observation windows were required, they were retracted as much as possible into the ports. Thereby the power loads are reduced to about 20-50 kW/m 2 in case of the Thomson Scattering 14 and CXRS observation systems.…”