Ampere-class bright field emission cathode operated at $100\mathrm{MV}/\mathrm{m}$

High-current bright sources are needed to power the next generation of compact rf and microwave systems. A major requirement is that such sources could be sustainably operated at high frequencies, well above 1 GHz, and high gradients, well above $100\mathrm{MV}/\mathrm{m}$. Field emission sources offer simplicity and scalability in a high-frequency era of the injector design, but the output rf cycle charge and high-gradient operation remain a great and largely unaddressed challenge. Here, a field emission cathode based on ultra-nano-crystalline diamond, an efficient planar field emission material, was tested at $100\mathrm{MV}/\mathrm{m}$ in an $L$-band injector. A very high charge of 38 pC per rf cycle was demonstrated (300 nC per rf pulse corresponding to an rf pulse current of 120 mA). This operating condition revealed a two-dimensional space charge limited emission where the one-dimensional Child-Langmuir limit was surpassed. An injector brightness of ${10}^{14}\mathrm{A}/(\mathrm{rad}\mathrm{m}{)}^2$ was estimated for the given operating conditions.

Figure 1

Raman spectrum and SEM micrograph of as-grown sample.

Figure 2

Schematic of the ACT beamline.

Figure 3

Comparison of transverse electron patterns (a) as simulated in gpt and (b) as obtained experimentally, both at YAG3 position. The input emitter distribution was generated and imported using a pattern captured in dc, see Fig. 1 in Ref. [24]. Image (b) was processed with FEpic and counted emitters are labeled by red marks. All other images with local maxima identified and labeled as emitters can be found in the Appendix.

Figure 4

The QvsE curves for Phases 1 and 2 separated into two regions corresponding to the experiment runs. In the legend, the numerical values label the maximum conditioning gradient achieved per QvsE curve (referred to as $E_h$). The inset shows an in situ taken image of the major breakdown event, located on the outer edge of the cathode puck at 1 o’clock (labeled with an arrow), that stopped the Phase 1 session.

Figure 5

Comparison between emission patterns on YAG3 at $70\mathrm{MV}/\mathrm{m}$ taken in Phases 1 (left) and 2 (right, rotated CCW by 50 deg). The images are placed with respect to the origin: since there is slight $y$ offset in the left image, the right rotated image rests below against the left image.

Figure 6

(a) FN plot and (b) $R^2$ plot exampled for $45\mathrm{MV}/\mathrm{m}$. Here, $E$ stands for rf gradient.

Figure 7

Field emission conditioning parameters: (a) field enhancement factor, (b) local field on the cathode surface, and (c) effective emission area for both the FN-like (low field) and non-FN (high field) regions.

Figure 8

Maximum charge and the number of emitters determined at the maximum of every conditioning QvsE curve as functions of $E_h$. The purple horizontal area represents one standard deviation around the mean value of the emitter number count.

Figure 9

Millikan plot for $100\mathrm{MV}/\mathrm{m}$ (blue circles) showing the parallel shift denoted with an arrow and is indicative of a space charge dominated regime that differs from the Child Langmuir law (red line). Here, $E$ stands for rf gradient.

Figure 10

Brightness dynamics inside the gun for (a) Phase 1 and (c) Phase 2 and along the entire beamline from the cathode to YAG3 for (b) Phase 1 and (d) Phase 2. For reference, the gun exit corresponds to the time stamp of 0.45 ns for all the figures.