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P Keall - One of the best experts on this subject based on the ideXlab platform.
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a study of the effect of in line and perpendicular magnetic fields on beam characteristics of Electron Guns in medical linear accelerators
2011Co-Authors: D Constantin, Rebecca Fahrig, P KeallAbstract:Purpose: Using magnetic resonance imaging (MRI) for real-time guidance during radiotherapy is an active area of research and development. One aspect of the problem is the influence of the MRI scanner, modeled here as an external magnetic field, on the medical linear accelerator (linac) components. The present work characterizes the behavior of two medical linac Electron Guns with external magnetic fields for in-line and perpendicular orientations of the linac with respect to the MRI scanner. Methods: Two Electron Guns, Litton L-2087 and Varian VTC6364, are considered as representative models for this study. Emphasis was placed on the in-line design approach in which case the MRI scanner and the linac axes of symmetry coincide and assumes no magnetic shielding of the linac. For the in-line case, the magnetic field from a 0.5 T open MRI (GE Signa SP) magnet with a 60 cm gap between its poles was computed and used in full three dimensional (3D) space charge simulations, whereas for the perpendicular case the magnetic field was constant. Results: For the in-line configuration, it is shown that the Electron beam is not deflected from the axis of symmetry of the gun and the primary beam current does not vanish even at very high values of the magnetic field, e.g., 0.16 T. As the field strength increases, the primary beam current has an initial plateau of constant value after which its value decreases to a minimum corresponding to a field strength of approximately 0.06 T. After the minimum is reached, the current starts to increase slowly. For the case when the beam current computation is performed at the beam waist position the initial plateau ends at 0.016 T for Litton L-2087 and at 0.012 T for Varian VTC6364. The minimum value of the primary beam current is 27.5% of the initial value for Litton L-2087 and 22.9% of the initial value for Varian VTC6364. The minimum current is reached at 0.06 and 0.062 T for Litton L-2087 and Varian VTC6364, respectively. At 0.16 T the beam current increases to 40.2 and 31.4% from the original value of the current for Litton L-2087 and Varian VTC6364, respectively. In contrast, for the case when the Electron gun is perpendicular to the magnetic field, the Electron beam is deflected from the axis of symmetry even at small values of the magnetic field. As the strength of the magnetic field increases, so does the beam deflection, leading to a sharp decrease of the primary beam current which vanishes at about 0.007 T for Litton L-2087 and at 0.006 T for Varian VTC6364, respectively. At zero external field, the beam rms emittance computed at beam waist is 1.54 and 1.29π-mm-mrad for Litton L-2087 and Varian VTC6364, respectively. For the in-line configuration, there are two particular values of the external field where the beam rms emittance reaches a minimum. Litton L-2087 rms emittance reaches a minimum of 0.72π and 2.01π-mm-mrad at 0.026 and 0.132 T, respectively. Varian VTC6364 rms emittance reaches a minimum of 0.34π and 0.35π-mm-mrad at 0.028 and 0.14 T, respectively. Beam radius dependence on the external field is shown for the in-line configuration for both Electron Guns. Conclusions: 3D space charge simulation of two Electron Guns, Litton L-2087 and Varian VTC6364, were performed for in-line and perpendicular external magnetic fields. A consistent behavior of Pierce Guns in external magnetic fields was proven. For the in-line configuration, the primary beam current does not vanish but a large reduction of beam current (up to 77.1%) is observed at higher field strengths; the beam directionality remains unchanged. It was shown that for a perpendicular configuration the current vanishes due to beam bending under the action of the Lorentz force. For in-line configuration it was determined that the rms beam emittance reaches two minima for relatively high values of the external magnetic field.
