Magnetrons

Magnetron principles

A magnetron is a device that converts high voltage DC electrical power into microwave power. The internal arrangement of a magnetron is shown below.

The magnetron is constructed from a circular arrangement of microwave cavities that form the anode. The cathode is arranged so that it is concentric to the anode vane tips. When the air is extracted, and the cathode is hot, electrons are emitted, from its surface, into the space between the cathode and the anode vane tips. If a DC voltage is applied between the cathode and the anode and a magnetic field is applied at right angles to the electric field, then the electrons will follow a curved path as shown above. Microwave power is generated as the electrons interact with the anode resonator structure.

Anode technology

Different anode designs are used in e2v magnetrons. Each type has its own particular advantages that make it suitable for use in specific applications. The following table shows general characteristics of these anode designs.

• The Strapped Vane Anode offers good efficiency, low starting jitter, excellent mode control and low cost
• The Long Anode gives the ultimate high power capability while retaining excellent frequency stability and mode control
• The Rising Sun Anode is particularly suited to millimetre-wave magnetrons and has superb short-pulse capability

Cathode technology

The operational life of a well-designed and properly used magnetron is ultimately dependent upon the cathode. e2v uses a wide range of cathode technologies, in order to achieve optimum device performance and life. These include:

• Nickel mush, oxide cathodes, for high voltage and high power applications such as LINACs
• Patented ”2-part” barium aluminate cathodes and BA cathodes with metal overlays for demanding high current applications in millimetre magnetrons
• Bright emitter tungsten cathodes for continuous wave industrial heating magnetrons
• e2v’s patented long-life ”ridge” oxide cathode that has helped to more than treble the average life of its marine magnetrons
• Directly-heated cathodes that start with less than two seconds preheating and with the capability to operate at duty cycles of up to 25%.

Magnetic circuit technology

The applied magnetic field sets the operating voltage of the magnetron.

Very high power magnetrons use water-cooled solenoids to provide a uniform magnetic field in the interaction space between the cathode and the anode. In some applications, such as LINACs, the solenoid current is varied to enable stable magnetron operation over a wider power range.

AlNiCo magnetic materials are used where accurate setting of operating voltage and good temperature stability of field is required.

e2v pioneered the use of samarium-cobalt magnets in magnetrons and has been using this material in production magnetrons for some years. The advantages of samarium-cobalt over AlNiCo include:

• Dramatic reduction in size and weight
• Virtual immunity to accidental demagnetisation due to magnets being brought too close to ferrous material
• Much higher magnetic field – this is particularly important for millimetre-wave magnetrons.

Frequency agility

Frequency agility for ECCM and anti-glint has been a requirement of many radar designers. This has prompted us to develop several agile mechanisms to offer tuning rates above those that are possible with simple mechanical cam-actuated tuners. Examples of these include the multipactor-tuned magnetron, the tuning-fork tuned magnetron and the piezo-tuned magnetron. In each case, the mechanism has been proved by volume production experience.

Phase priming and injection locking

Magnetron oscillation builds up from random noise at a frequency determined by the anode. A low power signal injected into the magnetron before it begins to oscillate will control the phase, but not the frequency of oscillation. This is known as phase priming.

Higher levels of injected signal lock both the phase and oscillation frequency. In general, it is most convenient to inject the locking signal into the output of the magnetron, via a circulator. Magnetron ”chains” are used to give the system a higher overall gain.

One example of this type of system is the PLM5800 series of Ku-band, 2-magnetron amplifier chains incorporating circulators. These offer:

• Output power up to 400 W in Ku-band
• Duty cycle in excess of 10% possible
• Three-second starting
• Weight less than 1 kg
• Gain in excess of 20 dB (total system gain).

Typical magnetron impedance characteristics

Some of the following parameters are often found in magnetron specifications.

Thermal drift

The frequency of a magnetron is broadly proportional to the size of the resonant magnetron cavity. When the amount of power into the magnetron is changed, either by switch-on or by a change in operating conditions, the amounts of power dissipated in the anode (and cathode) change with consequent changes in temperature. Since this changes the physical size of the cavity, the frequency of the magnetron is altered. Most of this drift happens within a few seconds of the change; after 10 to 30 minutes (depending on type) the frequency stabilises.

Any change in the ambient conditions that affect the anode temperature also causes a frequency change. This could be changing air temperature or pressure, a change in mounting plate temperature, or in coolant flow rate or temperature. This change is usually specified for each magnetron in kHz/°C. This value is almost invariably negative for magnetrons, that is frequency falls with increasing temperature.

Frequency pushing

The oscillating frequency is affected by the electron density in the interaction space of the magnetron – this is a function of the anode current. If the top of the current pulse is not flat, this will result in modulation of the frequency as well as of the power level.

Typical frequency pushing curve for 10 kW 3rd generation marine magnetrons (MG 5241).

The datasheets for some types include maximum limits on frequency pushing, expressed in MHz/A (megahertz per ampere) over a specified current range. Unless otherwise specified, the frequency pushing is measured with the magnetron feeding a matched load, and can be greater under mismatched conditions.

Frequency pulling

This is a measure of change of frequency with change of phase of load mismatch, and it is clearly desirable to minimise this characteristic in most magnetrons. The pulling figure is usually defined as the maximum frequency change when a fixed external mismatch (usually 1.5:1 VSWR but sometimes 1.3:1 VSWR) is moved one half wavelength in the output waveguide.

The pulling figure is a characteristic determined by the degree of coupling between the anode and output systems. Although a high degree of coupling gives good power output and efficiency, it gives poorer jitter and pulling characteristics. Consequently, the magnetron designer must choose the best compromise.

As the V/I curve is nearly horizontal, any change in the operating conditions will have little effect on the anode voltage, but a large effect on the current. Abnormal magnetron operation is often indicated by incorrect anode current, even though the anode voltage has not changed noticeably. The effects of changing load and magnetic field are also indicated.

Time jitter

Time jitter (or starting jitter) is the random variation in time delay between the leading edge of the applied voltage pulse and the leading edge of the detected RF output pulse. To a large extent, time jitter occurs as a function of the interface between the particular modulator and magnetron. Magnetron specifications refer to a desired range for the ”rate of rise of voltage” (RRV) for stable operation. RRV is defined as the steepest slope of the leading edge of the applied high voltage pulse, measurable above 80% amplitude, and is usually expressed in kV/µs (kilovolts per microsecond). If the value of RRV is too high, there is insufficient dwell time at the normal firing potential of the magnetron to permit a smooth transition into oscillation, consequently random delays occur in establishing stable oscillation. This is usually expressed as pulse-to-pulse variation in nanoseconds rms.

In extreme cases, where the delay in starting produces an RF output pulse having less than 70% of the energy content of a normal pulse, the pulse is considered to be ”missing”. In addition to specifying a maximum permissible time jitter, magnetron specifications contain the parameter of missing pulses, expressed as a maximum percentage of the total number of high voltage pulses applied over a three minute test period.

Frequency jitter

If it were possible to remove frequency variations due to pushing, pulling, thermal drift, temperature coefficient, shock, vibration and all other external effects, there would still be a small amount of frequency modulation (FM) on each magnetron transmitted pulse, and from pulse to pulse. This residual FM is random in nature and results from a number of minor uncontrollable factors.

In most system applications, random FM is small enough to be unimportant. However, in MTI radars it is a parameter that must be considered in calculating the maximum attainable MTI improvement factor for the system.