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Diagnostic Capabilities of Line-Integrated Neutron Pulse Height Spectra Measurements Daniele Marocco

Diagnostic Capabilities of Line-Integrated Neutron Pulse Height Spectra Measurements Daniele Marocco. Associazione Euratom-ENEA sulla Fusione, C.R. Frascati, C.P. 65, Frascati I-00044, Roma, Italy.

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Diagnostic Capabilities of Line-Integrated Neutron Pulse Height Spectra Measurements Daniele Marocco

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  1. Diagnostic Capabilities of Line-Integrated Neutron Pulse Height Spectra Measurements Daniele Marocco Associazione Euratom-ENEA sulla Fusione, C.R. Frascati, C.P. 65, Frascati I-00044, Roma, Italy D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  2. Two main quantities characterize neutron emission in fusion experiments: • Neutron emissivity • Neutron spectrum • On present devices they are measured by separate diagnostics: • Neutron cameras (multi channel diagnostics providing neutron emissivity along a plasma poloidal section) • Neutron spectrometers (single channel diagnostics providing line-integrated neutron spectra) • Thanks to digital technologies new systems using compact spectrometers can be developed which will allow to combine neutron spectra and 2-D neutron profile measurements • The present work aims at exploring the capability of a collimated compact spectrometer detector array equipped with a digital acquisition system Preface D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  3. Introduction: • Plama neutron emissivity • Plasma neutron emission spectrum • Diagnostics for neutron spectrometry: organic liquid scintillators • Diagnostics neutron emissivity measurements: neutron cameras • Research activity Outline D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  4. Main nuclear reactions in plasma experiments: D + D t (1.01 MeV) + p (3.02MeV) D + D 3He (0.82 MeV) + n (2.45MeV) D + T 4He (3.56 MeV) + n (14.03 MeV)  Nearly equal probability: the emission of 2.5 MeV neutrons indicates the birth of 1.0 MeV tritons When undergone by the fusion product tritons from the first D-D reaction branch = triton burn-up Introduction: neutron emissivity • Emissivity: plasma local neutron yield (n s-1 m-3) expressed as nA nB = particle densities; dAB= Kronecker symbol; <s v>AB= neutron reactivity D. Marocco Fusion science and engineering Doctorate