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tu b 204b 02 a study of the effect of inline and perpendicular magnetic fields on beam characteristics of medical linear accelerator Electron Guns
2010Co-Authors: D Constantin, Rebecca Fahrig, P KeallAbstract:Purpose: Integrated MRI‐linacs have the potential for real‐time volumetric imaging and targeting. Both inline (main MRI field parallel to linac beam) and perpendicular (main field orthogonal to linac beam) configurations are considered. The MRI fringe fields affect beam generation and transport, particularly the Electron gun, where the Electron energy is the lowest and thus magnetic fields have the largest effect. This work characterizes the Electron gun behavior in external magnetic fields for inline and perpendicular configurations. Methods: Two Electron Guns were studied, the Litton L2087 and Varian VTC6364. Based on fringe field measurements of a 0.5T open bore MRI scanner (GE Signa SP), field strengths of 0–0.16T were computed with the finite elements method. Space charge beam simulations were performed for both inline and perpendicular configurations. Emitted current and beam deflection were determined. Results: For the inline configuration, the Electron beam remains aligned with the gun axis. As the field strength increases, the emitted current has an initial plateau of constant value after which its value decreases to a minimum near 0.06T. The minimum of the emitted current is 26% and 21% from the zero field value for Litton and Varian Guns respectively. Above 0.06T, the emitted current increases monotonically. For the perpendicular configuration, the Electron beam is deflected from the gun axis even at small field strengths. The beam deflection increases with the magnetic field value, leading to a sharp decrease of the emitted current which completely vanishes at about 0.007T for Litton and 0.006T for Varian. Conclusion: For the inline configuration, there is always emitted current along the gun axis thus leading to the possibility to adapt the gun geometry for optimal beam generation compatible with the inline configuration. For the perpendicular configuration, magnetic shielding of the Electron gun is required to avoid beam bending. Support: NIH T32‐CA09695
K. Surles-law - One of the best experts on this subject based on the ideXlab platform.
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Ion back-bombardment of Gaas photocathodes inside DC high voltage Electron Guns
2005Co-Authors: J. Grames, D. Charles, J. Brittian, M. Stutzman, J. Clark, J. Hansknecht, P. Adderley, M. Poelker, K. Surles-lawAbstract:DC high voltage GaAs photoGuns are key components at accelerator facilities worldwide. New experiments and new accelerator facilities demand improved performance from these Guns, in particular higher current operation and longer photocathode operating lifetime. This conference submission explores bulk GaAs photocathode lifetime as a function of beam current, active photocathode area, laser spot size and the vacuum of the gun and beam line. Lifetime measurements were made at 100 μA, a beam current relevant for accelerators like CEBAF, and at beam currents of 1 mA and 5 mA, a regime that is interesting for high current Free Electron Laser (FEL) and Energy Recovery Linac (ERL) operation.
M Matoba - One of the best experts on this subject based on the ideXlab platform.
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development of soft x ray sources with Electron Guns of field emission and thermal cathode types and their application to counter experiments
1996Co-Authors: Hidenobu Ijiri, Hideaki Ohgaki, Yusuke Uozumi, T Sakae, M MatobaAbstract:Soft X-ray sources have been developed with Electron Guns of field emission- and thermal cathode-types. Those sources are used mainly to investigate the Electron drift and avalanche properties in a gas counter. Thin films of aluminum, titanium, iron, copper and nickel are used as the X-ray targets. Beams of quasi-monochromatic X-rays are obtained with these targets. The emitted X-rays consist of characteristic and bremsstrahlung radiations, which are checked by using a high purity germanium detector. Generation of pulsed X-rays is also tried. As an example of the application of the X-ray source, results of measurements of the SQS (Self Quenching Streamer) phenomenon in a gas counter have been shown.
P G Oshea - One of the best experts on this subject based on the ideXlab platform.
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negative transconductance in apertured Electron Guns
2007Co-Authors: J R Harris, P G OsheaAbstract:Passing an Electron beam through an aperture can serve to reduce the beam current or change the transverse beam profile. For a sufficiently intense beam, space charge will drive a radial expansion of the beam, which may cause the current passing through the aperture to increase even though the current arriving at the aperture is decreasing. When a gridded Electron gun is used, this may be expressed by stating that the transconductance of the apertured gun is negative. Here we explain this effect, and explore some of the key factors governing when it can occur and influencing its strength.
