Diagnostic Capabilities of Line-Integrated Neutron Pulse Height Spectra Measurements

Transcription

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

2 Preface 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

3 Outline Introduction: Plama neutron emissivity Plasma neutron emission spectrum Diagnostics for neutron spectrometry: organic liquid scintillators Diagnostics neutron emissivity measurements: neutron cameras Research activity

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

5 Introduction: Neutron Emission Spectrum The neutron emission spectrum in a tokamak reflects the velocity distribution of the fusing ion pairs Thermal plasma: Gaussian-shaped neutron spectrum (width W Ti) Non thermal plasma: Non-Gaussian tails and Doppler energy shifts

6 Diagnostics For Neutron Spectrometry 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 12 C(n, α) threshold reaction ( 8 MeV) Scintillator detectors All measurements performed using a single collimated line of sight through the plasma: single line-integrated spectra

7 Liquid Scintillators (1/2) 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) γ pulses can be discriminated through pulse shape analysis E n scintillator PMT dn/de Scintillator energy response function E n E Scintillator non linearity, finite energy resolution, multiple scattering from hydrogen atoms, alter the response function: specific codes or experimental measurements are needed

8 Liquid Scintillators (2/2) 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 Unfolding

9 Neutron Cameras: ITER RNC-VNC System 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 PHS covering a whole poloidal plasma section: possibility to observe spatial and time evolution of neutron spectra

10 Neutron Cameras: JET Neutron Profile Monitor (1/2) 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 γ -rays

11 Neutron Cameras: JET Neutron Profile Monitor (2/2) The JET profile monitor due to limitations imposed by analog electronics: Works just as a counter calibrated to provide neutron counts in specific energy windows (1.8 MeV 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 As a part of this project the development of a single channel digitizer (SCD) to be installed at JET has been carried out

12 Doctorate Research: Scope & Program Scope: Investigate the diagnostic capability of multiple line integrated neutron PHS (scintillators+digital electronics) by scanning 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) D. Marocco Fusion science and engineering Doctorate

13 Program: SCD and elaboration software set up (1/2) 200 MSamples/s &14-bit resolution FPGA- based (Altera 1S25) (mainly used for data compression) Data transfer to PC through PCI Estimated Sustainable Count Rates: ~ 500 khz DT ~ 900 khz DD DPSD rack unit

14 Program: SCD and elaboration software set up (2/2) LabVIEW TM code managing off-line data analysis : n and γ separation through digital charge comparison Separated n & γ count rates and PHS provided via pulse integration γ n n/γ separation plot

15 Program: SCD Benchmarking 12 khz acquisition 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

16 Program: SCD installation at JET Scintillator PMT // Splitter Analog PSD Module SCD 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

17 Program: Data Acquisition and Analysis Acquisition of data (Na-22 γ sources 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

18 Program: Data Acquisition and Analysis - 14 MeV to 2.5 MeV Ratio 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) n n T D = Y Y 2 σv σv DD DD 2D information on 1 MeV tritons slowing down can be obtained from the comparison of the time evolution of DD and DT signals DT DT channel #15 SHOT # MeV neutrons +14 MeV contribution 14 MeV neutron signal due to triton burn-up reactions Proton energy (MeV)

19 Program: Data Acquisition and Analysis - Fast neutron tails Rough estimate of the fast neutron component in each channel boxing the PHS channel #15 PHS unfolding: A Line-integrated ion temperature profile can be obtained during the ohmic phase Line-integrated profile information on intensities and temperature of different ion components can be derived during the additionally heated phase the

20 Program: Modeling Activity (1/3) Aim: develop a combined unfolding - inversion technique to derive a 2D neutron energy profile starting from a set of line integrated PHS (feasibility study) b k = brigthness measurement from chord k e j = neutron emissivity ψ = normalized poloidal flux coordinate ill-posed problem: small variations in input data produce high variations in the solutions regularization techniques are needed

21 Program: Modeling Activity (2/3) Each brightness measurement b k can be thought as the energy integral of a line integrated neutron spectrum S k b = unfolding of the line integrated PHS k sk ( E) de representing the measurement of a camera LOS Each emissivity e J can be thought as the energy integral of a local energy spectrum h J e j = h j ( E) de Local energy spectrum to be derived Inserting back these definitions in the emissivity linear equation system s ( E) = Lh( E) With respect to typical inversion problems the response matrix L connects energy functions rather than real numbers

22 Program: Modeling Activity (3/3) 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 a reference 2D neutron enegy profile (phantom) Derive a 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

23 Francesco Activity BringMyDaddyBack Home! F. Marocco My cradle Rome when mammy and daddy are lucky

24 Liquid scintillators: detection principle Liquid organic Scintillators: molecular compounds characterized by a molecular structure in which unbound π-electrons are prone to excitation by incident radiation S 3 S 2 Fluorescence S γ S 0 Delayed fluorescence S 3 S 1 T T 2 T S 0 D. Marocco Fusion science and engineering Doctorate

25 Liquid scintillators: n/γ separation 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 4000 Greater energy loss rate More delayed fluorescence in the output channel γ n Pulses that decay more slowly then those from lighter particles 10 Δt F ΔtS time (ns) 500 D. Marocco Fusion science and engineering Doctorate

26 Charge comparison channel 100 Δt F Δt S time (ns) 500

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

28 The Elaboration Process

29 N b k = l kj e j k =1, M j=1 b = L e If the bk coefficients are affected by noise the system could be inconsistent meaning that there is no emissivity that exactly solves the system. Le b 2 L T b = L T Le Least squares soultion Le b 2 + α D e 2 Thikonov regularization L T b = (L T L + αd T D)e

30 Program: Modeling Activity (3/3) Ion Temperature Profile Magnetic Surfaces Layout 2D Ion Temperature profile Gaussian Shaped Spectra Reference 2D Energy Profile Los Layout Combined unfolding - inversion Inverted 2D Energy Profile Line Integrated Spectra Line Integrated PHS: Synthetic Measurements Detector Response Functions Noise: Statistics Background Random

31 Neutron Cameras Diagnostic providing line-integrated neutron counts (brightness, ns -1 m -2 ) along a large number lines of sight (LOS) The emissivity over a poloidal section of the plasma is obtained by Inversion/tomographic techniques