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Advanced Neutron Diagnostics for MAST

This study focuses on advanced neutron diagnostics for MAST, including neutron emission, neutron sources, fast ions, and neutron cameras. The aim is to understand the behavior of fast ions, their impact on plasma instabilities and heating efficiency, and develop a neutron diagnostic system.

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Advanced Neutron Diagnostics for MAST

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  1. Study of advanced neutron diagnostics for MAST M Cecconello1, S Conroy1, G Ericsson1, M Weiszflog1, R Akers2 and M Turnyanskiy2 1EURATOM/VR Association, Department of Physics and Astronomy Division of Applied Nuclear Physics, Ångström Laboratory, Uppsala University, Sweden 2EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, UK

  2. An appetizer from JT60U Fast ions, neutron emission and neutron diagnostic in MAST Proposed neutron camera for MAST and MAST Upgrade TRANSP neutron sources used in MCNP MCNP modelling and some results Collimators, detectors, DAQs and magnetic compatibility A look at the near future and conclusions Outline

  3. Redistribution of fast ions due to TAE in JT60U M Ishikawa et al Nucl. Fusion 47 (2007) 849–855

  4. Neutron emission on MAST D + D Þ3He (0.82 MeV) + n (2.45MeV), Q = 3.27MeV Neutron rate Sn = 1 – 10 x 1013 s-1 Most of the fusion neutron production is due to beam-thermal reactions the beam-beam term accounts for 10 -20 % of the total while the thermal-thermal is negligible. Effect of NBI heating and induced toroidal rotation: reduction in the beam-plasma interaction energy (due to the relative velocity of beam-thermal), leading to a reduced fusion reactivity. Energy shift of the neutron spectra.

  5. Fast Ions and Neutron Emissivity The neutron emissivity profile is strongly dependent on the fast ion spatial and energy distribution. TRANSP simulated poloidal projections of co passing fast ion distributions with V||/V~0.7-1 with and w/o anomalous fast ion diffusion TRANSP simulated neutron source

  6. Fast Particles Studies on MAST Study of fast ions driven modes is important to understand the redistribution and/or losses of fast ions that can affect the heating efficiency, the stored energy and cause damage to the PFCs. Document fast particle driven collective instabilities AE: TAE, EAE, BAE, Alfvén cascades, high frequency CAE Energetic Particle modes: chirping modes, long-lived modes Effects of fast ions upon core instabilities Typically n = 1 internal kink: sawteeth, fishbones, infernal mode Use on/off axis beams as source of fast ions Exploit specific capabilities of MAST Super-Alfvénic beams, VNBI ~ 2.5 VA Large fast ion fraction with high β (above ITER values) Externally driven modes (TAE antenna), n = 1 – 3

  7. Sn (x 1013 s-1) t (s) PNBI = 1.5 MW [1] M.P. Gryaznevich et al Nucl. Fusion 48 (2008) 084003 Example of instabilities and neutron yield on MAST Long Lasting Modes during NBI and current ramp-up “The LLM does not seem to cause notable plasma energy degradation in MAST discharges. However […] changes in the LLM frequency, possibly indicat[e] an enhancement of fast particle losses.” [1]

  8. MAST Present Neutron Diagnostic 235U Fission Chamber (from PPPL)1 - pulse (5 ms) and current (10 μs) modes - Campbelling mode (0.5 ms) - calibrated with a 252Cf neutron source and backed up by activation foil measurement - no saturation expected with the increased neutron rate from NBI upgrades Used for total emission strength measurements (fusion reaction rate) What have studied the possibility of: relative emissivity profile Þtransport properties & MHD study the energy spectra Þreacting ions energy distribution 1(Planned installation of a spare 235U and a 238U for neutrons with En > 1 MeV.)

  9. Proposed neutron diagnostic for MAST and MAST U Camera system consisting of: Horizontal fan of 14 lines of sight Vertical fan of 9 lines of sight Collimator with liquid scintillator detectors Radial position of the detectors between 3 and 4 m Preferred location: MAST NPA location

  10. Horizontal lines of sight

  11. neutron camera Horizontal lines of sight and PINI injectors MAST Vessel CFC Beam Dumps MAST S-WEST PINI Injector (2.5MW/5s) MAST SW Injector (2.2MW/0.4s) R = 4 m MAST SOUTH PINI Injector (2.5MW/5s)

  12. Vertical lines of sight MAST MAST Upgrade Massive PF coils in the way of a vertical stack of lines of sight.

  13. TRANSP Simulation of MAST Neutron Source Pulse 18821 @ 0.25 s Quasi-stationary H-mode plasma: PNBI = 3.2 MW (co-NBI) Ip = 0.6 MA Te» Ti» 0.8 keV Neutron emissivity radial profile (cm-3s-1)

