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XTOD: Total Energy Monitor

This project aims to create a Total Energy Monitor to measure FEL pulse energy and variations. It utilizes thermal detectors for accuracy and reliability, with specific design specifications and material choices to ensure optimal performance and radiation hardness. The concept involves FEL absorption in a substrate, signal detection through temperature rise, and sensor sensitivity for precise energy readings. The system includes a CMR sensor on Si, calibration with a pulsed laser, and measures to address potential radiation damage and energy loss mechanisms. The design focuses on accuracy, dynamic range, and repeatability within specified error budgets.

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XTOD: Total Energy Monitor

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  1. XTOD: Total Energy Monitor Goal: To measure the total FEL pulse energy, and its pulse-to-pulse variations, based on the temperature rise on absorption

  2. Motivation: Why thermal detectors? • Complementary technologies to address different regimes and to minimize risk through cross-calibration

  3. Design Specifications • Radiation hardness for pulses from 0.8 to 8 keV required. • Dynamicrange: • Operation up to maximum FEL power of 2 mJ/200fs pulse required • Operation up to 10 mJ/pulse with attenuator • Minimum detectable energy 1µJ/pulse required, <0.1 µJ desirable • Repeatability: <1% at 0.2 mJ, and at full power • Sensor needs to handle beam jitter up to ±100 µm at 800 eV • Speed: 10 Hz required, 30 Hz (120 Hz?) desirable • Absolute accuracy <10% at 0.2 mJ • Range of validity • Instantaneous field of view 3  3 mm2 • Field of regard ±5 mm horizontal and vertical

  4. Detector Design Considerations • Radiation hard  Low-Z materials • Sensitivity, speed  Low T • Halo transmission  Small, ~mm3 Fluence to melt [J/cm2] Photon Energy [eV] FEL: 2-3mJ max, ≤1mm Spontaneous halo: 20 mJ

  5. Total Energy Monitor Concept • FEL absorption in substrate • Spontaneous halo transmission • Signal TEtotal/C • Measure T with thermistor • Sensitivity ∂R/∂T • Protected from FEL • Cooldown to bath T • Substrate sets decay time FEL (Etotal) Thermistor (Top, ∂R/∂T) Substrate (heat capacity C, speed) Heat sink (Tbath)

  6. Material Choices • Colossal Magneto-Resistive (CMR) film at metal-insulator transition (Nd0.66Sr0.33MnO3) • High sensitivity • T coefficient of resistance TCR = 1/R ∂R/∂T = 10%/K • Tunable transition T • Top ≈ 130K • Successful growth on Si • STO/ BTO buffers • Si Substrate • Radiation hard • High thermal diffusivity

  7. 5 Top=100K Top=150K 4 Top=200K 3 T at Sensor [K] 2 1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Time [ms] Finite Element Simulations of T Evolution t = 0 t = 0.1 ms t = 0.25 ms • Peak signals are of order 1K/mJ • Decay times compatible with FEL repeat interval Sensor FEL 0.5mm Si

  8. Sensitivity and Dynamic Range Pulser tests • Signal-to-Noise ratio is high: • For Vbias = 1V, TCR=10%/K at R=10kΩ en=10 nV/√Hz, BW=10 kHz • S/N >105 for saturated FEL: (Etotal=2 mJ, no 1/f noise) • S/N >100 without FEL (Etotal= 10 µJ, spontaneous halo only) • Note: Increased 1/f noise with Vbias • Dynamic range covers full energy range of FEL • Repeatability <1%

  9. Radiation Damage in Si? • FEL remains below single-shot damage threshold at Tmelt • Potential long-term fatigue can be measured and addressed • B4C or other low-Z protection layer on Si, or Gas attenuator Pure Si absorber Si with 20 µm B4C Si damage tests at TTF

  10. Energy Calibration • Calibrate sensor with 532nm pulsed laser when withdrawn • Measure incident and reflected beam • Absolute calibration to <5% with pyroelectric pulse energy meters Sensor on Si Attenuator: Filter wheel Beam focus Beam splitter on Gimbal mount Ophir PE-10 Pulse meters: 3% accuracy Incident beam calibration Minilite pulsed Nd-YAG laser at 532 nm 10 mJ/ 5 ns pulse max Reflected beam monitoring to assess radiation damage

  11. Alternative Energy Loss Mechanisms • Radiative Cooling: <1% • Scintillation? <<1% • Nelson, PRL27, 1262, 1966 • Electron Backscatter: <2% • Spallation? <1% • London, SPIE 4500, 51, 2001 Moshe APL 76, 1555, 2000

  12. Error Budget • Error limited by calibration or 1/f noise • Error within design specifications Calibration errors: <5% (limited by accuracy of pulse meter) Electronic noise error: <0.1% at saturation <5% at low energies (likely limited by 1/f noise) Energy loss error: <2% (limited by electron escape) Jitter error: <2% Total error: < 7% at saturation (2 mJ/ pulse) < 7% at 0.2 mJ < 8% at low energies (10 µJ/pulse)

  13. 4 1/2” flange Pulse tube Cold head Refrigeration and Positioning • Pulse-tube cryocooler: • Base T < 70 K • No cryogenic liquids underground • Low vibration (10µg/√Hz) • Operation at LCLS on xyz stage • 8” travel in z, ±5 µm xy-steps

  14. System Design Pulse tube XYZ-stage Direct imager Detector 532nm laser FEL in • Detector in FEL beam: • Cooled by pulse tube • Positioned by XYZ stage • Calibrated with 532 nm laser when withdrawn • Prototype being built

  15. CMR sensor FEL Si (+B4C?) Summary Total Energy Monitor • Thermal Nd0.66Sr0.33MnO3 sensor (CMR material at metal insulator transition) • Successful epitaxial sensor growth on Si  Radiation hard, fast • Low noise  High dynamic range • Accuracy <5-8% Set by optical laser calibration and 1/f noise • Prototype being built

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