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 < 0.05o (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 (1o 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 0.05o, 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 1000-1500 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 opticalfilter (existing)

background opticalfilter (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