J. D. Jarvis

Resonant tunneling and extreme brightness from diamond field emitters and carbon nanotubes

We report new results from field emission microscopy studies of multiwall carbon nanotubes and from energy spectrum measurements of beams from diamond field emitters. In both systems, we find that resonant tunneling through adsorbed species on the emitter surface is an important and sometimes dominant effect. For diamond emitters our observations include order-of-magnitude emission enhancement without spectral broadening, complex spectral structure, and sensitivity of that structure to the applied electric field. For carbon nanotubes we have observed electron beams from individual adsorbates which are estimated to approach the maximum beam brightness allowed by Pauli exclusion.

Effect of reflections and losses in Smith?Purcell free-electron lasers

We have included the effects of losses in the grating surface and reflections at the ends of the grating in the theory of Smith-Purcell free-electron lasers. Computations show that losses typically increase the start current by about 10%. The complex reflection coefficient for the evanescent wave is computed using numerical simulations, and is found to have a magnitude on the order of 30%. This typically increases or decreases the start current by about 10%, depending on the phase of the round-trip reflection.

Three-dimensional theory of the Smith–Purcell free-electron laser with side walls

We present an analytic theory for the exponential-gain (growth) regime of a Smith-Purcell free-electron laser amplifier (oscillator), which includes the effects of transverse diffraction in the optical beam. The optical mode is guided by the electron beam, having a mode width that depends upon the gain length. For the case of a wide electron beam, the dispersion relation converges with that of the 2-D theory. When the electron beam is narrow, the conventional cubic-dispersion relation is replaced by a five-halves dispersion. The dispersive properties of the grating divide device operation into four distinct regions, two amplifier and two oscillator. The number and location of physically allowed roots changes depending on operating region. Additionally, in the narrow-beam case, new challenges arise in satisfying the boundary conditions required for operation as an oscillator

Operation of an ungated diamond field-emission array cathode in a L-band radiofrequency electron source

We report on the operation of a field-emitter-array cathode in a conventional L-band radio-frequency electron source. The cathode consisted of an array of ∼106 diamond tips on pyramids. Maximum current on the order of 15 mA was reached and the cathode did not show appreciable signs of fatigue after weeks of operation. The measured Fowler-Nordheim characteristics, transverse beam density, and current stability are discussed.

Emittance measurements of electron beams from diamond field emitter arrays

Electron injector technology is presently dominated by a variety of photo- and thermionic electron injectors. Although new electron injectors based on field emission appear promising, their success is predicated on the development of reliable, high current density, low emittance, and spatially uniform, field emitter array cathodes. The authors report recent results of transverse-emittance measurements on a particularly promising cathode, the diamond field emitter array. A simple pepperpot technique is used to measure the divergence of the beam emitted from one such cathode at low current density. Based on these measurements, a 1 mm diameter uniformly emitting cathode will have a normalized transverse emittance of ∼1 mm mrad. Our results suggest that the beam quality of these cathodes is satisfactory for use in a variety of applications.

NEEDLE CATHODES FOR HIGH-BRIGHTNESS BEAMS

At the tips of sharp needles, the surface electric field is enhanced by many orders of magnitude. This intensifies thermionic emission and photoemission of electrons through the Schottky effect, and reduces the effect of space charge. The increased current density improves the brightness of electron sources by orders of magnitude. In addition, at very high fields (>109V/m), field emission and photo-field emission produce very high current density. Arrays of needles can be used to achieve high total current.

R&D Toward a Compact High-Brilliance X-Ray Source Based on Channeling Radiation

X-rays have been valuable to a large number of fields including Science, Medicine, and Security. Yet, the availability of a compact high-spectral brilliance X-ray sources is limited. A technique to produce X-rays with spectral brilliance B ∼ 1012 photons.(mm-mrd)−2. (0.1% BW)−1.s−1 is discussed. The method is based on the generation and acceleration of a low-emittance field-emitted electron bunches. The bunches are then focused on a diamond crystal thereby producing channeling radiation. In this paper, after presenting the overarching concept, we discuss the generation, acceleration and transport of the low-emittance bunches with parameters consistent with the production of high-brilliance X-rays through channeling radiation. We especially consider the example of the Advanced Superconducting Test Accelerator (ASTA) currently in construction at Fermilab where a proof-of-principle experiment is in preparation.

9.5: Resonant tunneling and extreme brightness from diamond field emitters and carbon nanotubes

We report new results from field emission microscopy studies of multi-wall carbon nanotubes and from energy-spectrum measurements of beams from diamond field emitters. In both systems, we find that resonant tunneling through adsorbed species on the emitter surface is an important and sometimes dominant effect. For diamond emitters our observations include order of magnitude emission enhancement without spectral broadening, complex spectral structure, and sensitivity of that structure to the applied electric field. For carbon nanotubes we have observed electron beams from individual adsorbates which approach the maximum beam brightness allowed by Pauli exclusion.

Diamond field emission arrays (DFEAs) for high-power free electron lasers

Compared to the present prevalent use of photocathodes for free-electron lasers (FELs), the field-emitter array (FEAs) possesses several advantages as an electron source, namely, high brightness, ruggedness, no drive laser requirement and minimal heat generation. And, compared to Spindt-type molybdenum (Mo) and silicon-tips FEAs, diamond FEAs (DFEAs) have strong carbon-carbon covalent bonding (high activation energy for electromigration), are chemically inert, mechanically and thermally stable, with a low sputter coefficient (resistant to bombardment of positive ions) and low electron affinity for efficient electron emission.

Compact long wavelength free-electron lasers

The idea of using the Smith-Purcell effect to build a compact (table-top) long wavelength (0.1 -1 mm) free-electron laser is quite old. However, it is only recently that a complete theory for the operation of such devices has been proposed. The current state of the theoretical and experimental efforts to understand these devices will be summarized.