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

Abstract

We report on the first successful operation of a field-emitter-array cathode in a conventional Lband radio-frequency electron source. The cathode consisted of an array of ∼ 10 6 diamond diamond tips on pyramids. Maximum current on the order of 15 mA were 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. Numerical simulations of the beam dynamics are also presented.PACS numbers: 41.75.Fr Over the past years, field-emission (FE) electron sources have been the subject of intense investigations due to several advantages they offer over photoemission and thermionic sources. The main advantages of FE sources stem from their ability to produce very lowemittance bunched beams, their capability to generate high-average current beams, and the absence of requirement for an auxiliary laser system. A single-tip FE cathode emits electrons from a very small transverse area and can therefore produce beams with extremely small, near quantum-degenerate, transverse emittances [1,2]. When arranged as large arrays, field-emission-array (FEA) cathodes can provide substantial average currents [3] to the detriment of emittance which then scales linearly with the FEA macroscopic radius [4,5].Pulsed field-emission occurs when a FE cathode experiences a time-dependent field, e.g., when located in a resonant radiofrequency (RF) cavity. Taking the example of a cylindrical-symmetric resonant pillbox cavity operating on the TM 010 mode with axial electric field E z (r = 0, z, t) = E 0 cos(2πf t), where f and E 0 are respectively the field frequency and peak amplitude, field-emitted bunches have a root-mean-square (rms) duration σ t ≃ ω −1 [β e E 0 /B(φ)] 1/2 where ω ≡ 2πf . The latter pulse duration is obtained by taking the current density to follow the Fowler-Nordheim's (F-N) law [6] j(t) = A(φ)β 2 e E(t) 2 exp[−B(φ)/(β e E(t))] where A(φ) and B(φ) are functions of the work function φ of the cathode material and β e is a field-enhancement factor [7,8]. Nominally, the bunch rms duration is a significant fraction of the RF field period typically resulting in beams with large energy spread. This limitation can however be circumvented by exposing the FE cathode to superimposed electromagnetic fields operating at harmonic frequencies with properly tuned relative phases and amplitudes. A practical implementation of this technique consists in a RF gun supporting two harmonic modes with axial electric fields [9].In this letter we report on the first operation of a diamond FEA (DFEA) cathode in a conventional L-band RF gun nominally operated with a Cesium Telluride (Cs 2 Te) photocathode. The DFEA is composed of ungated diamond pyramids which have proven to be rugged. Depending on the size and pitch of the pyramids, tests under DC voltages have showed field emission to begin at macroscopic fields E 0 ≃ 5 MV/m, and peak currents per tip as high as 15 µA has been obtained [10].The geometry of the DFE...

Beam dynamics simulations of optically-enhanced field emission from structured cathodes

“…The cathode has a similar geometry to the one experimentally investigated in Ref. [9] and also includes an anode (see Fig. 1, left column).…”

Bunch shaping experiments at the high brightness electron beam source laboratory (HBESL)

“…The deflecting mode cavity is a normal conducting, water cooled, 3.9 GHz copper cavity that is powered by a 50 kW klystron. While our experiment used a UV laser (∼100fs) illuminating a Cs 2 Te photocathode, several experiments involving multiphoton emission [5], ellipsoidal bunch generation [6], diamond field emitter array cathode [7], carbon nanotube cathodes, channelling radiation [8]and inverse compton scattering [9] have been done or proposed at HBESL. The alpha-BBO crystal is placed right before the UV enters the cathode and the amount of UV sent to the crystal is controlled using a remote-controllable aperture.…”

Initial beam dynamics simulations of a high-average-current field-emission electron source in a superconducting radiofrequency gun

“…The cathode is located at z = 0 and its outer radius was taken to be 3 mm. The work function was taken to be Φ = 4.5 eV (carbon-based field emitter) and a field enhancement factor β e = 700 was selected consistent with, e.g., values observed in carbon-nanotube field emitters [5]. We note that lower field-enhancement factors could be employed at the expense of increasing the peak field on the cathode.…”

“…FE has gained much interest over the last decade owing to new development in field-emission displays and vacuum electronic devices. Additionally, FE has also been extensively investigated as electron sources for bright [4] or high-current [5] electron beams. The theory behind FE is well established and the current density is related to the applied electric field E z (t) via the Fowler-Nordheim's law [6,7]…”