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Neutron diagnostics for fusion experiments

Neutron diagnostics for fusion experiments. Georges Bonheure. Introduction Time-resolved neutron emission Time-integrated neutron emission Neutron profiles Neutron spectra Summary. Outline. Introduction: fusion neutrons.

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Neutron diagnostics for fusion experiments

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  1. Neutron diagnostics for fusion experiments Georges Bonheure

  2. Introduction Time-resolved neutron emission Time-integrated neutron emission Neutron profiles Neutron spectra Summary Outline

  3. Introduction: fusion neutrons Neutrons produced in fusion reactions:D + T -> (4He + 3.56 MeV) + (n + 14.03 MeV) Q = 17.59 MeVD + D -> (3He + 0.82 MeV) + (n + 2.45 MeV) Q = 4.03 MeVT + T -> 4He + 2n Q = 11.33 MeVWhat do neutrons do? D

  4. JET: outside view Record: Q = 0.8 Steady state: Q = 0.3 total output : max 16 MW The largest tokamak: JET (Joint European Torus: www.jet.efda.org)

  5. The future ITER site now! www.iter.org

  6. Neutron source: progress in parameters The jump to ITER Plasma volume Neutron source strength Neutron flux at first wall ITER ~ 10x JET 100 m3 Neutron fluence ITER ~ 104 x JET (1025 n m-2) 1010 – 5.7 1018 n s-1 850 m3 1014 – 1020 n s-1 Biggest increase in neutron fluence! > Radiation hardness

  7. The plasma as a neutron source Ion temperature: Ion density ratio:

  8. Access: ITER diagnostics are port-based where possible Each diagnostic port-plug contains an integrated instrumentation package

  9. Introduction: fast neutron diagnostic systems • The variety of measurements that are possible are generally restricted due to: • Limited access to plasma • Harsh radiation environment X, g • Strong magnetic fields, powerful high frequency wave generators and power supply • Heat loads, mechanical stress • Timescale of measurements • Activation, tritium, beryllium

  10. Neutron diagnostic systems: 4 types of systems Time-resolved total emission (non-collimated flux) Fusion power Absolute emission Calibration of time-resolved emission Time-integrated emission (fluence) Spatial distribution of emission tomography 2D-cameras (collimated flux along camera viewing lines) Spectrometers (collimated flux along radial and tangential viewing lines) Plasma temperature and velocity Combination of these measurements characterizes the plasma as a neutron source

  11. Short range of interactions: characteristic scale is the nucleus size: 1 fermi (fm) 10-13 cm! Elastic scattering: A(n,n)A Inelastic scattering: A(n,n’)A* Radiative capture: n + (Z,A) -> g + (Z,A+1) Fission: (n,f) Other nucl.reactions: (n,p),(n,a),… High energy particle production (En > 100 MeV) Interaction of neutrons

  12. Fission counters: 238U and 235U counters embedded in moderator and led shield Operate both in counting and current mode Dynamic range: 10 orders of magnitude 3 pairs installed at different positions around JET Low sensitivity to X and g radiation No discrimination between 2.5 and 14 MeV neutron emission Calibrated originally in situ with californium 252Cf neutron source, periodically cross-calibrated using activation technique U235 U238 1. Time-resolved neutron emission

  13. Calibration with JET Remote Handling System 252Cf source strength: 109 n/s Duration : 3 days

  14. 1. Time-resolved neutron emission • For mixed 14 MeV and 2.5 MeV neutron fields: • Silicon diode • Fluence limit ~ 1012 cm-2 • Natural diamond detectors (NDD) • Chemical vapor deposited (CVD) diamond detectors • Radiation hardness >3.1015 cm-2 New radiation hard detectors are tested in JET

