In SituObservation of Dark Current Emission in a High Gradient rf Photocathode Gun

Abstract

Undesirable electron field emission (a.k.a. dark current) in high gradient RF photocathode guns deteriorates the quality of photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (∼100 µm) dark current imaging experiment has been performed in an L-band photocathode gun operating at ∼100 MV/m of surface gradient. Dark current from the cathode has been observed to be dominated by several separated strong emitters. The field enhancement factor, β, of selected regions on the cathode has been measured. The post scanning electron microscopy (SEM) and white light interferometer (WLI) surface examinations reveal the origins of ∼75% strong emitters overlap with the spots where rf breakdown have occurred.Electrons can tunnel through a surface barrier modified by the presence of an electric field, resulting in a field emission (FE) current [1][2][3][4]. While the existence of this physical phenomenon allows the operation of field emission electron sources [5][6][7][8], it has a negative (parasitic) impact on the performance of vacuum resonator-based dc and rf systems such as traveling wave tubes, photocathode guns, and particle accelerators [3,[9][10][11][12]. The troublesome field emission current is referred to as dark current. It is an incoherent source of electrons that impacts the energy budget of a device, and is a source of undesired secondary electrons and ions [9,13,14]. Historically, dark current has been considered to be a trigger of breakdown in vacuum devices which may interrupt the normal operation of the device and even jeopardize the entire facility [3].To date many questions surrounding FE still remain, especially in the rf case which limit the improvement of electron sources and high gradient accelerators for TeVscale linear colliders [15] and compact X-ray electron sources [16,17]. For example, a large discrepancy exists between emitter properties obtained through direct observation using advanced surface analysis tools and those indirectly obtained from fitting the experimental data to the Fowler-Nordheim (F-N) equation [3,18]; the temporal evolution of the FE area under high electromagnetic fields is mostly unknown [19]; and empirical methods and procedures to suppress or enhance dark current lack theoretical support. All these questions result from the lack of a means for in situ high-resolution FE observation. In earlier FE studies under a dc field, emitter mapping with better than 1 µm resolution has been achieved by scanning an anode along the cathode [20][21][22]. However, imaging the field emitters at high resolution while they are emitting under an rf field is extremely challenging due to the wide emitting phase (the timing with respect to the applied rf field) and energy spread range of the dark current [13,[23][24][25]. In this Letter, we present observations of in situ dark current emission in a high gradient photocathode gun using a dedicated dark current imaging beamline. The study was conducted at Argonne Wakefie...

High-gradient breakdown studies of an X -band Compact Linear Collider prototype structure

“…Diagnostics used in the experiment include three directional couplers to monitor the input, reflected, and transmitted rf signals and two Faraday cups to collect upstream and downstream dark currents during rf conditioning. These signals were monitored pulse by pulse for breakdown detection [17,38,39]. Normal rf pulse signal and typical breakdown signal are shown in Fig.…”

Confirmation of Transit-Time Limited Field Emission in Advanced Carbon Materials with Fast Pattern Recognition Algorithm

“…Many studies [1][2][3][4][5][6] have convincingly demonstrated that the electron emission from a large surface area field emission cathode placed in a macroscopic electric field E is not uniform. The emission area is only a small portion of the total surface area of a cathode since most of the emission is confined to a small number of emission spots randomly distributed over the cathode surface.…”

“…Second is fundamental: only a properly established j-E (and not I-V ) relationship can be used to define the validity range of a classical Fowler-Nordheim (FN) emission and clarify the role of other mechanisms that could cause deviation from the FN emission, i.e. cause non-conventional behavior that is being observed across a large body of experimental work [1][2][3][4][5][6][7][8][9][10] .…”

Fast Pattern Recognition for Electron Emission Micrograph Analysis

“…Many studies [1][2][3][4][5][6] have convincingly demonstrated that electron emission from a large surface area field emission cathode placed in a macroscopic electric field E is not uniform. The area contributing to emission is only a small portion of the total surface area available for emission.…”

“…Second is fundamental: only a properly established j-E (and not I-V ) relationship can help define the validity range of a classical Fowler-Nordheim (FN) emission and clarify the role of other mechanisms at play that cause deviation from the FN emission, i.e. cause nonconventional behavior, that is have being observed across a large body of experimental work [1][2][3][4][5][6][7][8][9][10] .…”