**4. Pulsed plasma antenna circuitry**

A plasma antenna operating in the microwave frequency range requires higher operating currents (Greater than 1A) and consequently can overheat when used continuously. Our plasma antennas are able to work CW at high frequencies (>1 GHz) because of a concept invented by Igor Alexeff and Theodore Anderson that uses fast high-current pulses; instead of DC current.

The plasma initiates quickly in less than a microsecond, but when plasma ionizing current is turned off, the ions take about a millisecond to recombine with electrons. Therefore plasma density stays high for almost a millisecond even though the ionizing current is no longer on. We use an even short pulse width (1 μs), and therefore less power is required to run the antenna.

We developed a pulsed voltage doubler circuit, allowing us to use a lower voltage DC power supply for the input power to the pulsing circuit using a modified Marx Generator. A Marx Generator is a pulsed voltage multiplier. A series of capacitors was charged in parallel and then discharged in series through spark gaps.

**Figure 12** shows a two stage voltage doubler circuit. We have built and tested a modified Marx Generator that replaces the spark gaps with an IGBT electronic switch.

Keeping the second spark gap in the circuit results in a faster rise time than in our previous pulsing circuits. The voltage doubler allows the use of a lower voltage DC power supply than would otherwise be required.

A simple non-voltage multiplying IGBT pulsing driver circuit is shown in **Figure 13**. A CMOS timer IC is used to generate short 1 μs pulses with a repetition time of 750 μs. An IXYS brand 2500 V IGBT is the high voltage switch. This pulsing arrangement allows us to use a factor of 750 less DC current and power from the DC power supply.

We have been driving our plasma antenna tube with the fast, high-current pulses described here to allow operation at higher frequencies than is possible with CW current. Our pulsing circuit had required that the both electrodes of the plasma tube operate at high voltage; with the positive electrode at constant maximum DC voltage (2–3 kV). We have modified the circuit to allow the negative electrode to remain grounded while a positive 1 μs pulse is applied to the anode electrode. This change offers several advantages:

Improved electrical safety because one electrode remains grounded and the other electrode is supplied with short pulses. Less capacitive loss in the current leads allowing the use of faster pulses; since one electrode is grounded and can be attached to a ground plane.

Eliminating the need for a negative lead wire means less high voltage wiring. Lower EMI caused by high current pulses in wiring.

Grounding one side of the plasma tube requires that the high voltage switcher (IGBT) be able to float up and down in voltage with the tube's anode. To do this we

have electrically isolated the switcher by using a battery to power the electronics

*Prototype pulsing circuit potted in epoxy. High voltage leads are at top and bottom, and the battery connection is*

**Figure 14** shows our first prototype with steel-filled epoxy; not the best choice of epoxy but one that is working quite well so far. Potting the circuit is advantageous for airplane and aerospace applications, providing mechanical ruggedness as well as electrical isolation. Epoxy has much higher thermal conductivity than air, but not as good as a metal heat sink. The steel filled epoxy has about a factor of 10 higher thermal conductivity than air. Highly thermally conductive epoxies can have a factor of 100 higher conductivity than air. Potting in epoxy in our case allowed operation without an additional metal heat sink, saving space and eliminating the need for electrical isolation between the IGBT and a metal heat sink. We tested the ruggedness of our epoxy-potted circuit by dropping it on a concrete floor from a

We ran the circuit shown in **Figure 13** (with no additional heat sink) with peak

After ½ hour of operation, the epoxy and IGBT were warm to the touch but with

**5. Power, current, and voltage requirements in pulsing excitations**

about 1 A with 5 μs pulse width and a time between pulses of about 1 ms. This duty cycle of 1/200 results in an average power of about 5 W.

The plasma antenna requires a relatively high voltage, low current power

Short pulses are applied to the terminals of the plasma tube. Peak current is

The power losses of the supply voltage connected to the plasma antenna are because the VSWR numbers in many cases indicate a very good match between the

Anyone trying to build a plasma antenna according should consult a licensed electrical safety expert before proceeding. After consulting a licensed electrical safety expert, proceed as follows. Use a three-wire grounded power cord and

Overall average power drain from the battery driving the plasma antenna will be much less than 5 W. Two standard 9 V batteries and/or one 6 V.75 AH SLA battery can operate a plasma antenna. The smart plasma antenna can operate on a 12 V car battery which is enough voltage to ionize the plasma in 12 tubes and run the

and by potting the IGBT and electronics in epoxy.

height of 6 ft. without damage to the circuit.

current of 20 A and a pulse period of 1000 μs.

no indication of over-heating.

supply.

**Figure 14.**

*Plasma Antennas*

*DOI: http://dx.doi.org/10.5772/intechopen.91944*

*on the right.*

computer.

**83**

feeds and antennas.

#### **Figure 12.**

*1000 V DC to 2000 V pulse circuit using modified Marx generator with spark gaps in the top photo and with an electronic switch and spark gap in the bottom figure. Two electronic switches was also built but not shown.*

#### **Figure 13.**

*Basic pulsing circuit. A DC power supply charges the capacitor, the IGBT pulsing circuit switching delivers short 1 μs pulses to the plasma tube.*

**Figure 14.**

**Figure 12.**

*Selected Topics in Plasma Physics*

**Figure 13.**

**82**

*1 μs pulses to the plasma tube.*

*1000 V DC to 2000 V pulse circuit using modified Marx generator with spark gaps in the top photo and with an electronic switch and spark gap in the bottom figure. Two electronic switches was also built but not shown.*

*Basic pulsing circuit. A DC power supply charges the capacitor, the IGBT pulsing circuit switching delivers short*

*Prototype pulsing circuit potted in epoxy. High voltage leads are at top and bottom, and the battery connection is on the right.*

have electrically isolated the switcher by using a battery to power the electronics and by potting the IGBT and electronics in epoxy.

**Figure 14** shows our first prototype with steel-filled epoxy; not the best choice of epoxy but one that is working quite well so far. Potting the circuit is advantageous for airplane and aerospace applications, providing mechanical ruggedness as well as electrical isolation. Epoxy has much higher thermal conductivity than air, but not as good as a metal heat sink. The steel filled epoxy has about a factor of 10 higher thermal conductivity than air. Highly thermally conductive epoxies can have a factor of 100 higher conductivity than air. Potting in epoxy in our case allowed operation without an additional metal heat sink, saving space and eliminating the need for electrical isolation between the IGBT and a metal heat sink. We tested the ruggedness of our epoxy-potted circuit by dropping it on a concrete floor from a height of 6 ft. without damage to the circuit.

We ran the circuit shown in **Figure 13** (with no additional heat sink) with peak current of 20 A and a pulse period of 1000 μs.

After ½ hour of operation, the epoxy and IGBT were warm to the touch but with no indication of over-heating.
