Discussion of Engineering Activities for C-Mod MSE upgrades Plasma Science & Fusion Center July 10, 2008 File: MSE-design-overview

Reduce spurious variability in polarization angle measured by MSE due to thermal stress-induced birefringence to < 0.05 o (in MSE frame of reference) this meeting by reducing temperature excursions, and/or thru in-situ, before/after shot calibration. Provide remote & reliable capability to open & close MSE shutter this meeting Provide means to measure polarized ‘background’ light emission in real time. Increase photon-gathering power of MSE If intensity calibrations identify a particular ‘culprit’ for loss of light. Improved spatial resolution (FY09-10). Overall MSE objectives, FY08-10 File: MSE-design-overview

Reduce temperature excursions of in-vessel MSE Optics File: MSE-design-overview Required physics ‘pre-analysis’ & lab measurements Allowable temperature excursion and ramp rate (1 o C / hour?). Do same specifications apply to L2 and L3? Is any temperature control required at L1? Specify some thermal ‘scenarios’ that temperature-control system must handle, e.g. torus cooling/heating; ECDC; plasma heating. Design analysis issues Is cooling/heating at periphery of lenses sufficient, or must we also control the radiation environment, i.e. control temperature of entire canister? Selection of tubing & means of attaching it to lenses. Disruption tolerance. Selection of coolant. Temperature monitoring, esp. invessel thermocouples. Consideration of copper plating to reduce temperature gradients.

Reduce temperature excursions of in-vessel MSE Optics, cont’d File: MSE-design-overview Design analysis issues Interface thru port flange. External heat exchanger. Control & data acquisition. Computer interface. Testing TBD

In-situ, before/after shot calibration system File: MSE-design-overview Major design objectives Calibration accuracy to 0.05 o, within ~15 sec of a shot. We probably require calibration at two polarization angles. Reliability: use on 50% to 75% of all C-Mod shots in a run campaign, i.e. about cycles between servicing. `Failure not an option.’ Remote shutter to protect lens L1 against boronization. Two basic design options Fixed system: polarizers at periphery of lens L1. Articulated system: translate a polarized light source into MSE field-of-view. Should provide a spatial or polarization ‘reference’ to compensate for small, uncontrolled movements of the mechanism. Option A: moving element is a mirror; fixed light source. Option B: moving element is a full polarized light source.

In-situ, before/after shot calibration system File: MSE-design-overview Major engineering challenges Disruption forces Provide articulated push/pull thru vacuum interface Limited space & mechanical interferences Heating by plasma Vacuum compatibility Thermal expansion. Provide illumination source through vacuum interface Temperature enviroment -20 to +120 Celsius?

In-situ, before/after shot calibration system File: MSE-design-overview Physics ‘pre-analysis’ Fixed system Easiest to implement. No moving parts  attractive solution. But … does a fixed system based on a polarized light source only at the periphery of lens L1 provide sufficient accuracy? Would not fully mimic light pattern from DNB, but maybe good enough. Tasks: lab measurements + optics calculations. Option A: moving mirror. Is required positional stability of mirror any less onerous than corresponding stability of Option B (= moving polarized light source)? Work: lab tests. Optics calculations to specify mirror shape & location of polarized light sources. Option B: moving polarized light source. Fully mimics light pattern from DNB. Work: lab tests & optics calculations to verify that two polarization angles are necessary & sufficient.

Remote shutter capability File: MSE-design-overview Our present manual system is inadequate. Requires manned access to cell. Completely incompatible with between-shots boronization. Has worked poorly: failed to provide access to all three positions (open, closed, linear-polarizer) in two recent run campaigns. An in-situ calibration system will incorporate a remote shutter. Note that we need to protect both lens L1 and the polarized light source. If we choose not to install an in-situ calibration system, we still need the remote shutter capability. The pneumatic mechanism developed for the polarimeter is a good basis for the MSE remote shutter and/or the articulated in-situ calibration mechanism.

Capability to measure polarized background light We observe significant levels of polarized background light in C-Mod. Glowing hot surfaces + plasma emission. Varies on a rapid time scale. Interpolating ‘beam-off’ periods does not provide adequate accuracy. Seems to be broadband emission. It is ‘straightforward’ to measure background in real time. Sacrifice ~4 of 16 fibers. Cost about $5k / channel. Might have to rework fibers in ferrules.    emission from DNB broadband, polarized plasma emission MSE optical filter (existing) background optical filter (proposed)

Improve light-gathering power The MSE light emission levels have always seemed weak. Recent intensity calibration suggests we are realizing only 10-20% of expected photon flux. Previous measurements by Howard Yuh indicated that the basic MSE optics (not including PEMs, linear polarizer, filter, or fibers) was ~80% transmissive. Fibers are ~20 years old. Left over from TFTR. If we can identify a particular element that is faulty, we will repair / replace it. This work should not interfere with any other upgrade.

Improve spatial resolution In various FWP and 5-year plans, we have promised improvements to the spatial resolution of MSE. Size of DNB is most important. More spatial channels is a secondary consideration. I am reluctant to invest in more spatial channels until we prove that the schemes to eliminate spurious effects of temperature-induced birefringence are eliminated. Increasing number of spatial channels requires either a major re-design of the optical relay system – to allow a larger fiber dissector & more room for fibers; or greatly increased photon-gathering power, to allow us to use 1 x8 rather than 2 x 8 fiber arrays.

Issues Schedule Division of responsibilities, PPPL vs PSFC