1、Challenges for MSE on Alcator C-ModS. D. ScottOctober, 2005Updated December 12, 20051. Calibration of an MSE channel using an APD detector. Calibrate an MSE channel in the usual way with the rotational stage. Replace the PMT with the APD. Re-calibrate. Check that the calibration with the APD doesnt
2、change if we vary the intensity.2. Why does the polarization angle measured with the WGP vary with radius? Is this effect real or does it indicate a problem with MSE? Approach: in-vessel calibration of the WGP for each MSE channel. Schedule: Jan-Feb 2006.3. Why does the measured polarization angle v
3、ary with intensity? Why is the effect of intensity most pronounced at low intensity rather than high intensity? Approach: illuminate a PMT with photodiode driven by a signal generator. Drive pre-amp directly with a signal generator. Also, see if effect goes away if the PMT is replaced by an avalanch
4、e photodiode. Schedule: December. Schedule: November3. Why is the measured PEM retardance not equal to the demand retardance? Is it due to the fact that the rays hit the PEMs at positions other than the center? Calculation: compute the average retardance for both PEMs for all MSE channels by launchi
5、ng rays from a given MSE channel (i.e. the backlit image area), recording where these rays hit the PEMs, and then assume that the retardance is parabolic along one axis of the PEM and independent of position along the other axis. Compare these computed retardances with the measured retardances can w
6、e reproduce the channel dependence, which shows a lower retardance for core channels (7,8,9) than edge channels (0,1,2). Schedule: November4. How do mirror reflections affect the rotation of polarization through the MSE optical system? Approach: ray-trace individual channels, recording k-vector and
7、mirror normal at each mirror for all rays passing through lens L1. Apply formula (to be derived!) that determines the rotation of polarization for each ray as a function of the k-vector and mirror normal. Question #1: can we understand the channel-to-channel variation of the rotation, i.e. the fact
8、that different channels observe 2-3 degree differences in polarization angle for the same incident direction of polarization. For this calculation, we should launch rays from the entire viewing area of a given channel. Question #2: is this effect (rather than variation of PEM retardance) responsible
9、 for the variation in angle we observe when we move the light source within the viewing area of a single channel? Schedule: limited by derivation of the correct formula. Should take only a few days, once formula is derived. 5. How much could we increase the MSE signal strength by replacing the PMTs
10、with avalanche photodiodes? How much will the statistical error decrease? Tasks:o Select APD vendor and place order for one APDo Purchase pre-amp from Nova Photonics.o Disassemble one spare PMT assembly to see how it is constructed.o Disassemble assembly from Levinton.o Design optical system (2) no
11、magnetic field; and (3) no tokamak-induced noise, power supply droop, interference with PEMs, etc. then we have effectively ruled out all of these potential issues as explanations of the MSE calibration difficulties.6. Spectrum measurements Objective: confirm that the spectrum matches the expected s
12、pectrum, and get information about ratios of upper state populations. Approach: Use long-pulse DNB to obtain improved spectra during beam-into-gas and beam-into-plasma at a few values of the TF / EF. Details to be determined with Bill Rowan. Schedule: as soon as DNB interlock becomes available. Hope
13、fully, this can be done this run period, i.e. before January 5, 2006. Proposed Procedure to Calibrate the WGP In-vesselOne of two things are true:1. The WGP generates polarized light with about the same polarization angle along each of the 10 MSE channels, but MSE incorrectly measures this angle, th
14、ereby generating the a measured angle that varies with channel number; or 2. For reasons we dont understand (and contrary to the lab measurements), the WGP generates different polarization angles along each of the 10 MSE channels, and MSE correctly measures these angles. So I think we should perform