  14. Synthetic MAST Neutron Source Neutron emissivity radial profile (cm-3s-1)

  15. TRANSP Simulation of MAST U Neutron Source Scenario C (long pulse operation) PNBI = 10 MW (1 PINI co, 1 PINI counter, 2 PINIs off-axis co) Ip = 1.2 MA Te» 2.4 keV Neutron emissivity radial profile (cm-3s-1)

  16. MCNP Model of MAST and of the Neutron Source Horizontal section Vertical section Neutron Source Volume sampled for neutron spectra.

  17. MCNP Neutron Flux and Spectrum Calculations MCNP calculates: the flux of neutrons per MAST neutron neutron flux (cm-2s-1) the energy spectrum of the flux of neutrons per MAST neutron

  18. Source Neutron Energy Spectrum For Maxwellian distributions, the energy distribution of the neutrons is very nearly Gaussian

  19. Neutron Flux throughout MAST Area (midplane) log10(GMCNP)

  20. Horizontal Lines of Sight & Neutron Emissivity MAST 14 13 12 11 10 9 8 7 6 5 4 3 2 1

  21. Horiz. Lines of Sight & Neutron Emissivity MAST U 14 13 12 11 10 9 8 7 6 5 4 3 2 1 For MAST Upgrade the same lines of sight are used.

  22. L D Neutron Collimator and Shielding for MCNP Polyethylene sphere: L = 50, 70 and 90 cm D = 11.24 and 35.68 mm A = 1 and 10 cm2 Detector located at the sphere centre Detector area defined by the collimator diameter Collimator cross-section is circular

  23. Horizontal Lines of Sight: Field of View @ R = 3 m L = 50 cm, A = 10 cm2 L = 90 cm, A = 1 cm2 radial field of view width » 8 cm radial field of view width » 40 cm

  24. Collimated Neutron Energy Spectrum @ R» 3.2 m L = 50 cm, A = 10 cm2 MCNP Line of sight # 6 L = 90 cm, A = 1 cm2

  25. Horizontal Collimated Neutron Fluxes @ R» 3.2 m L = 90 cm, A = 1 cm2 central column shadowing

  26. MCNP Direct Fluxes and Line Integrated Emissivity Line integrated emissivity along the line of sight Direct neutron flux (no neutron scattering)

  27. Horizontal Collimated Neutron Fluxes @ R» 3.2 m Neutron flux components total direct backscattered shield scattered

  28. MAST Vertical Lines of Sight and Field of View L = 90 cm, A = 1 cm2, R = 3 m vertical field of view width » 4 cm

  29. Vertical Collimated Neutron Fluxes @ R» 3.2 m L = 90 cm, A = 1 cm2

  30. 10 x A , 10 x Sn 10 x A , Sn A = 1 cm2, Sn = 1013 s-1 Expected Count Rates on MAST e detector efficiency

  31. Count Rate and Time Resolution on MAST Assuming a complete camera system, the count rate required for a neutron emissivity profile measurement with a Poissonian statistical uncertaintyd = N-1/2 and a time resolutionDt is: Dt = 1 ms, d = 0.1 ÞC ³ 0.1 MHz With the present system (»3 MW NBI power)we can push the time resolution to 10 ms with a statistical uncertainty of 10 % (central channels), by using a vertically elongated collimator. This is NOT as an uncertainty of 10 % on the emissivity profile! For MAST Upgrade a the time resolution of less than 10 ms, with d = 0.1, might be achievable due to the much higher expected NBI power.

  32. Different Collimator Cross-Sections circular elongated L = 90 cm, A = 1 cm2 L = 90 cm, A = 4 cm2

  33. Neutron Collimator and Shielding Need of neutron collimation, shield against scattered neutrons and against g rays emission associated with capture of thermal neutrons by surrounding materials (neutron shield, magnetic shield, support structure). Polyethylene (CH2) 2.23 MeV g are produced by H(n,g)D Boron loaded Polyethylene 0.48 MeV g are produced by 10B(n,a)7Li reduction of the 2.23 MeV gs Both give rise to significant g-background in the scintillator. MCNP estimate of g-background not yet performed.

