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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. Motivation: Why thermal detectors?.
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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
Motivation: Why thermal detectors? • Complementary technologies to address different regimes and to minimize risk through cross-calibration
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
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
Total Energy Monitor Concept • FEL absorption in substrate • Spontaneous halo transmission • Signal TEtotal/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)
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
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
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%
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
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
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
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)
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
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
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