  15. 1. Time-resolved neutron emission GEM based neutron detection m

  16. Neutron activation method 2. Time-integrated neutron emission Samples used as flux monitors are automatically transferred to 8Irradiation ends Sample activity measurements: 1) gamma spectroscopy measurements >>> most widely used reactions at JET: DD neutrons - 115In(n,n’)115mIn, DT neutrons - 28Si (n,p)28AL, 63Cu(n,2n)62Cu, 56Fe(n,p)56Mn >>>detectors : 3 NaI, HPGe (absolutely calibrated) 2) delayed neutron counting (235U,238U,232Th) >>>detectors: 2 stations with six 3He counters Neutron transport calculations with MCNP to obtain the response coefficient for the samples Calibration: accuracy of the time-resolved measurements is typically ~ 8-10% for both DD and DT neutrons (7% at best using delayed neutron method) – after several years of work !! • MIX composition: • Se-16%, Fe-20%, Al-16%, Y-48%

  17. Activation technique PRINCIPLE • Escaping light charged particles p, t, d, 3He or a hit selected targets and produce nuclear reactions of type A(z, n)B*, A(z,γ)B*,… • B* radioactive decay (gamma photons) are measured using high purity germanium detectors HpGe detector Example of JET results: Gamma spectrometry of a natural Titanium target Activation probe (targets holder) 48Ti(p,n)48V Ep > 4.9 MeV A measurement challenge: Escaping alpha particles

  18. 3. Neutron profiles: 2D cameras • Two multi-collimator arrays (60tons each) with 19 channels available in total , 10 horizontal and 9 vertical • Adjustable collimators: Ø10 and 21 mm • Detectors: • Liquid organic scintillators NE213 with pulse shape discrimination • BC 418 plastic scintillators • CsI scintillators for γ rays • Calibration: embedded sodium (22Na) sources • γ / n separation control: movable americium beryllium 241Am/Be source

  19. Digital pulse shape discrimination technique • Benefits • Detailed post processing possible (events identification, pile-up,…) • Deconvolution of spectrum information • Increase dynamic range in both energy and count-rate g n n/γ separation obtained with a 14 bits- 200 MegaS/s DPSD prototype One NE213detector of neutron camera is exposed to a plasma pulse

  20. Study of tritium diffusion nT/nD Pulse 61372: ne0 = 4.5 1019m-3 Pulse 61161: ne0 = 1.9 1019m-3 R (m) R (m) Time (s) Time (s) Theoretical predictions for D, v can be verified against measurements

  21. 4. Neutron spectroscopy • Time of flight • Proton recoil • 1) ‘thick hydrogenous target’ (high efficiency) • No information on recoil angle : energy spectrum recovered by unfolding • 2) ‘thin hydrogenous target’ (low efficiency) • Analysis of recoil proton momentum Trade off: energy resolution vs detection efficiency

  22. Neutron spectroscopy: time of flight (TOFOR) Energy resolution for DD neutrons: ~5% Detection efficiency: 8 10-2 cm2 Count rate: < 500 kHz Simulated with GEANT code

  23. 4. Neutron spectroscopy NE213 TOFOR MPR TANDEM TG Diagnostics Garching April, 2009 23 23

  24. Neutron spectroscopy: spectral unfolding techniques Comparison between different unfolding techniques: • Maximum entropy (MAXED) • Minimum fisher regularisation (MFR)

  25. Summary: neutron diagnostic systems Time-resolved total emission Total: fission counters 14 MeV: Silicon diodes fission counters Diamond detectors Time-integrated emission Foil activation Foil activation 2D-cameras Liquid scintillators NE213 Plastic scintillators BC418 Diamond detectors Stilbene, NE213, U238 fission counter, fast plastic Spectrometers • Time of flight • Proton recoil systems: • NE213 and stilbene • Magnetic proton recoil To be defined

  26. Final remarks • With the move towards ITER • role of fast neutron diagnostics will increase • Capabilities of those systems need to accommodate an increase in fluence by 4 orders of magnitude and in flux by 1 order of magnitude • JET has an extensive set of fast neutron diagnostics, more than 2 decades of accumulated experience, and it will continue to play a leading role in development of fast neutron measurements for fusion applications • Active research areas include new radiation hard detectors, new electronics and acquisition systems, spectrometers, tomography and unfolding techniques • Neutron measurements contribute to advanced physics studies e.g in the field of plasma particle transport • For references see in: http://pos.sissa.it/ ‘Neutron diagnostics for reactor scale fusion experiments’

  27. Thanks…

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