15、 an in-vessel calibration of the polarization direction of the WGP for each of the 10 MSE channels. Approach:1. Position the WGP in front of L1. 2. Remove the linear polarizer from the MSE diagnostic, and turn off the PEMs. 3. Mount the rotational stage with circular LED array etc. on the optical be
16、nch along the DNB trajectory as usual. 4. Backlight a selected MSE channel to position the center backlit image at the center of the rotational stage. Also, using the alignment fixture, aim the rotational stage directly at lens L1. 5. Perform a 36- or 72- point measurement as the rotational stage go
17、es through 360 degrees and take data with MSE. But we will use only the amplitude, i.e. we will effectively determine the angle at which the measured intensity reaches a maximum.6. Now insert the MSE linear polarizer and turn on the PEMs. Perform a 36- or 72-point measurement as the rotational stage
18、 goes through 360 degrees and take data with MSE. MSE should measure the same angle irrespective of the angle of the rotational stage (except that when the intensity gets very small, when the rotational stage transmission axis is nearly perpendicular to the WGP transmission axis, we may get a small
19、change due to the known variation of measured angle with intensity). Before step 7, mark the current position of the shutter on the MSE turret, so that when we make measurements in the future, we can return to exactly the same position of the WGP.7. Rotate the shutter so that lens L1 views the torus
20、 unobstructed, i.e. move the WGP polarizer out of the way. Repeat step 6, i.e. perform the usual in-vessel calibration of the channel. 8. Now do an in-vessel check of the angle-of-incidence effect: with the WGP still out of the way, rotate the entire rotational stage about its vertical axis so that
21、it is no longer pointed directly at lens L1. If possible, it would be great if we could rotate it by 10 or even 15 degrees. Do another 36- or 72- point measurement with the rotational stage. Ideally, we would take measurements at a rotational angle of say 5, 10, and 15 degrees. 9. Return the WGP to
22、its original position, i.e. to the fiducial mark as recorded just before step 7. Repeat steps 4-8 for each of the MSE channels or a subset of the MSE channels, as time permits. If time is limited, we should do the edge channel (0), a central channel (3 or 4) and channel 8.If time is limited, we prob
23、ably cant all of the proposed measurements for all of the channels. Of the various measurements, step #5 is more important than 6, 7, and 8. Step #6 is probably the least important, since we expect a null result. If in fact we see a null result on the first channel we measure, i.e. no change in angl
24、e as the rotational stage is rotated, then there would be little reason to repeat step #6 for the other channels. MSE Calibration Issues Studied to DateMost of the issues discussed in this list have been described in Howard Yuhs doctoral dissertation. Yuhs dissertation is always a good place to star
25、t. Quantum mechanics: effect of adding B to E=v x B on Stark EffectMemo 34: Analysis of Breton anglesMemo 39: Polarization angle for combined MSE + ZeemanMemo 40: computing intensity in e1 + e2 directionMemo 41: elliptically polarized emissionMemo 42: effect of Zeeman splitting on MSE analysisMemo 4
26、6: Pressure and magnetic field in long pulse DNBAPS 2004: Invessel Calibration of the Alcator C-Mod MSE DiagnosticDNB meeting (11/2004): Effect of Zeeman Splitting on MSE Analysis Retardance of photoelastic modulatorsMemo 7: Is the retardance correct?Effect of variations in PEM retardance (October 2
27、005) Polarized impurity radiation from beam excitationMP436: Tests of fluorine and molecular emission contamination of MSE signals and calibrationMP436 Summary (September 2005) Beam-excited molecular D2 emission during beam-into-gas calibrationMP436: Tests of fluorine and molecular emission contamin
28、ation of MSE signals and calibrationMP436 Summary (September 2005) Non-ideal mirror propertiesMemo 2: Effect of polarizer angle on MSE analysisMemo 20: Proper treatment of effect of mirrorsMemo 21: Angle of pi radiation in MSE reference frameMemo 23: More geometry Scattered light along MSE optical t
29、rain Phase shift between PEM drive signal and measured Fourier components of the MSE signalMemo 35: Calculation of MSE offsets Faraday rotation in MSE optics and results of wire grid polarizerMemo 36: Axial magnetic field from two permanent ring magnetsMemo 37: Invessel Verdet MeasurementsConsiderat
