1. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 1 Neutron yield measurements and absolute calibration issues in JET towards ITER requirements S. Popovichev UKAEA Fusion, Culham Science Centre, Abingdon, UK in collaboration with A. Murari, L. Bertalot, G. Bonheure, S.Conroy, M.J. Loughlin and contributors to the EFDA-JET workprogramme

2. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 2 Outline:  Introduction: ITER & JET  JET Neutron Yield Monitors: u Time-Resolved u Time-Integrated (Activation system)  Calibrations issues, results, challenges  Summary

3. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 3 Requirements, needs, potential solutions and status of ITER NFM has been already reported and published repeatedly, Requirements, needs, potential solutions and status of ITER NFM has been already reported and published repeatedly, e.g. see: G.J.Sadler, J.M.Adams et al., Varenna, 1997 M. Sasao, A.V. Krasilnikov, T. Nishitani et. al., Plasma Phys.Control.Fusion, 46 (2004), S107 K. Asai, T. Iguchi, K. Watanabe et. al., RSI, vol.75 (10), 2004 A.V. Krasilnikov, M. Sasao, Yu. A. Kaschuck et. al., NF, 45 (2005), 1503 (see also materials of 9th ITPA) The definition, design, developments, integration of neutron systems into ITER is already under way to a large extent. The definition, design, developments, integration of neutron systems into ITER is already under way to a large extent. Introduction: ITER & JET

4. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 4 Ii is very difficult to satisfy all requirements for neutron flux detectors using one type of detector. ******** Possible solution is to have several detectors and independent different techniques which will complement each other. JET is a good example of this! The complexity of choice and design of neutron yield detectors in ITER comes from very wide range of machine operation The complexity of choice and design of neutron yield detectors in ITER comes from very wide range of machine operation (in situ calibration; H, DD, Trace T and full DT (50:50) plasma) The use of different fuels leads to: The use of different fuels leads to: large variation in neutron flux (about 10 orders!) large variation in neutron flux (about 10 orders!) changes in neutron energy spectra, e.g. as does the use of different changes in neutron energy spectra, e.g. as does the use of different plasma heating techniques. plasma heating techniques. Introduction: ITER & JET

5. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 5 The JET tokamak is the most suitable test bed for the development of neutron systems due to:  its plasma parameters (and it is a ITER-like, ~1/3 of ITER)  unique capability to operate with DT fuel.  JET has a comprehensive set of absolutely calibrated neutron diagnostics making possible the cross check between “new” and JET systems. Neutron systems proposed and designed for ITER could be tested at JET. JET has already started a number of interesting developments/tests, e.g. testing and successful operation of NDD & Stilbene detectors (RF) and NE213 & CVD (ENEA, Italy) detectors during TTE-2003 and coming campaigns). Introduction: ITER & JET

6. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 6 JET Neutron Yield Monitors:  Time-Resolved: Total (2.5 MeV + 14 MeV) Neutron Yield 14 MeV Neutron Yield  Time-Integrated (Activation system): 2.5 MeV and 14 MeV Neutron Yield

7. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 7 JET Time-resolved Total Neutron Yield Monitor Detectors : 3 pairs of fission chambers - U 235 &U 238 Operate in both pulse counting and current modes Usable for neutron emissions from 10 10 to 10 20 n/s Relatively insensitive to the neutron energy Absolutely calibrated to 10% (in-situ using 252 Cf source and by activation technique) DAC system consists of:  12 “slow” data-channels (typically  t ~ 5-10 ms)  which suitably merged to single time-trace  3 “fast” windows (typically  t ~ 150  s, could be down to 20  s)

8. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 8 14 MeV Neutron Yield Monitors The Si diode detectors are employed at JET for this purpose since 1987. Several Si diodes with different surface area are installed in different locations at JET allowing the 14 MeV neutron rates from 10 13 n/s up to 10 18 n/s to be measured. The detection sensitivities of such kind of system is ~ 1 count per 10 10_ 10 12 JET 14 MeV neutron (depends on an area of diode, location in the machine, energy threshold used) The Natural Diamond (NDD) and Chemical Vapor Deposited Diamond detectors have been successfully tested at JET during TTE campaign in 2003. At present three CVD (to measure total and 14 MeV neutron yield) and one NDD are installed at JET.

9. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 9 Three CVD diamonds are installed in 2005:  A 2.5 MeV neutron monitor is built using a Polycrystalline CVD covered with a 6 LiF film detecting the alphas emitted by thermal neutron capture in 6 Li. A neutron moderator surrounds the detector. CVD is located at the level of the torus midline, approximately 7.8 m from the plasma centre.  A 14 MeV neutron spectrometer is built using a single crystal diamond. It is sitting near Main Vertical Port, approximately 4 m from the centre of plasma.  A 14 MeV neutron monitor built using a high efficiency Polycrystalline CVD which is connected to a fast electronic chain. Location is near Main Horizontal port at the level of plasma midplane, approximately 6 m from the plasma centre. One NDD is installed in the Torus Hall close to Main Horizontal port (the same detector box as for CVD3). 14 MeV Neutron Yield Monitors

10. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 10 Activation Technique  It is based on the analysis of the radioactivity induced by the plasma neutrons in selected samples.  Primary purposes:  a reliable calibration of fission chambers  to measure the 14 MeV neutron emission.  Careful neutron transport calculations are needed to relate the total neutron yield from the plasma to the neutron fluence at the sample position.  At JET, two codes have been used for transport calculations.  Only after several iterations and extended investigations the agreement between them was achieved. To simplify the neutron transport problem is beneficial to choose irradiation ends as close as possible to the plasma (e.g. JET 3Upper irradiation end)

11. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 11 JET Activation System  conventional gamma-radiation measurements >>> most widely used reactions at JET: DD neutrons - 115 In(n,n’) 115m In, DT neutrons - 28 Si (n,p) 28 AL, 63 Cu(n,2n) 62 Cu, 56 Fe(n,p) 56 Mn >>> detectors : 3 NaI, HPGe (absolutely calibrated)  The accuracy of the yields measurements is typically ~ 8-10% for both DD and DT neutrons (7% as best for delayed neutrons)  delayed neutron counting of beta-delayed neutrons from fission events ( 235 U, 238 U, 232 Th) >>> usually used to measure DD neutron yield on conditions that Y DD >> Y DT >>> detector system: 2 stations with six 3 He counters 8  At JET - 8 Irradiation ends located: in 5 octants ---> test of toroidal symmetry of neutron emission5 with inboard and outboard positions -> radial plasma position and upper and lower positions --> vertical plasma displacement

12. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 12 Introduction: ITER & JETIntroduction: ITER & JET JET Neutron Yield Monitors:JET Neutron Yield Monitors: –Time-Resolved –Time-Integrated (Activation system) Calibrations issues, results, challenges Calibrations issues, results, challenges Summary Summary

13. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 13 Calibration issues (1) The absolute calibration of FCs was first obtained in 1984 in situ using 252 Cf source. Angular distribution of FC response were measured for 4 radii R values (t total meas > 10h). The overall accuracy of the calibration ±10%. Second in situ run of calibration was performed in 1985 after some changes were made to the diagnostic disposition. The insensitivity to the neutron energy spectrum was also demonstrated using other sources ( 241 Am-Be and 14 MeV neutron generator). Although this results was recognized to be appropriate only for those machine conditions at the calibration time. Later, activation measurements were used to deliver the absolute calibration with an improved accuracy ~ ±7%. Those measurements indicated that with the installation of new heavy equipment near JET main access ports, the response of individual FC is changing considerably over the time and had become dependent on the neutron energy spectrum.

14. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 14 Fission chambers are checked against each other on the shot by shot basis and an alarm is raised if the response of one of the detectors by more than ~5% from their average. The FC monitor data are always cross checked against the data from other available independent neutron yield measurements (e.g. Neutron Profile Monitor, MPR ) Comparison with code predictions (e.g. TRANSP) and in-vessel dose rate calculations and measurements also confirms the adequacy of neutron calibration procedure. Calibrations issues (2)

15. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 15 Calibration results (1) FCs calibration check during JET TTE campaign 2003 Yn total (DD+DT) from activation, neutron/shot Yn total (DD+DT) FCs, neutron/shot

16. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 16 Calibration results (2) Si diodes calibration for JET TTE campaign 2003 Si diode, counts/shot Activation Y14, neutron/shot

17. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 17 Calibration results (3) NDD and CVD Diamond detectors response to Tritium JET shots ## 61084-61236

18. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 18 Calibration results (4) Comparison of 14 MeV Yn measured by Neutron Profile Monitor (Bicron detectors) and MPR with Si diodes data (~450 TTE shots) Yn 14 (Si diodes), neutrons/shot Yn 14 Neutron Profile monitor, n/shot Neutron Profile Monitor Magnetic Proton Recoil Spectrometer

19. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 19 Calibration results (5) Two shots with T gas puff and different DD/DT ratios: 61097: large T influx 61374: small T influx Comparison between FC(red) and TRANSP (blue) Yn total (DD+DT) FCs, 10^16 n/shot Time

20. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 20 Challenges… FCs Hardware - Did not have serious technical/hardware problems so far. 14 MeV neutron monitors. 14 MeV neutron monitors. serious problems There are two most serious problems:  radiation damage  radiation damage of Si diodes (fluence limit of ~ 10 12 n/cm2), the detectors have to be replaced periodically; dynamic range is restricted  can be used only in pulse-counting mode so the dynamic range is restricted (so saturation issues arise). Therefore JET TTE solution of this problem was to install : Therefore JET TTE solution of this problem was to install : several (4) Si diodes several (4) Si diodes with different sensitivities and CVD & NDD CVD & NDD detectors with high radiation hardness.

21. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 21 Challenges… FCs Calibrations FCs Calibrations :  fission chamber calibration can change considerably with installation of the new equipment near to FC  fission chamber calibration can change considerably with installation of the new equipment near to FC (e.g. ~17.5% change because of installation of EFCC in 2002)  the individual fission chamber calibration factor are quite sensitive to the neutron energy spectrum (e.g. different response to DD and DT neutrons). It has been noticed during PTE1 (1991), DTE1 (1997) and TTE (2003). Extensive MCNP calculations are needed to quantify these effects.

22. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 22 JET FCs TTE 2003 Calibrations Total neutron yield, (FCs average = Activation), n/shot Total neutron yield, individual FCs, n/shot FCs Total neutron yield, (Rn< 10 16 n/s i.e. mostly DD neutrons )

23. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 23 JET FCs TTE 2003 Calibrations Total neutron yield (FCs average = Activation), n/shot Total neutron yield, individual FCs, n/shot FCs Total neutron yield, (Rn> 10 16 n/s) i.e ~50-50% DD & DT neutrons)

24. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 24 JET Fission Chambers response 235 U counter response – linear energy scale 238 U counter response – 19.4 cm Pb shield

25. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 25 Summary The neutron emission from JET is reliably measured by a comprehensive set of neutron yield monitors available. These monitors can operate usefully over a wide range of neutron intensities (FCs are covering 10 decades). The absolute calibration relies on activation measurements and is performed to an accuracy of ~ 10%. Monte Carlo transport calculations have to be always performed in support (and understanding) of the calibration data.

26. S. Popovichev 10th Meeting of ITPA Topical Group on Diagnostics, Moscow, 10-14 April 2006 26 Issued to be addressed: It will need to perform the Monte Carlo Neutron transport calculations with detailed ITER model will be needed (especially for activation measurements). It will need to perform the Monte Carlo Neutron transport calculations with detailed ITER model will be needed (especially for activation measurements). Particular attention has to be given to the calibration methods/procedures of ITER neutron diagnostic (several different methods should be used) Particular attention has to be given to the calibration methods/procedures of ITER neutron diagnostic (several different methods should be used) >> Direct calibration by neutron generator is practical only before start-up when the ITER vessel will be cold. Will be this calibration valid for hot vessel? >> Direct calibration by neutron generator is practical only before start-up when the ITER vessel will be cold. Will be this calibration valid for hot vessel? >> An activation system for in situ and for routine cross calibration is strongly recommended. >> An activation system for in situ and for routine cross calibration is strongly recommended. What else? Time for discussion!