"This is a breakthrough in electron source technology," Bill Milne of the University of Cambridge told nanotechweb.org. "The total expected size and weight reduction from the electron source, tube and collector is around 50%. This is highly significant because today, each conventional microwave amplifier based on thermionic sources is 30 cm long and weighs 1 kg, and a typical communications satellite carries around 50 amplifiers. The size and weight savings would lead directly to more devices being carried on each satellite or lighter satellites with cheaper launch costs, since it costs £10,000 to send 1 kg into space."

In a conventional microwave amplifier, a hot cathode source generates a constant stream of electrons. The input radio-frequency (RF) signal that requires amplification is applied to a coil, or helix. "The electrons travelling through this helix experience the electric field from the input signal and slow down or speed up accordingly, forming bunches through a process known as velocity modulation," explained Milne.

Typically, only about 30% of the beam is converted to bunches and the efficiency is even lower at higher frequencies. The amplifier applies additional energy to the bunches before a second helix converts the electron bunches to an amplified RF signal. "This amplified output is then coupled to an antenna (e.g. a dish) for transmission thousands of miles back to earth," said Milne.

In the nanotube devices, in contrast, a carbon-nanotube array acts as a cold cathode electron source. The source emits bunches of electrons directly by turning on and off in response to the input RF signal, in a process known as temporal modulation. The devices produced electron bunches with a peak current of 12 A/ cm2 at a pulse frequency of 1.5 GHz.

"There is no need for heating (since carbon nanotubes are cold cathodes), no need for two-thirds of the length of the interaction tube (since the bunches are directly obtained at the carbon nanotubes), and the collector can be simplified as all electrons have the same velocity and fewer electrons are being dumped," said Milne. "With hot cathodes, there is a need for a multi-stage collector as the electrons have different speeds (i.e. are dispersed) from the inefficient bunching process."

Each cold cathode contained 2,500 nanotubes, spaced at twice their height apart to give maximum electrostatic efficiency. To create high current densities, the nanotubes must be similar in shape, height and diameter so that a large proportion of them can emit electrons simultaneously. With this in mind, the researchers developed a wafer-scale carbon-nanotube growth technology that produced emitters with a standard deviation in diameter of 4% and a standard height deviation of 6%.

The carbon nanotubes must also be well crystallized so that they have a high electrical conductivity. So the team came up with post-growth processing techniques to increase the graphitization of the nanotubes.

Milne and colleagues are currently developing prototype devices with higher frequencies. "Higher frequencies - above 30 GHz - are of interest today because of the greater availability of communication channels," he said. "Recently, we developed and operated a 32 GHz microwave diode and triode."

The scientists reported their work in Nature.