ESS Workshop Bunch Shape Measurements Bunch Length Detector
ESS Workshop “Bunch Shape Measurements” Bunch Length Detector Based on X-Ray Produced Photoelectrons Peter Ostroumov February 11, 2013
Content § BSM based on secondary electrons for CW beams § Motivation for X-BLD § Properties of X-rays produced due to the interaction of proton/ion beams with material § Bunch length detector based on X-ray produced photoelectrons – Principle of operation – Photocathode – Registration of photoelectrons § Experimental results, bunch shape of ion beams § Possible modifications of the detector for application in high-power accelerators 2
Bunch Shape Monitor Based on Secondary Electrons § Very well established technology for proton, H-minus, ion linacs § Requires a wire biased to -10 k. V to produce and accelerate secondary electrons § Limited beam pulse length (~50 to 150 sec) and beam repetition rate to avoid destruction of the wire (for high-intensity beams) § Proton beam space charge and magnetic field can impact on electron trajectory – requires more studies for high current beams § About 12 years ago at Argonne we developed BSM for CW low intensity ion beams 3
BSM for CW Beams § Beam fundamental frequency = 12. 125 MHz, deflector frequency =97 MHz § CCD Camera 4
BSM for Low Intensity CW Beams 5
Electron Beam Image, no RF 6
CW RFQ Off-line Commissioning, July-August 2012 7
300 ke. V/u CW RFQ, Beam Characterization in July 2012 § § 125 µA helium beam accelerated in CW RFQ Bunch frequency = 60. 625 MHz RF deflector frequency = 121. 25 MHz Red –simulations, Blue- measurements with BSM 8
Motivation for X-ray Based BLD § Stripping of heavy-ion beams in high-power driver accelerators – Time focus of the bunched beam on the stripper results in the lowest longitudinal emittance growth – Heavy-ion beam density is high and measurement of the bunch time profile must be non-interceptive – A liquid Li stripper is planned in future heavy-ion linacs – Beam-stripper interaction produces significant amount of X-rays § X-BLD for application in high-power proton, H-minus accelerators – Does not require wire or target permanently inserted into the beam. A gas flow can be used or a solid “needle” can be dropped across the beam – Bunch time profile can be monitored on-line which is not possible using conventional methods 9
X-Ray Production Mechanisms by Bombarding Target with Ion Beam § 2 mechanisms – Characteristic x-rays. – Brehmsstrahlung or "braking radiation" § Characteristic x-rays. – Electrons from higher states drop down to fill the vacancy, emitting Xray photons with precise energies determined by the electron energy levels § Bremsstrahlung (braking) radiation – Charged particles (electrons, ions, protons) are suddenly decelerated upon collision with the metal target – Protons and alpha particles produce negligible bremsstrahlung radiation 10
Properties of X-rays from Target n Potentially, picosecond-level resolution can be acheived n Intensity of K-shell X-rays is 2 order of magnitude higher of all other X-rays n For test purpose at low-intensity ion linac we use copper target n For high-intensity beams: use, for example, xenon gas flow 11
10 Me. V/u Beams on Copper Target § X-ray spectra excited by 10 -Me. V/u projectiles passing through a 965 mg/cm 2 -thick Cu foil, measured with a Si(Li) spectrometer. § The dashed lines indicate the positions of the Cu Kα and Kβ (single vacancy) diagram lines. 12
X-ray Based Bunch Length Detector: Principle of Operation § Main goal was to detect X-rays, produce photoelectrons and measure bunch time structure 13
X-ray Based BLD § General view of the X-BLD installed at the ATLAS facility. 1 – target translator, 2 – target, 3 – photocathode assembly, 4 – RF deflector, 5 – detection unit (chevron MCP), 6 – CCD-camera. 14
X-BLD Installed in ATLAS Beamline CCD camera Phosphor screen Target, photocathode & Deflector 15
Photocathode Properties Back-surface secondary electron quantum yield for a 1020 -Angstrem Cs. I transmission photocathode § The best response of the photocathode is with 1 k. V X-Ray 16
