Measurement of back-bombardment temperature rise in microwave thermionic electron guns

Kowalczyk, Jeremy M. D.; Hadmack, Michael R.; Madey, J. M. J.

doi:10.1063/1.4817539

Cited by 5 publications

(4 citation statements)

References 4 publications

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“…Second, whilst the RF is on (∼5 µs in medical systems), the temperature of the cathode can be expected to rise steadily at a rate dependent on the power being delivered by back accelerated electrons. 2,17,23 As such, back-bombardment is one of the principal effects which limit the achievable pulse length in thermionic guns used in both accelerator physics and in therapeutic systems. Conventional DC medical electron guns can be expected to have two advantages compared to the RF type cathode described here when considering backbombardment.…”

Section: E Back-bombardment Powermentioning

confidence: 99%

A novel electron accelerator for MRI‐Linac radiotherapy

et al. 2016

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Purpose: MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. Methods: Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified. Results: For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-widthtenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields. Conclusions: Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility. C 2016 American Association of Physicists in Medicine. [http://dx

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Section: E Back-bombardment Powermentioning

confidence: 99%

A novel electron accelerator for MRI‐Linac radiotherapy

et al. 2016

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“…The difference in the temperature rise of CeB 6 compared to LaB 6 is ascribed to the difference in the thermal properties between CeB 6 and LaB 6 as listed in Table II thermionic RF gun (59 K over 5 ls pulse duration at 2856 MHz). 21 The current density rise is estimated from the numerical simulation to be 4.2 A cm À2 for CeB 6 and 5.3 A cm À2 for LaB 6 . This means that the CeB 6 cathode current density rise due to BB electrons will be 21% less than in LaB 6 ; this behavior is due the emission slope of the materials, as can be seen in Figure 6(b).…”

Section: B Simulation Resultsmentioning

confidence: 99%

Emission properties and back-bombardment for CeB6 compared to LaB6

Bakr

Kawai

Kii

et al. 2015

Journal of Applied Physics

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The emission properties of CeB6 compared to LaB6 thermionic cathodes have been measured using an electrostatic DC gun. Obtaining knowledge of the emission properties is the first step in understanding the back-bombardment effect that limits wide usage of thermionic radio-frequency electron guns. The effect of back-bombardment electrons on CeB6 compared to LaB6 was studied using a numerical simulation model. The results show that for 6 μs pulse duration with input radio-frequency power of 8 MW, CeB6 should experience 14% lower temperature increase and 21% lower current density rise compared to LaB6. We conclude that CeB6 has the potential to become the future replacement for LaB6 thermionic cathodes in radio-frequency electron guns.

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“…We recently developed a method to make temperature measurements of thermionic electron gun cathodes with 40 MHz bandwidth [1] to aid in calculations to counteract back-bombardment heating (BB), but the accuracy of that method can be improved with knowledge of the emissivity across a broad spectral range that is not available in the literature. Additionally, the emissivity equals the absorptivity (Kirchhoff's law of Note that a periscope consisting of two mirrors to bring the light up to the input level of the monochromator is not shown thermal radiation [2]) and determines how efficiently a particular wavelength of light will be absorbed by the material: useful for choosing a laser wavelength to manipulate the material's surface temperature with time to counteract BB.…”

Section: Introductionmentioning

confidence: 99%

Emissivity of Lanthanum Hexaboride Thermionic Electron Gun Cathode

et al. 2014

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The emissivity of the thermionic electron gun cathode material lanthanum hexaboride (LaB 6 ) at an operating temperature of 1622 K has been measured for wavelengths from 550 nm to 2400 nm. The emissivity is calculated from scanning monochromator spectral measurements calibrated with a tungsten lamp and the published emissivity of 0.82 at 1600 K for LaB 6 at 650 nm. The results show a higher emissivity at shorter wavelengths (up to 0.86 at 729 nm) and a lower emissivity in the near-IR to mid-IR (down to 0.41 at 2146 nm). These measurements were motivated by the need to make fast, accurate optical measurements of the temperature of the LaB 6 cathode in our thermionic microwave gun, and to select the wavelength most readily absorbed by the cathode for manipulation of its surface temperature as a means of counteracting back-bombardment heating.

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Measurement of back-bombardment temperature rise in microwave thermionic electron guns

Cited by 5 publications

References 4 publications

A novel electron accelerator for MRI‐Linac radiotherapy

A novel electron accelerator for MRI‐Linac radiotherapy

Emission properties and back-bombardment for CeB6 compared to LaB6

Emissivity of Lanthanum Hexaboride Thermionic Electron Gun Cathode

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