  5. The neutron emissionspectrum in a tokamak reflects the velocitydistribution of the fusing ion pairs Introduction: Neutron Emission Spectrum Non thermal plasma: Non-Gaussian tails and Doppler energy shifts Thermal plasma: Gaussian-shaped neutron spectrum (width W  √Ti) D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  6. Large neutron spectrometers: • Magnetic proton recoil (MPR): neutrons from the plasma are converted into recoil protons by means of a thin hydrogen-reach target; the recoil protons are momentum discriminated through a magnetic field • Time of Flight (TOF): measurements of the times of correlated neutron scattering events in a start and stop detector • Compact neutron spectrometers: • Diamond detectors (14 MeV only): based on the 12C(n,) threshold reaction (8 MeV) • Scintillator detectors Diagnostics For Neutron Spectrometry All measurements performed using a single collimated line of sight through the plasma: single line-integrated spectra D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  7. PMT En scintillator dN/dE En E • Based on neutron scattering with hydrogen atoms: • Recoil protons excite scintillator molecular compounds with consequent ligth emission • Ligth pulses are converted into electron signals and amplified coupling the detector to photomultipliers (PMT) • gpulses can be discriminated through pulse shapeanalysis Scintillator energy response function Liquid Scintillators (1/2) • Scintillator non linearity, finite energy resolution, multiple scattering from hydrogen atoms, alter the response function: specific codes or experimental measurements are needed D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  8. For a non monocromatic neutron beam a Pulse Height Spectrum(PHS) given by the superposition of the different energy response function is provided by the scintillator • The actual neutron spectrum is obtained by means of unfolding codes PHS Liquid Scintillators (2/2) Unfolding D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  9. A neutron camera system equipped with organic liquid scintillators and digital electronics is foreseen for ITER: • Radial neutron camera (RNC) to be installed in equatorial port plug#1: • Ex-port system: 12 LOS (x 3 on planes at different toroidal angles) • In-port system: 9 LOS • Vertical neutron camera (VNC) to be installed in a lower divertor port: • 10 LOS Neutron Cameras: ITER RNC-VNC System • PHS covering a whole poloidal plasma section: possibility to observe spatial and time evolution of neutron spectra D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  10. 2 concrete shields each including a fan-shaped array of collimators: 10 collimated channels with a horizontal view; 9 channels with a vertical view • Each LOS equipped with: • NE213 liquid organic scintillator • Analogue pulse shape discrimination (PSD) electronics for simultaneous measurements of the 2.5MeV-14MeV neutrons and g -rays Neutron Cameras: JET Neutron Profile Monitor (1/2) D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  11. The JET profile monitor due to limitations imposed byanalog electronics: • Works just as a counter calibrated to provide neutron counts in specific energy windows (1.8 MeV - 3.7 MeV for DD; > 7 MeV for DT). No PHS during discharges • Is limited to count rates ~200 kHz • An ENEA project for a full digital upgrade of all neutron profile monitor channels has been approved Neutron Cameras: JET Neutron Profile Monitor (2/2) As a part of this project the development of a single channel digitizer(SCD) to be installed at JET has been carried out D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  12. Scope: Investigate the diagnostic capability of multiple line integrated neutron PHS (scintillators+digital electronics) byscanning with the SCD all the JET neutron profile monitor channels • Program: • Set up of SCD and elaboration software • SCD benchmarking • SCD installation at JET • Data acquisition and Analysis (on-going) • Modeling activity: feasibility study of deriving a 2D neutron energy profile using a combined inversion-unfolding technique (to be started) Doctorate Research: Scope & Program D. Marocco Fusion science and engineering Doctorate

  13. 200 MSamples/s &14-bit resolution • FPGA- based (Altera1S25)(mainly used for data compression) • Data transfer to PC through PCI Estimated Sustainable Count Rates: ~ 500 kHz DT ~ 900 kHz DD Program: SCD and elaboration software set up (1/2) DPSD rack unit D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  14. g n • LabVIEWTM code managing off-line data analysis : • n and g separation through digital charge comparison • Separated n & g count rates and PHS provided via pulse integration Program: SCD and elaboration software set up (2/2) n/g separation plot D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  15. 12 kHz acquisition Program: SCD Benchmarking • The system has been benchmarked at the PTB accelerator (Braunshweig–Germany) with 2.5 and 14 MeV neutrons against an analog acquisition chain using a fully caraterized liquid scintillator: • comparable energy resolution up to ~ 420 kHz D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  16. PMT Scintillator // Splitter Analog PSDModule SCD Program: SCD installation at JET The digitizer can be connected to a single profile monitor channel in place of the analog PSD module normally used to detect 14 MeV neutrons D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  17. Program: Data Acquisition and Analysis • Acquisition of data (Na-22 gsources and plasma discharges) obtained placing the digitizer on each channel of neutron profile monitor presently on–going • Identify a data sub-sets with quasi-similar plasma conditions and: • Characterize single PHS • Compare PHS from different channels (channel to channel variations & temporal evolution) • Perform PHS unfolding D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  18. Program: Data Acquisition and Analysis - 14 MeV to 2.5 MeV Ratio channel #15 Rough estimate of the actual DD and DT counts for each channel using PHS Counts converted to brightness by means of intrinsic efficiency of the detectors and inverted: A 2D profile of the fuel ratio can be obtained (Ratio Method) 2D information on 1 MeV tritons slowing down can be obtained from the comparison of the time evolution of DD and DT signals 2.5 MeV neutrons +14 MeV contribution 14 MeV neutron signal due to triton burn-up reactions D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  19. Program: Data Acquisition and Analysis - Fast neutron tails channel #15 Rough estimate of the fast neutron component in each channel boxing the PHS PHS unfolding: • A Line-integrated ion temperature profile can be obtained during the ohmic phase • Line-integrated profile information on intensities and temperature of the different ion components can be derived during the additionally heated phase D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  20. Aim: develop acombined unfolding - inversion technique to derive a 2D neutron energy profile starting from a set of line integrated PHS (feasibility study) bk = brigthness measurement from chord k ej = neutron emissivity  = normalized poloidal flux coordinate Program: Modeling Activity (1/3) ill-posed problem: small variations in input data produce high variations in the solutions regularization techniques are needed D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  21. Each brightness measurement bk can be thought as the energy integral of a line integrated neutron spectrum Sk unfolding of the line integrated PHS representing the measurement of a camera LOS • Each emissivity eJcan be thought as the energy integral of a local energy spectrum hJ Program: Modeling Activity (2/3) Local energy spectrum to be derived • Inserting back these definitions in the emissivity linear equation system With respect to typical inversion problems the response matrix L connects energy functions rather than real numbers D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  22. If a feasible method is identified its robustness will be tested through simulation using a reference plasma and neutron camera layout (ITER, JET): • Set up areference 2D neutron enegyprofile (phantom) • Derivea set of synthetic meaurements (line-integrated PHS): • Integration along LOS • Folding with the detector response functions (including statistical, background and random errors) • Apply the combined unfolding-inversion algorithm to obtain an “inverted” 2D neutron energy profile • Compare the phantom and the inverted profile Program: Modeling Activity (3/3) D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  23. Bring My Daddy Back Home ! Francesco Activity F. Marocco My cradle – Rome – when mammy and daddy are lucky

