DC to Microwave Conversion

Introduction

The DC power collected from the solar panel array must be converted to microwave power in order for it to be transmitted from geostationary orbit (GEO) to the collector stations on earth.  Below we will consider the operating frequency for transmission, the different radio frequency (RF) sources available, the magnetron selected for this purpose, and the challenges of implementing a magnetron phased array.

Selection of Operating Frequency for Transmission

The primary factors that must be considered in the selection of an operating frequency are atmospheric, the generation of the signal, and the reception and conversion of the signal.  Taking these factors into consideration, it is best to choose as high of a frequency as possible, as it allows for a smaller antenna, tighter beam, and smaller ground collector.  The optimum operating frequency was determined to be 5.8 GHz.

 

Generation

Relatively high efficient power generation can be done for nearly any desired frequency above the hundreds of kilohertz, vacuum devices typically operate between the hundreds of kilohertz and hundreds of gigahertz range.  For higher frequencies, lasers can be used with good efficiency for infrared and visible frequencies.

 

Alternatives

Aside from RF frequencies between approximately 1 to 10 GHz, millimeter waves could be used, however the atmospheric attenuation and increased intricacy of the transmit and receive antennas is disproportionate to the reduced antenna size; this rules out gyrotrons as an RF source, as they oscillate at millimeter wave frequencies.  Lasers, though substantially reducing the size of both the transmit and receive areas, presents many more technical problems that would need to be resolved, would be very attenuated by the atmosphere, and are somewhat less efficient in the energy conversion than comparable RF devices, not to mention the political problems of putting a high energy laser in orbit.

 

RF Power Generation

Solid State Devices

There are many different solid state devices that are currently being used to generate microwaves for various applications.  Generally such oscillators are used in low power application, i.e. under 100 kW, and usually much less than that.  The primary advantages of such solid state devices is their small size, and versatility.  Solid state RF oscillators are, however, very inefficient, too much so to be practical for space based solar power (SSP) applications.  Because of these issues, solid state microwave oscillators will not be looked at any further.

Klystrons

Klystrons are high-Q RF amplifiers.  A klystron consists of three parts: an electron gun, an RF amplification region, and a collector.  The electron gun is where the beam is formed and focused. It includes a filament, a cathode, focusing electrodes, and a modulating anode.  When voltage differential is applied between the anode and the cathode, a large electron current flows.  The input cavity produces an RF voltage across its gaps when power is applied to the input loop.  The RF amplification region contains a number of cavities and drift spaces.  In the output cavity, the RF energy is coupled through an iris to a waveguide.

 

Figure 1. Components of a Klystron

Klystrons are commonly used in applications of high power RF where precise control of frequency and phase is required, such as television stations, linear accelerators, and some radar applications.  Klystrons have a major drawback of low efficiency, typically around 40% to 50%, with some of the newer high end models having efficiencies as high as 70%; too low for SSP.  Klystrons typically operate at frequency from the hundreds of megahertz to the hundreds of gigahertz.

Figure 2. 805 MHz, 1.25 MW Klystron at the Los Alamos Neutron Science Center

Traveling Wave Tubes

A traveling wave tube (TWT) is another example of an RF amplifier.  A TWT consists of an electron gun, an elongated vacuum tube, and a collector at the opposite end.  Electrons are emitted from a heated cathode through an anode down the tube, there is an RF input near the anode, and an RF output near the collector at the opposite end of the tube.  A magnetic field is applied to focus the electrons into a beam as they pass through a coupled RF cavity that stretches from the RF input to the RF output.  The traveling electrons eventually collide with the collector.  The RF output can typically achieve a gain of  70 dB of voltage compared to the input signal.  TWTs will operate from frequencies ranging from hundreds of megahertz to tens of gigahertz, and are typically used in applications requiring a high amplification of a weak signal at high power, such as in some communication situations and radar.  Typical efficiencies are slightly better than for klystrons, TWTs are generally 50% to 75% efficient.

 

Figure 3. Basic Construction of a Traveling Wave Tube

Magnetrons

Magnetrons are vacuum devices which act as oscillators at microwaves frequencies, and are capable of high power outputs.  The basic construction of a magnetron consists of a cathode surrounded by an anode containing cavities, which are resonant at the frequency of the magnetron.  A filament is heated so that electrons are emitted from the cathode.  An externally applied magnetic field oriented along the axis of the magnetron, along with the electric field oriented radially from the anode to the cathode causes the emitted electrons to spin and circle the cathode in the air gap between the anode and cathode.  After a short startup time, the electric fields developed in each successive cavity will differ by pi radians.  When this occurs, the magnetron is said to be operating in pi mode.

Figure 4. Cross-Sectional Diagram of an 8-Vane Magnetron

Operating frequencies range from the tens of megahertz to the hundreds of gigahertz.  Magnetrons have extremely high efficiencies compared to similar RF devices, typically 70% to 90%, with some high-end magnetrons having efficiencies as high as 99%.  This high efficiency comes at a price of low frequency precision and poor phase and frequency control.  This restricts the applications of magnetrons mostly to heating and CW radar, although recently the US Air Force has taken an interest in using them for electronic warfare and the Active Denial System.  Magnetrons are also one of the cheapest RF solutions, costing only about $13 per kilowatt.

 

RF Source Comparison

It is very clear that the magnetron is the best choice for this application, as precise frequency control is unnecessary, but high efficiency is very desirable.