  34. Neutron Detector Liquid scintillators BC-501A (NE213) from Saint-Gobain Crystals Neutron Þ Recoil protons Þ photons High efficiency (5 %) and high count rate capabilities Good PSD capabilities for n/g discrimination Calibration for absolute neutron yield measurements Response function for neutron spectroscopy (energy calibration) Good contact with Physikalisch-Technische Bundesanstalt (2.5 MeV monoenergetic neutron sources, g sources). Requires temperature control for stability and flash point (< 25 °C) Hamamatsu R762 PMT SiPM (SensL) Insensitive to B field LED Gain stabilization Temperature monitor

  35. Data Acquisition Count rate of analog PSD systems is restricted by the gate integration time of the electronic circuit to 105 kHz. PMT Fast Amplifier Flash ADC Flash ADC with 1Gsamples memory, 10 bit resolution Sampling rate of 200 MHz (Dt = 5 ns), 8 GHz maximum Individual pulses are recorded (Dt = 350 ns) Raw data transfer in PC connected to flash ADCs after acquisition is finished Neutron/g discrimination and post processing is carried out via software on the PC

  36. B (T) Magnetic Field at Detector Location in MAST Poloidal field is mainly perpendicular to the PMT side MAST Bf < 1 mT MAST Upgrade Bf < 2 mT Magnetic shielding not too difficult.

  37. What happens next ? 2008 - 2009 Proof-of-principle at MAST Single line of sight for radial and vertical scans Test of different collimator geometries Neutron emissivity profiles measurements EFDA Priority Support: 0.33 ppy and 156 k€ for development 2009 Finalize design of neutron camera for MAST Extensive MCNP simulations Shielding and support, DAQS development Emissivity profiles from inversion of neutron camera data 2009 Application for partial funding to the Swedish Research Council (VR) Application in April, answer in November 2010/2011 ? Installation and Operation

  38. Sn (x 1013 s-1) t (s) Neutron Camera on MAST and MAST U Reduced time resolution but spatially resolved neutron emissivity profiles Fast ions spatial distribution Reacting ions energy distribution function

  39. Conclusions Single line of sight proof-of-principle for MAST is doable and can provide and indication of the neutron emissivity profiles in a reasonable amount of pulses. Neutron camera with L = 90 cm, A = 10 cm2 is required for achieving in MAST high performing plasmas a time resolution of 10 ms with enough spatial resolution (at least 10 horizontal LoS, 9 vertical LoS) and sufficient statistic (10 % uncertainty). In MAST Upgrade, depending on the total NBI power available, the time resolution can be pushed below 10 ms (10 % uncertainty). In principle the same collimator structure can be used for both MAST and MAST Upgrade Liquid scintillators with DPSD seem ok Acknowledgments Special thanks to B Lloyd, G Cunningham, N Conway, M Dunstan, R Scanell, M Walsh and the MAST team.

  40. MAST Vertical Stack Not a viable solution due to PF coils presence.

  41. Different Neutron Emissivity Profiles compared Line integrated neutron emissivity MAST 18821 MAST Synthetic

  42. Digital Pulse Shape Discrimination Post-experiment data processing pile-up reprocessing, dedicated n/g separation simultaneous n/g discrimination and PHA correction for PMT gain variations g n Y. Kaschuck, Nuclear Instruments and Methods in Physics Research A 551 (2005) 420–428

  43. counts pulse height Liquid Scintillator as Neutron Spectrometer source measured recoil proton edge Courtesy of A. Zimbal Physikalisch-Technische Bundesanstalt Compton electron edge The energy spectrum of neutrons provides information on their production mechanisms and the energy distributions of the reacting ions.

  44. Liquid Scintillator Energy Calibration Low Z material: g interactions are Compton only L(Eg) = L (En) but Eg» 2-3 En En : L(En) = L(Eg) electron energy equivalent L(E) is linear in Ee non linear in Ep Compton electron recoil proton eeeMeV

  45. MAST U Vertical Lines of Sight and Field of View L = 90 cm, A = 1 cm2, R = 3 m vertical resolution » 4 cm

  46. Magnetic Field at Detector Location in MAST Upgrade B (T) The poloidal field is mainly perpendicular to the PMT side The toroidal field is negligible (< 2 mT)

  47. Photo-multiplier Magnetic Shielding Outer shield: soft iron (1 mm thick, rin = 23 mm) Inner shield: m-metal (0.8 mm thick, rin = 21 mm) Shielding factor up to 104 B(T) 150 mT 0.2 mT

  48. Neutron Energy Spectrum outside MAST Vessel

  49. TRANSP Simulation of MAST Neutron Source - 3 Pulse 18808 @ 0.28 s PNBI = 3.5 MW (co-NBI) Ip = 0.6 MA Te» Ti» 1.1 keV

  50. Counting modes Pulse Mode Current Mode Campbelling Mode

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