High Voltage Direct Drive amplifier for Plasma loudspeaker

[thadman]

I'd like to build a simple prototype plasma tweeter in the next few weeks to demonstrate the concept.

A design that pulses a high frequency resonance (>100khz) greatly reduces the voltage demands on the amplifier and will maximize energy transfer. However, such a circuit would require complex analysis.

I believe a simple, yet elegant design would result from directly coupling the output of the amplifier to the electrodes.

For the tweeter prototype, I'd like to use a separation distance of 1cm between the electrodes and operate them in anti-phase (push-pull).

Dielectric breakdown of the air occurs at a potential of 3kV/cm. Since the electrodes are defined by a separation distance of 1cm, we will require 1.5kV on the positive electrode and -1.5kV on the negative electrode. Once this potential is reached, the air will ionize and transition from a gas (~excellent insulator) to a plasma (~excellent conductor). Resistance will be VERY low.

For the amplifier prototype, I'd like use a push-pull voltage source. If the amplifier operates in push-pull mode, we can operate the electrodes in anti-phase. If the amplifier is a voltage source, we can control the potential between the electrodes.

I'd like to build two different amplifiers and contrast their performance.


1) Class A amplifier utilizing Vacuum tubes


Goals:

-maximum linearity
-minimum 100W


Advantages:

Vacuum tubes are characterized by a linear response and the ability to provide extremely high voltages. In addition, Vacuum tubes would be able to provide a continuous waveform to the electrodes. As a result, once the plasma was formed (transient state), it would be inherently stable.


Disadvantages:

Vacuum tubes are very inefficient and quite expensive. They will require significant cooling. However, I am interested in heat transport so I do not consider this a significant disadvantage.

A digital to analog converter will have to be used. This will require additional components and will introduce an error (however small).


Notes:

A stable plasma requires a specific "holding current". As a result, a solution must be provided which maintains the "holding current" through the electrodes while not interfering with the response of the tweeter. Since an AC waveform is being passed between the electrodes, there will be a point at which current is zero. I'm not sure if this is simply a "theoretical" problem since the waveform will not spend any finite amount of time at a position since it is continuous.

I believe an AC voltage source power supply will be required. How would you approach designing the power supply?

What Vacuum tubes would you recommend?

Do Ultralinear (high fidelity) Vacuum tubes exist which can provide 1.5kV on the electrodes?


2) Class D amplifier utilizing Insulated Gate Bipolar Transistors


Goals:

-Maximum linearity
-minimum 1kW
-easily scalable to 10kW


Advantages:

IGBTs are characterized by their high efficiency, fast switching, and the ability to handle extremely high voltages and large currents. I believe IGBTs would easily be capable of providing 1.5kV to the electrodes and can switch over 100,000 times per second. Additional IGBTs could be connected in parallel to provide any desired amount of current.

IGBTs would not require a digital to analog converter. They can be switched at the discrete steps corresponding to the exact representation of the digital wave from the source.


Disadvantages:

IGBTs could not provide a continuous waveform to the electrodes. In addition, semiconductors are inherently non-linear (can this be corrected with feedback?).


Notes:

A stable plasma requires a specific "holding current". As a result, a solution must be provided which maintains the "holding current" through the electrodes while not interfering with the response of the tweeter. The IGBTs are characterized by a stabilization time and will exist in the "OFF" position for a finite amount of time. Since the IGBTs are able to switch very quickly (>100,000 times per second), the time the IGBTs spend in the "OFF" position will be very small. I do not assume any current will be passing through the electrodes when the IGBTs are in the "OFF" position. How can I relate this to the holding current? Will the plasma still be destabilized even if this time is made very small?

An H-bridge utilizing IGBTs could provide current in either direction with a DC voltage source power supply. How would you approach designing the power supply?

What IGBTs would you recommend?

What should I use to control the IGBTs?

Thanks,
Thadman

[thadman]

edit: The Vacuum tubes will require a DC voltage source.