Time-resolved SEM monitor with large dynamic range for R&D of Linac4

Transcription

1 Time-resolved SEM monitor with large dynamic range for R&D of Linac4 M. Hori Max-Planck Institute for Quantum Optics University of Tokyo K. Hanke CERN HB 21 October 21

2 Time-resolved measurement of Linac4 chopper 3-MeV chopped beam 45 kev 3 M ev 5 MeV 1 12 MMeV 16 M ev H - source LEBT RFQ chopper line DTL CCDTL PIMS 5x18 particles 86 m Chopped beam <5 particles f=35 MHz

3 Our design philosophy for Linac4 detector 1. Simplicity and ease of use - extension of classic interception technique + secondary electron detection (let s not use lasers yet). 2. Robustness - stripper foils are most resistant to high beam intensities. 3. Use modern gating techniques, optics, and readout sensors and try to get the best performance.

4 Our work for Linac4 grew out of our previous R&D efforts at the CERN Antiproton Decelerator measuring pulsed antiproton beams using secondary electron emission monitors (SEM s).

5 Secondary electron emission monitor based on wires (a) X-projection preamplifiers Beam Y-projection preamplifiers 5 cm X-projection cathode grid Y-projection cathode grid UV, X-ray, or particle beam Vacuum chamber Readout pins X-projection grid Readout sockets Y-projection grid anode grids RSI 76, (25) (b) Ceramic frame Readout pins and sockets Gold-sputtered tungsten wires or carbon filaments diameter 5-3 um placed in UHV. Wires intercept 1-3% of the beam % travel through without being affected. Gold-coated tungsten wires Gold microstrips Secondary electrons detected by charge-sensitive preamplifiers with RMS equivalent noise charge 2.

6 Secondary electron emission monitor based on foils (a): 2 mm (c): Laser trimmed strip 5 µ m Aluminized or gold-sputtered polyester or Mylar foils. Ag+C paste Gold strip Copper strips Ag+C paste Gold strips Laser beam Cutting of 8-um wide strips in metal layer using Nd:YAG or excimer nanosecond laser trimmer. (b): (d): scan Transparent Mylar or polyester foils are left intact. Signal wires Glass epoxy µ Polyester t d = 1.5 m NIM-A 522, 42 (24)

7 Profile measurements at the Antiproton Decelerator Intensity (a.u.) 2 1 (a): Position (mm) 2 (c): X Y Y-position (mm) Beam halo X-position (mm) 1 (b): Intensity (a.u.) -2 x1 7 4 Antiprotons / pulse (d): Extracted pulses NIM-A 522, 42 (24) Clear profiles can be observed using the detection of secondary electron emission particles/pulse

8 Lasers can be used to simulate particle beams Attenuators Telescope nm BBO crystals CCD Nd:YAG pulsed laser Nd:YAG pulsed laser Evacuated beamline Monitor 1 Monitor 2 Single-shot measurement of beam profile at several point along beamline. Enables rapid determination of beam emittance, beam tuning. Intensity Intensity 2 (a) X-position (mm) 2 (c) X-position (mm) Y-position (mm) Y-position (mm) 16 (b) Intensity 16 (d) Intensity RSI 76, (25)

9 Photoelectron emission efficiencies for lasers of various wavelengths x1 4 Photoelectrons x1 4 Photoelectrons (a) 213 nm Laser energy (nj) (c) 355 nm 1 5 Laser energy (µj) x1 4 Photoelectrons x1 4 Photoelectrons (b) 266 nm 1 1 Laser energy (nj) (d) 532 nm 1 5 Laser energy (µj) RSI 76, (25) W-value of gold = 4.6 ev 213 nm (5.8 ev) g= nm (4.6 ev) g=1-5 proceeds via single-photon. relatively linear. 355 nm (3.5 ev) g= nm (2.3 ev) g=1-9 proceeds via two-photon, non-linear. Needed field =5-2 kw/cm2

10 These types of charge-sensitive preamplifiers can provide timing resolutions in the 1 ns to 1 us range only... 5 x=6 mm Signal (V) x=4 mm x=2 mm x= mm x=-2 mm x=-4 mm x=-6 mm Time elapsed (µs) RSI 76, (25)

11 Vacuum chamber Detector developed for Linac4 R&D Carbon target foil 1 cm Acceleration grid A Retraction mechanism - 3-MeV H ion beam Permanent ring magnet Permanent ring magnet Phosphor screen Fiber optic conduit F Fiber optic conduit G Secondary electrons Acceleration grids B-E RF coaxial line 5 V RF coaxial line 5 kv Type-HN coaxial feedthroughs Thermoelectrically cooled CCD NIM A 588, 359 (28)

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13 Side view of detector Up/down mechanism Electron emission target foil H- beam Acceleration grid 1 Fiber conduit Acceleration grid 2 Phosphor screen 5 kv / 1 ns RF feed

14 Front view RF striplines Phosphor screen Acceleration grid 1 Fiber conduit Acceleration grid 2 5 V / 1 ns RF feed 5 kv / 1 ns RF feed

