o ak r idge n ational l aboratory u. s. d epartment of e nergy 1 iter diagnostic rga's:...

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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

ITER diagnostic RGA's: background and current design status

Walt Gardner

Task Leader, ITER Diagnostic RGA

U.S ITER Project Team

26 March 2007

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W. L. Gardner 26-Mar-07

ITER diagnostic RGA’s

Prior to becoming a part of the U.S. package the diagnostic RGA’s were ill defined at best

RGA’s were thought to be straightforward. This is not true in the ITER environment

The diagnostic RGA systems are considered to be “group 1a* diagnostics: those needed for machine protection and basic machine control”

* The machine is unable to operate without a working diagnostic providing every group 1a parameter (1b for advanced operation). -- ITER PID v3.0 [adopted from the ITPA diagnostics WG]

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Seventeen potential locations were identified at a kick-off meeting in Cadarache (11-Oct-06)

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RGA measurement specifications taken from Table 4.22-2 of the ITER Project Integration Document

RESOLUTION

MEASUREMENT PARAMETER CONDITION RANGE or

COVERAGE Time

or Freq.

Spatial or Wav e No.

ACCUR ACY

16. Divertor

Operational Parameters Gas Composition

Before, during and after discharge

A = 1 – 100 amu 1 s A = 0.5 amu Several points

20% during pulse

18. Gas Composition in Main Chamber Gas Composition

During conditioning; prior to, during and after discharge

A = 1 – 100 amu 10 s A = 0.5 amu

Several points 50% during pulse

19. Gas Composition in Ducts Gas Composition

Before, during and after discharge

A = 1 – 100 amu 1 s A = 0.5 amu Several points

20% during pulse

RESOLUTION MEASUREMENT PARAMETER CONDITION RANGE or

COVERAGE Time or Freq.

Spatial or Wave No.

ACCUR ACY

16. Divertor Operational Parameters

Gas Composition

A = 1 – 100 A = 0.5

TBD 1 s Several points

20% during pulse

18. Gas Composition in Main Chamber

Gas Composition

A = 1 – 100 A = 0.5

TBD 10 s Several points 50% during pulse

19. Gas Composition in Ducts

Gas Composition

A = 1 – 100 A = 0.5

TBD 1 s Several points 20% during pulse

Table rearranged to better reflect column headings

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Will need more than one sensor type

Conventional RGAs [quadrupole mass specs (QMS)] Pros: Will cover the specified mass range, readily available, good

general mass spectra libraries, adequate time response in multiplex mode

Cons: Cannot resolve He & D2 peaks, slow in scan mode, poor resolution of hydrogenic species peaks

Penning Gauge Diagnostic (PGD)* Pros: Good hydrogenic peak resolution (H, D, T), separate He peak,

fast response (few ms) Cons: Sensitive only to species with optical emission peaks, hence,

no information on CxDy, CxTy & CxDy-zTz

Both the QMS and PGD diagnostic are currently being considered as complementary sensors within the diagnostic RGA system.

* Currently part of the Pressure Gauge package (EU)

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Expected measurement capabilities for the two RGA sensor types

Parameter QMS PGD Operating pressure range 10-12 – few x 10-2 Pa (FC &

SEM) Few x 10-4 – few x 10-1 Pa

Minimum partial pressure ≈ 10-7 Pa (FC); ≈10-12 (SEM) ≈ 10-4 Pa Peak resolution 1 AMU (peak width at 10%

height) 0.2 Å

Mass range 1-100 AMU Limited to species with optical emission lines

Time response Up to 0.5 ms/AMU Few milli seconds Magnetic field TBD TBD Operating temperature (max.) 150 ˚C (200 ˚C) 150 ˚C (200 ˚C)

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Issue: Pressure Operating pressures of up to 20 Pa (150 mTorr or

0.2 mbar) in the divertor region are greater than can be tolerated by either a QMS (few x 10-2 Pa) or a PGD (few x 10-1 Pa).

Hence, RGA’s monitoring the divertor must be differentially pumped

Recommend differential pumping for other RGA’s to perform calibration, operate during discharge cleaning, and complement the leak checking system

A preliminary design already exists for a [Type 2] Diagnostic Vacuum Pumping System (T2DVPS)

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RGA system here

Torus service vacuum

Proposed 2nd cryopump system (w/containment)

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..

