ITER diagnostic RGA's: background and current design status

1. ITER diagnostic RGA's: background and current design status Walt Gardner Task Leader, ITER Diagnostic RGA U.S ITER Project Team 26 March 2007

2. 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]

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

4. RGA measurement specifications taken from Table 4.22-2 of the ITER Project Integration Document Table rearranged to better reflect column headings

5. 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)

6. Expected measurement capabilities for the two RGA sensor types

7. 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)

8. RGA system here . Proposed 2nd cryopump system (w/containment) Torus service vacuum . . . .

9. 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)

10. 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)

12. 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)

13. 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?)

14. 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

16. Preliminary design layout of the Diagnostic RGA system Penning diagnostic & pressure gauge cluster • Not shown: • Tritium containment “box” • Magnetic shielding • Aperture, pipe from pumping duct, torus isolation valve, calibration system • Various service inputs and outputs Gate Valve Cryopump RGA head Cryopump

17. 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

18. 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

19. RF Discharge-based Optical Gas Analyzer (RFD-OGA) RF Cable Powered Electrode Insulator Tube for gas and light transmission 100 mm

20. Schematic for the RFD-OGA

21. 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

22. 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]