15 Electron - to - photon imager Four parallel 5-Ohm lines for effective impedance 12.5 Ohm allows fast switching of voltage potential CCD camera

16 Switching grid and phosphor screen 1-μm-diam gold-coated tungsten wires Ceramic substrate Phosphor screen 5-Ω gold striplines (a) (b) 3 mm Chrome frame 5-Ω gold striplines SMA connector RF connectors

17 Optical imaging system to photograph the beam profile Active area 5 mm diam. UHV compatible 5 million fibers. 13 x 13 pixels. Magnification 1.8 x Read noise 5 electrons Provides shielding against 16 bit ADC, 2, e- full range X-rays and neutrons. Speed sec full frame

18 Electronic gating and acquisition system V A V C Phosphor screen 1 MΩ 1 MΩ Grid D Grid E CCD Target foil 1 µ F 1 µ F Grid A Grid B V off Grid C V off V off V off 5 kv, 1-ns FET switch F V off -1 kv, 1-ns FET switch G -5 V avalanche diode switch I V off V off Timing signal Gate and delay generator H 3.5 kv avalanche diode switch J NIM A 588, 359 (28)

19 1.5 High voltages applied to electrodes Electrode voltage (kv) (a): (b): (c): Target foil Grid B Grid D Two FET switches Two avalanche diode switches Open circuit at end of coaxial lines (d): Phosphor Time elapsed (ns)

20 Photodiode signal (V) Electrode voltage (kv) (a): Phosphor (b): Grid D (c): Laser Time elapsed (ns) Time-resolved measurement of pulsed UV beam Rise and fall time of gates 5 ps. UV laser pulse 7 ps.

21 Relay imaging telescope 532 nm, 4 ns laser pulse Nd:YAG laser 164 nm, 6 mj BBO crystal A Harmonic seperator Polarizing beamsplitter Telescope f=13 mm lens Fresnel Rhomb SBS amplifier cell SBS generator cell Iris Telescope BBO crystal B 266 nm, 7 ps laser pulse To experiment Ultraviolet laser pulse generator to simulate Linac-4 beam SBS process to compress Nd:YAG laser pulses to < 7 ps, 266 nm UV laser 1 Megawatt peak power

22 Y position (mm) (a) Y position (mm) (b) CCD intensity (arb. u.) X position (mm) 5x1 7 photoelectrons 2 mm spatial resolution CCD intensity (arb. u.) 4 2 (c) X position (mm)

23 Magnetic field 1 Gauss, acceleration voltage 1 kv Stop motion pictures of the beam Y position (mm) (a) t = -5 ns (b) t = -4 ns (c) t = ns (d) t = 2 ns (e) t = 2.5 ns (f) t = 3 ns (g) t = 3.5 ns (h) t = 4 ns (i) t = 8 ns (j) t = 18 ns (k) t = 23 ns (l) t = 5 ns X position (mm) 5x1 8 photoelectrons, similar to Linac4 intensites. Swirling effect due to cyclotron motion.

24 Normalized CCD signal intensity (arb. u.) Magnetic field, acceleration voltage 1 kv 5x1 8 photoelectrons, similar to Linac4 intensites. Many prepulses and afterpulses appear at regular intervals!? Time elapsed (ns)

25 No magnetic field, lower acceleration voltage (some distortion due to space-charge seen) Y position (mm) Y position (mm) N e N e = 4 (a) t = -2 ns -1 1 = (g) t = -2 ns (b) t = -1 ns -1 1 (h) t = -1 ns (c) t = ns (d) t = 1 ns X position (mm) (i) t = ns (j) t = 1 ns (e) t = 2 ns -1 1 (k) t = 2 ns (f) t = 3 ns -1 1 (l) t = 3 ns X position (mm)

26 No magnetic field, lower acceleration voltage Normalized CCD signal intensity (arb. u.) (a): N e = 4 x 1 8 (b): N e = 2 x 1 7 Better time res. less afterpulses Time elapsed (ns)

27 Y position (mm) (a) (c) (b) (d) Beam profile for various focusing settings of the Orsay beamline X position (mm)

28 x1 4 3 Secondary electrons Protons per microbunch x1 4 Linearity measurements Beam diam 1 cm, f=1 MHz Integrated 1 pulses Linear response between Np=1 and 6x1 4 protons/pulse 6 Secondary electrons Protons per microbunch

29 Conclusions Spatial profile monitor with timing resolution was developed mm spatial, 1 ns timing resolutions. 2. Afterpulse and prepulses <.5% of main pulse 3. High sensitivity - measured Np=1 protons/pulse at Orsay Tandem. 4. Wide dynamic range - measured between Ne=5 and 5x1 8 secondary electrons or photoelectrons. 5. Detector performance is adequate for diagnosing chopped beam in Linac4-to-PS Booster mode. 6. Future R&D needed for Linac4-SPL level performance.

30 Remaining problems for higher-intensity operation (SPL): 1. Wire and foil damage 2. Space-charge effects 3. Prepulses and afterpulses These problems appear to be difficult to solve in an classic interception -type monitor. All these problems can be solved using a laser monitor. Use Nd:YAG laser beam with this monitor and selectively photoneutralize the H- ions