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Issue: Radiation

Very high radiation fields can degrade ceramic insulators over time

High radiation fields can degrade QMS solid state femtoamp high-gain amplifiers, which must be located near to the Faraday cup and SEM sensors. [Could use vacuum tubes and other radiation resistant components*]

The fiber optic for the PGD is susceptible to the formation of color centers in a high radiation field, which degrades transmission. Need to continuously anneal the fiber at 250˚C. In high neutron flux areas annealing will not work.

Tritium beta decay will cause a background shift in the QMS secondary electron multiplier; may only be a problem if using it for leak checking purposes

* Demonstrated on JET: R. Pearce, et al., Vacuum, 44(5-7), pp. 643-645 (1993)

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Issue: Magnetic field Theoretical evidence* that the sensitivity of QMS-

based RGA’s degrades significantly while resolution improves in fields > 0.05 T parallel to the mass filter axis

For transverse fields** an increase in resolution is measured for B = 0.017 T. However, enhanced sensitivity was observed and is attributed to flux leakage into the ionizer

There appear to be no studies on the effects of magnetic fields on the PGD [Small R&D activity]

Need to determine smallest acceptable field in relevant quadrupole geometry in order to calculate shielding needed. [Small R&D activity]

* Tunstall, J. J., et al., Vacuum 53, 211-213 (1999)** Srigengan, B., et al., IEE Proc.-Sci. Meas. Technol., 147(6), 274-278 (2000)

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Time response as a function of length for various diameters and masses

Appears to be adequate response for hydrogenic species when sampling the divertor ducts (length ≈ 5-7 m)

Slower response if sampling the divertor directly (L ≈ 13 m), the equitorial ports (L ≈ 10 m), and the upper ports (L ≈ 12 m). Not so good for higher masses

Response time = pipe vol (l)÷conductance (l/s)

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One proposed area to place an RGA system for monitoring pumping ducts

Cryopump

300-mm diameter pipe

Area proposed for RGA system (on skid or cart?)

●

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Other issues/considerations

Calibration RGA’s require periodic calibration. Once installed it would be

impractical to remove these systems to a central calibration facility. In-situ calibration to known gas mixtures and pressures is the preferred method

Mechanical QMS’s are sensitive to vibration

Electrical/Electronic Tore Supra experience is that their QMS is particularly

susceptible to RF noise from ICRF Thermal management Installation Maintenance

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Cryopump

Gate Valve

RGA head

Penning diagnostic & pressure gauge cluster

Preliminary design layout of the Diagnostic RGA system

Cryopump Not shown: Tritium containment “box” Magnetic shielding Aperture, pipe from pumping duct, torus

isolation valve, calibration system Various service inputs and outputs

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Can a better diagnostic be developed for measuring divertor gas composition? Desire local measurement of gas

composition at pressures up to ~20 Pa, in magnetic fields of several Tesla, and in high radiation flux

Recent interest from pellet fueling team in knowing T:(D+T) ratio on <100 ms time scale to control firing of T pellets vs. D pellets

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Possible solution: Capacitive RF discharge based device Japanese group has operated an RF glow discharge up

to 2T at P=100 mTorr (13.3 Pa) [Kaneko, T., et al., JJAP, 44(4A), 1543-1548 (2005)]

Optical signal could be collected (mirror array) and analyzed in manner similar to the PGD

Should be able to operate with only 10’s of watts of RF power and pre-discharge voltages of ~100 V.

Because RF impedance changes with pressure, one could use a calibrated impedance monitor to measure pressure (complement to other pressure measurements?)

Simple construction with radiation tolerant materials is possible

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100 mm

RF Cable

Insulator Powered Electrode

Tube for gas and light transmission

RF Discharge-based Optical Gas Analyzer (RFD-OGA)

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Schematic for the RFD-OGA

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Possible location under the divertor dome

Locate RFD-OGA discharge chamber on shelf (200 mm wide x >450 mm long x >200 mm high)

RFD-OGA light output could piggyback on Impurity Monitor mirror system in this case

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Summary

Have had and are having discussions with ITER - Cadarache on nailing down real estate and specifications

Have identified issues and are working to resolve those [no apparent show stoppers, some modest development needed]

Have generated a design schematic and rough layout of the RGA system

Have produced an “advanced” RGA [RFD–OGA] concept for monitoring gas composition in the divertor to control pellet injector firing (isotopic control) [if strong ITER interest, concept needs R&D $’s and place(s) to test concept]

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