With the explosive development of communication systems, the electromagnetic spectrum in the radio-frequency (RF) and microwave ranges is becoming increasingly congested. Additionally, in military applications, communication and guidance devices can be jammed or damaged by electronic warfare systems. To overcome these challenges, RF systems should acquire the capability to be rapidly switched to a different frequency or band and to block incoming high-power signals. The existing solutions, e.g. those based on semiconductors, do not work at high power that easily damage the semiconductors. Plasmas, in contrast, can offer solutions for high-power reconfigurable RF/microwave systems. Indeed, plasmas (in contrast to semiconductors) can be turned on/off, plasma properties relevant for electromagnetic interactions can be quickly altered by changing the power and/or frequency of plasma-sustaining source, and plasmas operate very well at high power.
Plasma Antennas
An antenna is a front end of any RF communication system. A plasma antenna has inherent advantages over conventional metallic antennas: the entire antenna can ‘disappear’ when the plasma is turned off, and plasma properties, such as electron density, can be electronically changed. Moreover, in an array of plasma elements, each element can be individually turned on/off and tuned, so that tunable phased arrays can be constructed.
Despite the potential advantages, there were, until recently, a few fundamental problems with plasma antennas. First, the electrical conductivity of a weakly ionized plasma is orders of magnitude lower than that of metallic conductors, which would presumably translate to low antenna efficiency and gain. However, our studies demonstrated that with a proper plasma-supporting power and thus electron density, the plasma antenna gain at its resonant frequency can be within a few dB of the gain of equivalent metallic antenna. This very encouraging result is due to the fact that the oscillating RF electric current only flows in a micron-thin skin layer of a metallic conductor, whereas the skin depth in a plasma conductor is comparable with the size of the plasma, and this difference in the effective cross sections between plasma and metallic conductors largely compensates for the difference in the electrical conductivities.
Another issue with plasma antennas is the noise level. While the noise is not important for transmission antennas that send out strong signals, a receiving antenna that has to receive weak signals must have a very low noise level to ensure an acceptable signal-to-noise ratio. The Johnson-Nyquist thermal ‘white’ noise is proportional to the absolute temperature of charge carriers, and this noise cannot be filtered out. Electrons are the charge carriers in plasmas, and conventional plasmas have an electron temperature on the order of 10,000-50,000 K, which corresponds to unacceptable noise levels. Can a way of making a plasma with relatively high electron density but low electron temperature be found? Our group explored a concept of plasma generation by repetitive short pulses and demonstrated that parameters and conditions can be found such that the electron temperature in the pause between the pulses drops much faster than the electron density does. Therefore, a time interval can be found during which the electron temperature is close to the room temperature while the electron density is still high. The direct noise measurements confirmed that the noise level behaves as expected from the electron temperature evolution. These encouraging fundamental results a plasma transceiver (i.e. transmitter/receiver) antenna can be operated in a time-gated manner, working as a transmitter during the plasma-sustaining pulses and as a low-noise receiver between the pulses.
Plasma Tunable Impedance Elements
Variable capacitors and inductors are the key elements of any tunable RF system. For example, tuning a radio to a different station is done by changing the capacitance and thus the resonant frequency of an LC circuit. Mechanically variable capacitors, e.g. those in which capacitor plates are physically moved with respect to one another, can handle high power, but are bulky and slow-reacting. Semiconductor-based variable capacitors, called varactors, are very compact and ensure fast tuning, but are easily damaged at high power. In contrast to these solutions, plasma-based variable capacitors can operate at high power and ensure fast tuning.
Imagine a radio-frequency electric discharge that creates a volume of quasineutral (i.e. with equal number densities of the electrons and positive ions) plasma between the two electrodes. The frequency-dependent dielectric permittivity of the bulk plasma is determined by the electron density and the collision frequency (which, in turn, depends on the gas pressure). Changing the plasma-sustaining power and/or frequency would change the permittivity in a wide range of positive and negative values. Additionally, between the bulk plasma and the electrodes there are two oscillating sheaths where there are almost no electrons, so that the sheaths act as vacuum capacitors. The effective capacitance of the sheaths can be changed by changing the plasma-driving frequency. As a result, changing the discharge power and/or frequency would change the impedance experienced by a probing signal with a different frequency. This idea was implemented and studied in our work, and we showed that, indeed, changing the discharge power and frequency leads to wide variation of the impedance for a probing frequency. Moreover, the impedance can change from capacitive to inductive, which is unique, since no conventional capacitor or varactor can exhibit such behavior.
These unique and encouraging results could serve as a basis for novel tunable high-power RF/microwave systems.
Plasma-Based Microwave Limiters and Switches
High-power electromagnetic pulses can cause irreversible damage to sensitive electronics and computers, and plasma can help to protect against such threats by absorbing or reflecting the incoming high-power signals. In the absence of a threat, the plasma would be off, enabling free transmission of signals, but a threat signal would automatically quickly ignite the plasma and be absorbed or reflected. It is also important that the plasma ignition happens early, when the incoming pulse has not reached its peak intensity yet.
Such a device has been developed and successfully demonstrated in our group. In the device, a small gas discharge tube was placed in a special type of a resonant cavity so that the tube location is where the electromagnetic field is concentrated. To further facilitate early plasma ignition, a DC bias was applied to the discharge tube. The experiments showed the desired performance: with the incoming microwave power of only 0.1 W or so, the plasma was ignited and the transmission was shut off almost completely.
Subsequently, by designing and making a broadband resonant cavity with overlapping simple resonant cavities and using several individually-controlled gas discharge tubes, our group demonstrated a limiter with broadband performance. That device can also serve as a broadband high-power switch with high linearity and low insertion loss.
This novel technology can be used in practical application with minimal additional development and optimization.
Metamaterials and Photonic Structures with Plasmas
In the last few decades, both science and applications of electromagnetics and optics have been revolutionized by so-called metamaterials, i.e. engineered structures with properties not achievable with naturally-existing materials. Structures with negative refractive index represent perhaps the most prominent example of such metamaterials.
In most cases, the frequency response of metamaterials is fixed, and unusual properties (such as a negative refractive index) exist at a single frequency or in a narrow band around that frequency. If plasma elements are embedded in a metamaterial, the ability to turn the plasmas on and off and to change their properties can potentially enable tunable and/or wideband metamaterials.
Additionally, time-modulation of the embedded plasma elements could lead to novel exciting properties.
Consider, for example, the recently discovered photonic time crystals. In periodic structures such as crystals, the electromagnetic properties vary periodically in space, and there is a frequency band gap, i.e. a certain range of frequencies cannot propagate in this structure. In photonic time crystals, the situation is reversed: periodic modulation in time creates a band gap in wavelength. The modulation depth required for such unusual behavior can be very small if the effective dielectric permittivity is near zero.
Plasmas appear to be a very good fit for photonic time crystals. Plasmas can be easily time-modulated at any desired frequency, and the reactance, corresponding to the effective permittivity, can be near zero (see Plasma Tunable Impedance Elements above). Demonstration of plasma-based time crystals would be an exciting result.
Time modulation can also enable nonreciprocal structures where an electromagnetic wave can propagate in one direction but not in the opposite direction, and plasma could potentially enable such structures with tunable properties.