Accelerator, Laser, and Coherent Optoelectronics R&D Lab (ALCOL)

Abstract

Electron beams emitted from a cold cathode are thermally stable and mono-energetic with a small phase-space volume. We have been developing a field-emission type RF-gun system for high brightness electron source applications, including electron scattering/diffraction and tunable coherent X-ray/THz generation. The system consists of a single-gap gun-cavity and an S-band klystron/modulator capable of powering the gun with up to 5.5 MW peak (PRR = 1 Hz, duration = 2.5 ms). The designed gun built with the symmetrised side-couplers has surface field on the cathode ranging 50 – 100 MV/m with 1.3 – 1.7 MW klystron-power and 1.2 field ratio (HFSS). ASTRA simulations also indicate that the gun produces the beam with transverse emittances of less than 1 mm-mrad with 10 – 20 pC bunch charge at 500 keV beam energy. Under the gun operating condition, particle tracking/PIC simulations (CST) show that a single-tip CNT field-emitter produces short pulsed bunches (~ 1/10 RF-cycle) with small emittance (£ 0.01 mm-mrad) and high peak current density  (³ 10,000 kA/cm2). After the gun is fully installed and commissioned, a CNT-tip cathode will be tested with RF-field emission.

Introduction

Beam brightness cannot be improved, but only spoiled along the downstream accelerator. Therefore, high quantum efficiency (QE) and low intrinsic emittance, plus long lifetime, are very necessary for injector-type electron sources. The beam emitters of the electron sources are required to have a high QE and uniform emission at the longest possible wavelength with fast response time (< 100 ps). Emittance can be diluted by space charge expansion seeded by non-uniform electron emission. In order to minimize intrinsic emittance, cathode surfaces should preferably be atomically flat within a few nanometers of peak-to-peak variation. Furthermore, operating lifetime of more than 1 year with a reasonable vacuum level of 10-10 Torr range and easy, reliable cathode cleaning or rejuvenation/re-activation is strongly preferred.

Linac-based coherent radiation sources need bright electron beams, but injectors comprised of conventional DC-guns and RF-bunchers for longitudinal pulse compression have inherent limitations in reducing transverse and longitudinal emittance. While thermionic and photo-cathodes have been most widely employed with injectors for accelerator machines, certainly there are some practical limits in applying them to a compact, high brightness electron source [1 – 4].

Usually, field emitters have been studied for the applications of long pulse operation. The main advantages of field emission are that it simply generates pulsed beam and that the beams emitted under the emitters have low transverse emittance. Normally, the thermionic cathodes and field emitters are tolerant of poor vacuum and they have a long operating lifetime. However, the beam from the electron sources usually has long a bunch length (> 100 ps) and large longitudinal emittance with a lack of beam-profile control. Although high beam currents could be created and accelerated by an electron gun with field-emitters, it is difficult to find stable and reproducible operating conditions. Recently, various cathode materials have been extensively developed for vacuum microelectronic applications [5]. While actual RF devices like field-emitter tube amplifiers are still based on tip arrays of refractory metals, cathodes with nano-crystalline diamond materials would be preferred, if the field emitters have sufficient uniformity and high current stability.

CNT-tip field-emitter cathode

  Flat panel display technology with commercial interest has stimulated research on the fabrication and characterization of nanostructured materials for field emission applications. However, it turned out that low cost fabrication over large areas is very challenging, particularly for applications demanding large emission currents requiring robust emitter materials. It was found that CNTs are quite suitable for the application as they are low-cost, robust, nano-structured material [6]. Previously, field emission of CNT cathodes was demonstrated with multi-walled carbon nanotubes (MWNTs) as well as single-walled carbon nanotubes (SWNTs) [7]. Carbon nanotubes have several advantages over other field-emitting materials. In contrast to commonly used emitters such as tungsten, a nanotube is not a metal, but a structure built by covalent bonds. The activation energy for surface migration of the emitter atoms is thus much larger than for a tungsten electron source. Therefore, the tip can withstand extremely strong fields (several V/nm). Depending on the exact arrangement of the carbon atoms, the cylinders are semiconducting or metallic conductors and the tubes can be closed or open at one or both ends. Furthermore, when compared with other film field emitters such as diamond or amorphous carbon structures, CNTs show a high aspect ratio, a small radius of curvature of the cap and good conductance [8]. The small diameter of CNTs (diameter of a single SWNT can be - 1 nm) enables efficient emission at low fields, despite their relatively high work function (> 4.5eV). At 1 – 3 V/pm of emission threshold fields, CNTs are most suited for low-power, high-current density applications and are more robust than diamond, with the ability to deliver current densities in excess of 1 A/cm2. Some prior work reported that a single CNT is capable of emitting 30 nA, so achievable current densities with CNTs might be orders of magnitude higher. The emitted current from a field emitter depends directly upon the local electric field at the emitting surface determined by very small variations in the emitter geometry and surface conditions. The main variables of emission performance with respect to the amount of electron current are alignment, purification, end type (open or closed), density and spacing of tubes. Remarkable progress has been made on high current emission of the CNT emitters for electron beam devices, but much more work is still needed to achieve the level of uniformity, brightness, and stability required for injector and accelerator application. 

The Fowler-Nordheim equation represents the emission current density on the flat cathode in one-dimensional space. However, FE tips, especially CNTs, have much more complicated physical shapes, so the presence of a CNT tip on the flat geometry can significantly distort the electrical field distribution around the tip area. The spatial field variation allows different sites on the tip to experience different electric fields. Considering the tip geometry and actual field distribution in the vicinity of the tip end, one can see that an accurate evaluation of the emission current of a CNT according to the applied voltage is a critical issue that has been long studied. The possible irregularities of spatial field distributions and geometrical shapes of CNTs resulting from uniformity of arrayed tips, emission characteristics relying on absorbed molecules, and uncontrollable distribution of electronic properties set up unknown factors on the CNT emission characteristics, which are not properly represented in the Fowler-Nordheim equation.

 The ratio of field distortion due to the presence of a charged object(s) in the space is parameterized by the electric field enhancement factor b = E/E0, where E0 (= U/L) is the electric field between the electrodes (U and L: the anode-cathode voltage and distance, respectively). For theoretical analysis, the local electric field, E, was approximated by various analytic forms (e.g. b » r(1 + d/D), where D is the tip-to-tip distance) of the field enhancement factor, b. In principle, as the aspect ratio (r = l/d) of CNTs exceeds 1000, field emission from nanotubes would occur at a much lower voltage applied than conventional field emitters. Typical computer simulation based on electrostatic finite element algorithms can provide very accurate solutions of the local field distribution, E, which is directly implemented in FN equation (Fig. 1). The transverse emittances and intrinsic energy spread of the electron bunch emitted from a cathode are mostly determined by initial beam conditions, when electrons are created at a cathode surface. Although the electron beam emitted from CNT tips has a small transverse emittance, the emission condition of CNTs is strongly dependent upon the field enhancement factor, which sensitively varies with the shape of the nanotube tip.