Effects of pulse-length and emitter area on virtual cathode formation in electron guns

Valfells, Á.; Feldman, D. W.; Virgo, M.; O’Shea, P.G.; Lau, Y. Y.

doi:10.1063/1.1463065

Cited by 122 publications

(138 citation statements)

References 14 publications

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“…This current suppression due to the density of charge per pulse is known as space-charge-limited current. Space-charge-limited current (I SCL ) for pulsed electron emission from a planar surface can be estimated by the single sheet model given by equation (1) [20]. Here, ε 0 represents vacuum permittivity, A denotes the area of emission (1.5 × 10 -8 m 2 ), V is the anode voltage (100 V), f laser is the repetition rate of the laser (3 kHz) and d is the anode-cathode spacing (3 mm).…”

Section: Resultsmentioning

confidence: 99%

High-density Au nanorod optical field-emitter arrays

et al. 2014

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Abstract. We demonstrate the design, fabrication, characterization, and operation of highdensity arrays of Au nanorod electron emitters, fabricated by high-resolution electron beam lithography, and excited by ultrafast femtosecond near-infrared radiation. Electron emission characteristic of multiphoton absorption has been observed at low laser fluence, as indicated by the power-law scaling of emission current with applied optical power. The onset of spacecharge-limited current and strong optical field emission has been investigated so as to determine the mechanism of electron emission at high incident laser fluence. Laser-induced structural damage has been observed at applied optical fields above 5 GVm -1 , and energy spectra of emitted electrons have been measured using an electron time-of-flight spectrometer.

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Section: Resultsmentioning

confidence: 99%

High-density Au nanorod optical field-emitter arrays

et al. 2014

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“…Here, we apply a simpler model in which we treat the photoemitted electrons as a single sheet which shields the cathode from the acceleration field. The surface charge density threshold σ lim at which the field at the cathode is cancelled is then easily derived analytically [36]:…”

Section: Image Charge Limited Emissionmentioning

confidence: 99%

Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun

Pasmans

Vugt

Lieshout

et al. 2016

Phys. Rev. Accel. Beams

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The femtosecond photoemission yield from a copper cathode and the emittance of the created electron beams has been studied in a 12 MeV=m, 100 keV dc electron gun over a wide range of laser fluence, from the linear photoemission regime until the onset of image charge limitations and cathode damaging. The measured photoemission curves can be described well with available theory which includes the Schottky effect, second-order photoemission, and image charge limitation. The second-order photoemission can be explained by thermally assisted one-photon photoemission (1PPE) and by above-threshold two-photon photoemission (2PPE). Measurements with a fresh cathode suggest that the 2PPE process is dominant. The beam emittance has been measured for the entire range of initial surface charge densities as well. The emittance measurements of space-charge dominated beams can be described well by an envelope equation with generalized perveance. The dc gun produces 0.1 pC bunches with 25 nm rms normalized emittance, corresponding to a normalized brightness usually associated with rf photoguns. In this experimental study the limits of femtosecond photoemission from a copper cathode have been explored and analyzed in great detail, resulting in improved understanding of the underlying mechanisms.

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“…This is called the space charge limit. The critical charge that can be emitted is determined by the RF field that is used to pull out the charge, which is given by the Child-Langmuir law [11]:…”

Section: Plasmon Enhanced Photocathodementioning

confidence: 99%

“…If, however, the amount of electrons ejected in one pulse is so large that they e↵ectively screen the cathode from the RF field, those electrons that are ejected last will encounter a field that will push them back into the metal surface. This is the space charge limit, a point at which the photoemission saturates, which is given by [11] Q max = ✏ 0 E rf ⇥ x . The only way then to reduce the emittance is to reduce the excess energy, which, in turn decreases metal's QE reducing the pulse charge.…”

Section: Introductionmentioning

confidence: 99%

Plasmon Enhanced Photoemission

Polyakov

2012

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Next generation ultrabright light sources will operate at megahertz repetition rates with temporal resolution in the attosecond regime. For an X-Ray Free Electron Laser (FEL) to operate at such repetition rate requires a high quantum e ciency (QE) cathode to produce electron bunches of 300 pC per 1.5 µJ incident laser pulse. Semiconductor photocathodes have su cient QE in the ultraviolet (UV) and the visible spectrum, however, they produce picosecond electron pulses due to the electron-phonon scattering. On the other hand, metals have two orders of magnitude less QE, but can produce femtosecond pulses, that are required to form the optimum electron distribution for high e ciency FEL operation. In this work, a novel metallic photocathode design is presented, where a set of nano-cavities is introduced on the metal surface to increase its QE to meet the FEL requirements, while maintaining the fast time response.Photoemission can be broken up into three steps: (1) photon absorption, (2) electron transport to the surface, and (3) crossing the metal-vacuum barrier. The first two steps can be improved by making the metal completely absorbing and by localizing the fields closer to the metal surface, thereby reducing the electron travel distance. Both of these e↵ects can be achieved by coupling the incident light to an electron density wave on the metal surface, represented by a quasi-particle, the Surface Plasmon Polariton (SPP).The photoemission then becomes a process where the photon energy is transferred to an SPP and then to an electron. The dispersion relation for the SPP defines the region of energies where such process can occur. For example, for gold, the maximum SPP energy is 2.4 eV, however, the work function is 5.6 eV, therefore, only a fourth order photoemission process is possible. In such process, four photons excite four plasmons that together excite only one electron. The yield of such non-linear process depends strongly on the light intensity.In this work, the structure consisted of rectangular nano-grooves (NGs) arranged in a subwavelength grating on a metal surface is presented that provides a dramatic increase in the metal's absorption, field localization, and field enhancement. When light is polarized perpendicular to the orientation of the grooves a standing SPP wave is excited along the vertical walls in the NGs, that act as Fabry-Perot resonators. By adjusting the geometry of 2 the NGs and the period of the subwavelength grating the resonance can be fine tuned to a desired position, for example, the laser fundamental wavelength, anywhere from the UV to the near infrared (NIR).Two types of gratings are presented: (a) a gold grating with period of 600 nm, and (b) an aluminum-gold grating with a period of 100 nm; both with resonance at 720 nm. In each case, strong on-resonance absorption was observed, with over 98% for grating (b). Unlike the grating-coupled SPP waves, where the angle is well defined by the momentum matching condition, the resonant NGs allow coupling to the standing modes at a range...

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Effects of pulse-length and emitter area on virtual cathode formation in electron guns

Cited by 122 publications

References 14 publications

High-density Au nanorod optical field-emitter arrays

High-density Au nanorod optical field-emitter arrays

Extreme regimes of femtosecond photoemission from a copper cathode in a dc electron gun

Plasmon Enhanced Photoemission

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