  24. 1 2 S3 S3 S2 S1 S1 T3 T2 g 2 3 T1 1 4 S0 S0 • Liquid organic Scintillators: molecular compounds characterized by a molecular structure in which unbound π-electrons are prone to excitation by incident radiation Fluorescence Liquid scintillators: detection principle Delayed fluorescence D. Marocco Fusion science and engineering Doctorate

  25. n g • The proportion of delayed fluorescence of the pulse is related to the triplet density in the wake of the incident particle (determined by the rate of energy loss,dE/dx, of the incident particle) Heavier particle Greater energy loss rate More delayed fluorescence in the output Pulses that decay more slowly then those from lighter particles Liquid scintillators: n/g separation D. Marocco Fusion science and engineering Doctorate

  26. Charge comparison

  27. Data first stored in a portion of the RAM (1.2 GB).When the programmed acquisition time is reachedthe acquisition stops and data are saved to disk Input ± 2.8 V Coupled in Interleaved mode (5 ns delay) giving an actual sampling rate of 200 MSamples/s on one input channel The acquisition process Operates on an endless ring of digitized data performing: Offset Removal Dynamic data window creation Window cutting Matches the different speed in input and output Packs window data and timing information (time of the first over threshold sample)

  28. The Elaboration Process

  29. If the bk coefficients are affected by noise the system could be inconsistent meaning that there is no emissivity that exactly solves the system. Least squares soultion Thikonov regularization

  30. Ion Temperature Profile Magnetic Surfaces Layout 2D Ion Temperature profile Gaussian Shaped Spectra Los Layout Reference 2D Energy Profile Detector Response Functions Line Integrated Spectra Program: Modeling Activity (3/3) Inverted 2D Energy Profile Noise: Statistics Background Random Combined unfolding - inversion Line Integrated PHS: Synthetic Measurements D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

  31. Diagnostic providing line-integrated neutron counts (brightness, ns-1m-2)along a large number lines of sight (LOS) • The emissivity over a poloidal section of the plasma is obtained by Inversion/tomographic techniques Neutron Cameras D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

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