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Leistungen
The carbon nanotube cathode is an array of individual and vertically aligned carbon nanotube (CN) array. A typical array includes 5µm height and 50 nm diameter CNs. The pitch between two neighbouring CNs is equal to twice the CNT height i.e. 10µm. Typically, the standard deviation in the radius and height distributions of the CNs is 4.1% and 6.3% respectively so that the aspect ratio (h/r) of most of the nanotubes in the array is around 200 +/- 7.5%.
After a dedicated post treatment process (E. Minoux, et al., "Achieving High Current Carbon Nanotube Emitter", accepted for publication in Nano Letters (2005)), each nanotube of the array is capable to emit a current of 100 +/-20µA. The maximum current density for an array operated in DC mode is around 1.5A/cm2 and becomes 12A/cm2 when this cathode is operated at 1.5GHz (K. Teo et al., Nature 437, 968 (2005)).
Recently, we fabricated an optimised array made of 1.5µm height and 30nm diameter CNs (pitch = 3µm) that delivers a current density of 4A/cm2 in continuous mode.
Such high-density cathodes should find application in microwave amplifiers but also in X-ray tubes.
We have developed wafer scale compatible carbon nanotube growth and processing techniques. These, for the first time, allow for the highly selective, clean and preferential growth of individual carbon nanotubes at pre-determined locations on a substrate. These nanotubes exhibit extremely high uniformity, typically 4% deviation in diameter and 6% deviation in height. Such structures are very useful as high performance electron sources, and thus can be applied in electron microscopy, lithography, holography and field emission displays, as well as the microwave amplifier application demonstrated in CANVAD.
To define the preliminary design of a Microwave vacuum amplifier, calculations of some parameters to design a diode and triode test vehicles have been done.
The mains parameters calculated were:
- the emitter-grid capacitance of the cathode/grid system.
- the electron transit time between cathode and grid.
- the distribution of the RF electrical field in a RF diode working at frequency of 1.5Ghz and in a RF triode working at 32Ghz.
- the geometrical characteristics of the input and output cavity of the 32GHz triode.
- the equivalent circuits of the input and output cavity.
First, the electron emission of the CNT cathodes has been measured in a RF diode having a resonance frequency of 1.5 GHz. In this device a 1.5GHz electrical field of 35MV/m is created in front of the CNT cathode when a 3kW peak power (duty cycle 1%) is injected in the cavity. Then,it has been measured an average emission current density of 1.5A/cm2 corresponding to a peak current density of 15 A/cm2.
Second, the electron emission of the CNT cathodes has been measured in the 32Ghz triode vehicle. In this device a DC electrical field of ~ 30MV/m is created in front of the CNT cathode by a DC polarisation of a grid. The field modulation is created by injection of a 32GHz RF signal of a few watts in the input cavity. A current density of 0.15A/cm2 modulated at 32GHz was measured in the output cavity.
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