Figure 5. Average RF Output Power vs. Frequency for Various Electronic Devices

Magnetron Used in This Project

Basic Parameters

  • Power Output:  10 MW, continuous wave (CW)
  • Frequency:  5.8 GHz
  • Efficiency:  90%
  • Mass: 1600 kg
  • Diameter:  2 m
  • Height:  6 m
  • Lifespan:  60 years
  • Cost:  $130,000

 

Efficiency and Lifespan

For a magnetron, efficiency and lifespan are closely related, as the primary of loss of efficiency is from electrons colliding with the anode, which can nearly be eliminated with careful engineering.  Other sources of loss of efficiency are RF leakage through the input coax and power lost in higher order harmonics, both of these can also be very much reduced with careful design of the magnetron.  Very high efficiencies can be achieved with magnetrons (i.e. >95%), but these levels of efficiency are difficult to engineer in magnetrons with greater than several hundred kilowatts of power output.  The U.S. Air Force has recently taken an interest in magnetron for use in electronic warfare and their Active Denial System.  The Air Force Research Laboratory is currently developing several magnetrons that are capable of 2.5 MW CW power outputs, with 85% efficiency.  With just a little more development, it is reasonable to assume that 10 MW magnetrons capable of 90% efficiency could be realized.

Common life expectancies for magnetrons for conventional applications magnetrons range from about 2000 to 10,000 hours of operation.  However, it should be noted that the first component to fail on a magnetron, as with all vacuum devices, is the vacuum.  Space in GEO provides a better vacuum than can be easily created on earth, and does not need to be maintained, which means that the life expectancy of the magnetron is limited only by the slow degradation of electron collisions with the anode and other forms of radiation damage present is space.  Furthermore, pulsing or frequently switching a magnetron on and off substantially decreases lifespan; this would not be a problem for SSP, as the magnetrons would be in continuous use.  60 years is a very reasonable estimate for the lifespan, though may function well even longer.

 

Q-Factor

Magnetrons inherently have poor Q-factors, and their frequency and phase are difficult to control.  Although, it has been shown both theoretically and experimentally that if the DC source is stabilized, Q-factors as high as 105 can be achieved.  Although a stable frequency is not as important in power transmission as it is in communication and other applications, it is still desirable to have as stable of a signal as possible because it allows for more efficient conversion on both ends.  In space it will be much easier to maintain a constant and stable DC voltage, as the solar panels will be receiving a nearly constant amount of insolation continuously.

 

Power Output

There are many different types of magnetrons in existence.  Relativistic magnetrons are capable of very high power output, upwards of 30 GW, and occupy less volume than a conventional magnetron of the same power capability—the physical size and weight of a magnetron is roughly proportional to its power output.  They are typically used for radar applications.  Relativistic magnetrons, however, have a theoretical maximum efficiency of 50%, but in actuality are usually only able to achieve efficiencies of 20-30%, making them unusable for SSP.  Conventional magnetrons, which have mostly been described in this section, have high efficiencies and other desirable characteristics.  Perhaps the greatest downside of using conventional magnetrons is their power output.  The largest conventional magnetrons have power outputs of a few megawatts.  Although these magnetrons can be made to have power outputs as high as 10 MW, this is too low for use in SSP, which requires the use of a magnetron phased array.

 

Magnetron Phased Array

Perhaps the greatest challenge of using conventional magnetrons lies in synchronizing a large number of them in phase, frequency, and voltage.  When magnetrons are directly coupled into a waveguide, even if operating at approximately the same frequency, they will differ in phase such that the combined output is only slightly higher than if only one magnetron was being used.  The frequency of a magnetron can be controlled to some extent by controlling the input DC voltage, likewise, when a weak reference microwave in injected from an outside source, the phase of the magnetron tends to lock to the outside signal.  Thus, in most applications where a magnetron phased array is employed, each magnetron is treated as a voltage controlled oscillator and a phased locked loop is used to control the phase and frequency of each magnetron.  A phase shifter is also needed to for each magnetron.  This method provides a phase stability within about 1 degree.

Figure 6. Block Diagram of a Phase Controlled Magnetron

The drawbacks of using this injection locking method of synchronization for SSP is that is results in a 10% loss of efficiency for each magnetron, and it adds a greater cost and weight with the added equipment.  This method by it self is not practical for SSP, but recent research has shown that the same effect can be achieved by allowing leakage of adjacent magnetrons in the array, so long as two magnetrons at opposite ends of the array are controlled in phase and frequency.  This method of mutual coupling for phased arrays of magnetrons was borrowed from similar semiconductor arrays, but has been shown experimentally to work with magnetrons.  Theoretically an arbitrary number of magnetrons could be locked using this method, so long as two are phase controlled.  In practice, however, this is not going to be the case.  There are too many factors that can contribute to the breaking of the locking, making additional phase controlled magnetrons necessary.  Unfortunately, there has been little experimental work on arrays consisting of more then ten magnetrons.  Though, interpolating the available data, it seems reasonable that two phase-controlled magnetrons per 100 should be sufficient to maintain phase locking of the array.  This would result in a decrease in efficiency of the magnetrons by only 0.2% overall, which is essentially negligible.

 

Figure 7. Concept of Phased Array Magnetrons with Mutual Injection Locking

References

http://www.radartutorial.eu/08.transmitters/tx13.en.html

http://www.radartutorial.eu/08.transmitters/tx08.en.html

http://www.radartutorial.eu/08.transmitters/tx12.en.html

http://www2.slac.stanford.edu/vvc/accelerators/klystron.html

http://www.kirtland.af.mil/afrl_de/index.asp

Shinohara, N. , & Matsumoto, H. (2010). Research on Magnetron Phased Array with Mutual Injection Locking for Space Solar Power Satellite/station. Electrical Engineering in Japan, 173(2), 21-32.