Photocathode § www. luxel. com 17
Photocathode Substrate § Slit dimensions are 10 x 1 mm 2 18
Photocathode Assembly 19
Photocathode – Properties of Photoelectrons Energy distribution of the photoelectrons: Photoelectron energy distribution for Cs. I photocathode excited by X radiation with an energy of 277 e. V. Photoelectron energy distribution for Cs. I photocathode excited by X radiation with an energy of 8080 e. V. • the both distributions are almost the same with a peak at 0. 5 e. V • nearly the same width at half maximum. • 80% of the photoelectrons are below 2 e. V. 20
Trajectory of 10 ke. V Photoelectrons Photocathode a) Electron trajectories X-ray source Collimator b) Potential contours Beam Deflector (focusing) plates 21
Commissioning of the Detector § The slit downstream of the photocathode was too wide (1 mm) – Reduced to 0. 15 mm § Stray magnetic field steers 10 k. V electrons – Shielding against magnetic field § RF frequency of the deflector is 97 MHz and is driven by the accelerator master oscillator § Primary ion beam must be tuned to avoid losses on the walls – Can produce X-rays on the walls which increases the background § Chevron MCP – Very good amplification, up to 108 – Problem: Narrow dynamic range, Dynamic range depends from amplification. Large dynamic range MCPs are available but expensive ~$10 K 22
Electron Beam Image on the Phosphor with no RF Applied § We installed a grounded slit (0. 15 mm) downstream of the photocathode Focused electron beam profile: Resolution is ~5 pixels = 0. 4 mm 23
Oxygen Beam (~1. 6 Me. V/u, 9 Me. V/u, 0. 2 to 0. 5 A) ECR PII Booster Detector location 24
Experience with X-BLD § Main mission of the XBLD development was successfully accomplished: – – We clearly see X-ray produced photoelectrons Successfully used for bunch time profile measurements Excellent properties of the secondary electrons from photocathode No issues with electron beam optics § Beam current at ATLAS is very low, less than 0. 5 µA that we were not able to use “point” source of X-rays § The resolution of our detector was limited by low energy beam size on the target and it is quite low: ~170 psec for 9 Me. V/u beam 25
Possible improvements of the XBLD 26
Time resolution § X-ray source size must be small, below 1 mm § X-rays must be collimated to reduce “visible” source size § Transverse size of the X-ray emitter contributes to resolution Beam X-ray source Photocathode 27
Use a Spectrometer to Select X-rays Produced by Filling of K-shell Vacancies (8 -10 ke. V) § Make sure that X-ray production takes less than 1 femtosecond Electron detecto r 28
High-Power Proton Accelerators Nozzle design to produce a gas jet 0. 15 x 10 mm slit grounded X-ray Spectrometer X-ray collimator Proton beam Gas jet, macroparticles, needles (pulsed) 29
Sub-Picosecond X-ray Streak Camera § Developed for detection of electron bunch length using synchrotron radiation 30
Next Steps § X-BLD with wire target and proton beam – Will greatly increase resolution, perhaps to 5 -10 psec level even for low energy (3 -5 Me. V) beams – ANL hardware components will be available for future experiments § § X-ray spectroscopy Gas target Falling needles or jet of macroparticles Higher proton energies 31
Summary § We have developed and tested an X-ray based Bunch Length Detector (X-BLD) for application in ion and proton accelerators § The sensitivity of X-BLD is high enough to measure bunch length even for ion beams with quite low intensities such as 0. 15 A § Temporal resolution of an X-BLD can be improved for high intensity ion beams by incorporation of an X-ray spectrometer into the device. § An electron beam detection based on MCP-phosphor system has low dynamic range and requires improvement. Secondary Electron Multipliers are proven to be better option. § High power proton and ion accelerators: – X-BLD can be applied for on-line monitoring of the bunch length. Requires either pulsed gas flow or dropping of micro-particles or “needles” across the beam – No thermal issues with the target 32
- Slides: 32