30、ions on MP to measure Faraday rotationFurther analysis of data from MSE invessel linear polarizer (May 2005)Inferring Faraday rotation with the wire grid polarizer (July 2005)Faraday rotation measurements from July 18 (July 2005)Results from WGP study of September 16 (September 2005) Finite bit A/D
31、resolution at low signal amplitudesEffect of finite bit resolution of A/D on MSE analysis Effect of circularly polarized lightMemo 5: Effect of circularly polarized light on MSE analysisMemo 33: correcting for circularly polarized light in mse_2020 Effect of Zeeman splitting on thermal H.Memo 14: Th
32、ermal Dalpha emission due to CX on beam ions Faraday rotation including effect of ripple on MSE lens assembly L3Memo 37: Effect of TF ripple on MSE lens L3 Apodization effects arising from short analysis durationsEffect of Aliasing on MSE analysisApodization effects on MSE analysis (may 2003)More on
33、 apodization effects on MSE analysis (May 2003) Effect of finite entrance slits on tilted bandpass filtersMemo 25: Effect of finite entrance slit on tilted bandpass filters Bending of DNB due to reionization in drift duct and fringe fieldsMemo 29: Effect of torus pressure on MSE calibrationPressure
34、and Magnetic field in long pulse DNB (March 2005) Effect of sigma contamination of pi line.PresentationsMSE Data Analysis 4 August 2003.ppt Proper treatment of subtracting background noiseMemo 4: How to subtract background lightMemo 9: corrected background subtractionImproved noise suppression in MS
35、E analysis (November 2002) Digitizal FFTs replacing lock-in amplifiers to provide measurements of PEM retardance and circularly polarized light Polarization fraction of measured MSE signalMemo 3: how to compute polarization fractionMemo 13: How to compute polarization fraction with background Effect
36、 of non-normal incidence at the PEMsMemo 1: Effect of non-normal PEM incidenceMemo 18: Linear polarizers and flat glass plates Effect of spatial resolution on MSE analysis for edge channels which see a large geometric multiplier between polarimeter angle and magnetic field pitch angleMSE spatial res
37、olution and effect on analysis (July 2004) Effect of temperature on temperature-tuned bandpass optical filtersFilter swap temperature scan (April 2004) Relationship between pitch angle and polarization angle in MSE reference frameMemo 32: The relation between real and MSE invessel optics pitch angle
38、s Understanding polarization angle of light transmitted through a linear polarizer at non-normal incidenceMemo 48: Linear polarizers at oblique angles of incidenceMemo 49: Tilted linear polarizers at oblique angles of incidenceMemo 49c: Tilted linear polarizers at oblique angles of incidence Invesse
39、l MSE calibrations using a linear rotating stageDesign of MSE calibration fixtureRevised MSE calibration based on GA experienceMemo 26: Reflections near the Brewster angleMemo 19: Preliminary polarizer calibration resultsMemo 27: Polarizer calibration of April 28-29Memo 30: Initial results from In-v
40、essel MSE calibration (June, 2004)MSE Calibration Preliminary (January 2005)Final MSE calibration Results (February 2005)Revised final MSE calibration results (February 2005) Measurements of filter bandpass responseMemo 24: Analysis of beam-into-gas (no TF) Results of beam-into-gas calibrationsMSE c
41、alibration 2003 preliminary (May 2003)MSE calibration 2003 (June 2003)Summary of MSE calibration of January 21, 2004Summary of MSE calibration of March 4, 2004Revised summary of MSE calibration of March 4, 2004 MSE measurements in plasmaUnusual behavior of measured polarization angle profile at high
42、 density (October 2002)Shot-to-shot scatter increases with plasma density (October 2002) Nonlinear dependence of MSE signal strength on DNB currentMSE signal strength vs dnb current Mechanical problemsDisruption stresses on mirrors (December 2002)Proposal to use metallic mirrors for MSE (December 20
43、02)MSE mirror repair (January 2003)More on Metallic Mirrors (February 2003) MSE spatial resolution and DNB aperturePreliminary DNB aperture proposal (February 2003)DNB size: what we can do about it (April 2003)DNB collimating aperture: preconceptual design (April 2003) Loss of MSE signal intensityMSE intensity study (June 2005)Effect of PMT voltage on sensitivity (July 2005)Recent studies of MSE signals (July 2005)