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gridded Electron Guns and modulation of intense beams
2006Co-Authors: J R Harris, P G OsheaAbstract:Gridded Guns are useful for producing modulated Electron beams. This modulation is generally limited to simple gating of the beam but may be used to apply structure to the beam pulse shape. In intense beams, this structure spawns space-charge waves whose dynamics depend in part on the relative strengths of the velocity and density variations which comprise the initial current modulation. In this paper, the strengths of beam current and velocity modulation produced in a gridded Electron gun are calculated, and it is shown that under normal conditions, the initial modulation is dominated by density variation rather than velocity variation
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effects of pulse length and emitter area on virtual cathode formation in Electron Guns
2002Co-Authors: P G Oshea, A Valfells, D W Feldman, M Virgo, Y Y LauAbstract:Recent experiments at the University of Maryland using photoemission from a dispenser cathode have yielded some interesting results regarding the effects of the area of emission and of the ratio between the pulse length and the gap transit time on the amount of current that may be drawn from an Electron gun before a virtual cathode forms. The experiments show that a much higher current density may be drawn from a short pulse or limited emitter area than is anticipated by the Child–Langmuir limiting current. There is also evidence that the current may be increased even after virtual cathode formation, which leads a distinction between a limiting current density and a current density critical for virtual cathode formation. The experiments have also yielded some interesting results on the longitudinal structure of the current pulse passed through the anode. Some empirical and theoretical scaling laws regarding the formation of virtual cathodes in an Electron gun will be presented. This work was motivated by the needs of the University of Maryland Electron Ring (UMER) [P. G. O’Shea, M. Reiser, R. A. Kishek et al., Nucl. Instrum. Methods Phys. Res. A 464, 646 (2001)] where the goal is to generate pulses that are well-localized in time and space.
V.c. Semyonov - One of the best experts on this subject based on the ideXlab platform.
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experimental study of matrix carbon field emission cathodes and computer aided design of Electron Guns for microwave power devices exploring these cathodes
1997Co-Authors: Y.a. Grigoriev, V I Rogovin, A I Petrosyan, V V Penzyakov, V.g. Pimenov, V.i. Shesterkin, V.p. Kudryashov, V.c. SemyonovAbstract:The experimental study of matrix carbon field-emission cathodes (MCFECs), which has led to the stable operation of the cathodes with current emission values up to 100 mA, is described. A method of computer aided design of TWT Electron Guns (EGs) with MCFEC, based on the results of the MCFEC emission experimental study, is presented. The experimental MCFEC emission characteristics are used to define the field gain coefficient K and the cathode effective emission area Seff. The EG program computes the electric field upon the MCFEC surface, multiplies it by the K value and uses the Fowler–Nordheim law and the Seff value to calculate the MCFEC current; the Electron trajectories are computed as well.
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Experimental study of matrix carbon field emission cathodes and computer-aided design of Electron Guns for microwave power devices, exploring these cathodes
1996Co-Authors: Y.a. Grigoriev, V I Rogovin, A I Petrosyan, V V Penzyakov, V.g. Pimenov, V.i. Shesterkin, V.p. Kudryashov, V.c. SemyonovAbstract:Experimental study of matrix carbon field emission cathodes (MCFEC) which has led to stable operation of the cathodes with emission current value up to 100 mA, is described. A method of computer-aided design of TWT Electron Guns (EC) with MCFEC, based on the results of the MCFEC emission experimental study is presented. The experimental MCFEC emission characteristics are used to define the field gain coefficient K and cathode effective emission area Seff. The EG program computes the electric field upon the MCFEC surface, multiplies it values by the K value and uses F-N law and the Seff value to calculate the MCFEC current; the Electron trajectories are computed as well.