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Apr 2012

Volume 83, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Rev. Sci. Instrum. 83, 041101 (2012); http://dx.doi.org/10.1063/1.3697599 (19 pages)

Michael A. Duncan

The laser vaporization cluster source in the "cutaway" configuration. The sample rod is mounted from above with a flexible nylon screw in a holding block. The pulsed gas valve is mounted in the stainless steel can (left) and the skimmer is mounted on the opposite wall.

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back to top Particle Sources, Optics and Acceleration; Particle Detectors

Monte Carlo code G3sim for simulation of plastic scintillator detectors with wavelength shifter fiber readout

P. K. Mohanty, S. R. Dugad, and S. K. Gupta

Rev. Sci. Instrum. 83, 043301 (2012); http://dx.doi.org/10.1063/1.3698089 (10 pages)

Online Publication Date: 3 April 2012

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A detailed description of a compact Monte Carlo simulation code “G3sim” for studying the performance of a plastic scintillator detector with wavelength shifter (WLS) fiber readout is presented. G3sim was developed for optimizing the design of new scintillator detectors used in the GRAPES-3 extensive air shower experiment. Propagation of the blue photons produced by the passage of relativistic charged particles in the scintillator is treated by incorporating the absorption, total internal, and diffuse reflections. Capture of blue photons by the WLS fibers and subsequent re-emission of longer wavelength green photons is appropriately treated. The trapping and propagation of green photons inside the WLS fiber is treated using the laws of optics for meridional and skew rays. Propagation time of each photon is taken into account for the generation of the electrical signal at the photomultiplier. A comparison of the results from G3sim with the performance of a prototype scintillator detector showed an excellent agreement between the simulated and measured properties. The simulation results can be parametrized in terms of exponential functions providing a deeper insight into the functioning of these versatile detectors. G3sim can be used to aid the design and optimize the performance of scintillator detectors prior to actual fabrication that may result in a considerable saving of time, labor, and money spent.
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29.40.Mc Scintillation detectors

Demonstration of a real-time interferometer as a bunch-length monitor in a high-current electron beam accelerator

J. Thangaraj, G. Andonian, R. Thurman-Keup, J. Ruan, A. S. Johnson, A. Lumpkin, J. Santucci, T. Maxwell, A. Murokh, M. Ruelas, and A. Ovodenko

Rev. Sci. Instrum. 83, 043302 (2012); http://dx.doi.org/10.1063/1.3698388 (6 pages)

Online Publication Date: 3 April 2012

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A real-time interferometer (RTI) has been developed to monitor the bunch length of an electron beam in an accelerator. The RTI employs spatial autocorrelation, reflective optics, and a fast response pyro-detector array to obtain a real-time autocorrelation trace of the coherent radiation from an electron beam thus providing the possibility of online bunch-length diagnostics. A complete RTI system has been commissioned at the A0 photoinjector facility to measure sub-mm bunches at 13 MeV. Bunch length variation (FWHM) between 0.8 ps (∼0.24 mm) and 1.5 ps (∼0.45 mm) has been measured and compared with a Martin-Puplett interferometer and a streak camera. The comparisons show that RTI is a viable, complementary bunch length diagnostic for sub-mm electron bunches.
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07.60.Ly Interferometers
29.20.-c Accelerators

Source fabrication and lifetime for Li+ ion beams extracted from alumino-silicate sources

Prabir K. Roy, Wayne G. Greenway, and Joe W. Kwan

Rev. Sci. Instrum. 83, 043303 (2012); http://dx.doi.org/10.1063/1.4704457 (6 pages)

Online Publication Date: 18 April 2012

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A space-charge-limited beam with current densities (J) exceeding 1 mA/cm2 have been measured from lithium alumino-silicate ion sources at a temperature of ∼1275 °C. At higher extraction voltages, the source appears to become emission limited with J ⩾ 1.5 mA/cm2, and J increases weakly with the applied voltage. A 6.35 mm diameter source with an alumino-silicate coating, ⩽0.25 mm thick, has a measured lifetime of ∼40 h at ∼1275 °C, when pulsed at 0.05 Hz and with pulse length of ∼6 μs each. At this rate, the source lifetime was independent of the actual beam charge extracted due to the loss of neutral atoms at high temperature. The source lifetime increases with the amount of alumino-silicate coated on the emitting surface, and may also be further extended if the temperature is reduced between pulses.
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07.77.Ka Charged-particle beam sources and detectors
29.25.Ni Ion sources: positive and negative
41.75.Ak Positive-ion beams

Frequency control in the process of a multicell superconducting cavity production

Valery Shemelin and Paul Carriere

Rev. Sci. Instrum. 83, 043304 (2012); http://dx.doi.org/10.1063/1.4705985 (9 pages) | Cited 1 time

Online Publication Date: 26 April 2012

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Modifications in the geometry of a superconducting RF cavity due to various processing procedures are presented in a convenient matrix formulation. Specifically, the effect of chemical etching, cooling down, and preloading are characterized, while the corresponding frequency shifts are calculated with a reliable software. This matrix method was used in the fabrication of the first cornell energy recovery linac (ERL) 7-cell cavity. Cavity fabrication can be broken down into three main stages: deep-drawing cups, welding the cups in pairs to obtain “dumbbells” and end groups, and, finally, welding the obtained components into a completed cavity. Frequency measurements and precise machining were implemented after the second stage. A custom RF fixture and data acquisition system were designed and validated for this purpose. The system comprised of a mechanical press with RF contacts, a network analyzer, a load cell and custom LABVIEW and MATLAB scripts. To extract the individual frequencies of the cups from these measurements, the established algorithm of calculations was analysed and corrected. Corrections for the ambient environment were also incorporated into the measurement protocol. Using the procedure presented, the frequency deviation of the completed 1.3 GHz 7-cell cavity was 360 kHz, corresponding to an average error about 75 μm in length for every cell.
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29.20.Ej Linear accelerators
02.60.Dc Numerical linear algebra
85.25.Am Superconducting device characterization, design, and modeling
29.27.Eg Beam handling; beam transport
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