Neutron spectrometry in fusion energy research

1. Neutron spectrometry in fusion energy research Göran Ericsson, Erik Andersson Sundén M.Cecconello, S.Conroy, M.Gatu Johnson, L.Giacomelli, C.Hellesen, A.Hjalmarsson, J.Källne, E.Ronchi, H.Sjöstrand, M.Weiszflog Uppsala University G.Gorini, M.Tardocchi, J.Sousa, A.Murari, S.Popovichev Milano-Bicocca, IST, JET • Outline: • Neutron emission in fusion experiments • Role of diagnostics, measurement conditions • Spectrometer design and techniques • The ToF technique; TOFOR • Thin-foil proton recoil technique; MPRu • Outlook and Conclusions Frontiers … Rome, 2009 1 See also poster by E.Andersson Sundén

2. Scatter Background T = total spectrum, B = thermal bulk NBI = neutral beam AKN = alpha knock-on Simulation JET; D Neutron emission • Fusion experiments with D and T fuel: • d + d  3He + n (2.45 MeV) • d + t  4He + n (14.0 MeV) • “Impurities” • d + 3He, 4He, 9Be, 12C, ...  n + X • Plasma parameters: Pfus, Ti, f(vion),… • Fuel ion velocity populations: • Thermal  f(En) Gaussian • RF heating  f(En) anisotropic, double humped • Beam heating, alpha heating, … • Spectral components (ITER): • Thermal bulk Sn 1, • Beam heating Sn 0.1, • RF heating Sn 0.01, • a heating Sn 0.001, • Neutron emission variations: • Intensity; 0 - 1020 n/s (ITER) • Temporal (ms), spatial (cm) n rate JET; D Thermal RF RF Simulation ITER; DT Rn [1015 s-1] Frontiers … Rome, 2009 2

3. Role and situation for diagnostics • Provide information on relevant plasma/fuel ion parameters • Feed-back for active control; ms time frame • Extended n source (100 m3), “continuous” n emission (min) • Collimated LOS, direct + scattered spectral contributions • Reliable, robust techniques • Harsh experimental conditions around the “reactor” • Neutron and gamma background • High-frequency EM interference • High levels of temperature, B-field • Competition over “real estate”; LOS, position, weight, space, … • Challenges for neutron spectroscopy • Results on ms  spectroscopy on MHz signal rates (Ccap) • High eOR close to reactor core • Access to weak emission components  high S/B ratio > 104 • Peaked, well-known response function (0 – 20 MeV) • Real-time information in ms  data acq., processing, transfer Frontiers … Rome, 2009 3

4. Neutron spectroscopy techniquesMost “standard” n spectr. techniques tested in fusion (JET) NE213, Stilbene, nat. + CVD diamond Reginatto, Zimbal, RSI 79 (2008)- PTB work Krasilnikov, Rev Sci Instr 69 (1997) Lattanzi, Angelone, Pillon , Fus Eng Des (2009) TOFOR - UU Gatu Johnson, NIM A591 (2008) Frontiers … Rome, 2009 4 TANDEM (TPR) - Harwell Hawkes, RSI 70 (1999) 1134 MPRu - UU Andersson Sundén, NIM A610 (2009)

5. Time-of-flight Optimized for Rate • Optimized for 2.45 MeV n in D plasmas • Continuous source of n: • Double scattering in S1 + S2 • 16m from plasma • 2m concrete floor • Fast plastic scintillators: • 5x S1 disks • 32x S2 “umbrella” • S2 tilt to compensate for Dtlight • e ≈ 1% • Background = randoms • B  Rn2 S:B  Rn • Limitations: • “Paralysis” at high Rn • Rate in S1 (≈ MHz) • Ccap ≈ 500 kHz (S:B ≈ 1) • Cmax ≈ 44 kHz (Rn = 1.7∙1016 n/s) • Emphasis on rate capability: • Digital free-running time stamping • Separate, non-correlated p.h. spectra En = 2mnR/ttof2 R n” n’ Frontiers … Rome, 2009 5 n flux

6. TOFOR – count rate capability • Limiting sensitivity: random coincidences • No correlated time – p.h. information •  Randoms corrected for on statistical level •  Uniform level from ttof < 0 • Digital time stamping electronics (IST, Portugal) • Dead time free: ALL signal events recorded (+ ALL randoms) • Event based correlations: reduce randoms, reduce timing walk S1 S2 Frontiers … Rome, 2009 6

7. Thin-foil Magnetic Proton Recoil TOFOR • Separation of functions: • n-to-p conversion in thin (mm) foil • Energy (momentum) separation in B-field • Counting in position resolved hodoscope (32 phoswich scint.) • Focal plane detector (FPD) can be shielded to any required level • Detectors need “only” count protons • Flexibility: • Multiple conversion foils – 2.5/14 MeV • Multiple p collimators • Background reduction: • Concrete + lead radiation shield • Phoswich scintillators, tdecay = 2, 180 ns • TR digital boards • Digital pulse shape discrimination • Performance: • Ccap >> MHz (Cmax = 0.61 MHz) • S:B  20000:1 (14-MeV in DT), Frontiers … Rome, 2009 7 5:1 (2.5-MeV in D)

8. Alpha heating in DT; MPR 1997 Scattered n Bgr/statistics Background 14-MeV p LED pulser MPR results • Observation of weak components • Alpha heating signature – knock-on n • Phoswich DPSD Phoswich DPSD Qshort Qlong • Preliminary phoswich DPSD analysis • Protons from T burn-up n (14-MeV) • Component at 1% of 2.5 MeV emission • LED for PMT gain monitoring system Frontiers … Rome, 2009 8 TR boards: 8 bit, 200 MSPS, 512 MB Baseline restoration, pile-up rejection Standard DPSD: 2D plot of Qlong/Qshort Strong candidate for ITER Min. ionizing e- 2.5-MeV p 14-MeV p 14-MeV n 0.3 mm 2.5 mm Amplitude

9. The future • Combined pulse-height/time digitizing boards for ToF • Compact spectrometers for neutron camera; NE213, CVDD • Neutron spectroscopy system for ITER: • 2.5-MeV n spectrometer for D operations; ToF • 14-MeV n spectrometer for high power DT; MPR/TPR • Real-time applications • Innovative, new concepts … Conclusions • Harsh experimental conditions; special requirements • Challenges for Fusion neutron spectrometry: • Count rate capability – provide plasma information • Background rejection – study weak emission comp. • Dynamic range/sensitivity – varying plasma cond. Frontiers … Rome, 2009 9