THE A6 RELATIVISTIC MAGNETRON REVISITED

Electro-Dynamic and Electronic Observables in Simulations of an

A6 Single Radial Output Magnetron

John G. Leopold, Anatoli Shlapakovski, Joseph Z. Gleizer,

Arkady Sayapin and Yakov E. Krasik

Physics Department

Technion – Israel Institute of Technology

Haifa 32000, Israel

Radiated power pulse shortening in relativistic magnetrons

A very old problem (cathode plasma expansion is the accepted explanation)

J. Benford and G. Benford, “Survey of pulse shortening in high power microwave sources,” IEEE Trans.

Plasma Sci., vol. 25, no. 2, pp. 311–317, June 1997

In our experiments (t ≤ 200ns, Vmax<250kV) we observe no

considerable plasma expansion in the magnetron cathode-

anode gap.

Pulse shortening has been observed in PIC simulations with

no plasma present but not properly understood

W. Arter and J.W. Eastwood, “Characterization of relativistic magnetron behavior by 3-D PIC simulation,”

IEEE Trans. Plasma Sci., vol. 26, no. 3, pp. 714-725, June 1998.

X. Chen, M. Esterson, and P. Lindsay, “Computer simulation of a high-power magnetron and possible

implications for RF pulse shortening,” IEEE Trans. Plasma Sci., vol. 26, no. 3, pp. 726-732, June 1998.

3D PIC calculations

Without Anode Caps

- WoAC

With Anode Caps - WAC

rA=17 mm, rR=34 mm, Vmax=190 kV

rC=6.25mm rC=9 mm

le = 18 mm 14.4 mm

The decrease in the voltage and increase in the current at

the same time when the radiated power drops, indicates

that the magnetron impedance is under-matched relative to

the generator impedance.

J.G. Leopold, A.S. Shlapakovski, A. Sayapin, and Ya.E. Krasik, “Revisiting power flow and pulse shortening in a relativistic magnetron”, IEEE Trans Plasma Sci.,vol. 43,no. 9, pp. 3168-3175, Sept. 2015. J.G. Leopold, A.S. Shlapakovski, A. Sayapin, and Ya.E. Krasik, “A six vane, single radial output slot relativistic magnetron revisited”, IEEE Proc. PPC, pp. 306, 2015.

Bax= 0.27 T

Without Anode Caps

- WoAC

Pulse shortening at 60-70 ns No pulse shortening

Axial current drops to negligible values

le = 35 mm 62 mm

Bax= 0.38 T

J.G. Leopold, A.S. Shlapakovski, A. Sayapin, and Ya.E. Krasik, “Pulse-Shortening in a Relativistic Magnron: The Role of Anode Block Axial Endcaps”,IEEE Trans Plasma Sci.,Vol. 44, No. 8,, 2016, pp. 1375-85.

With Anode Caps - WAC

• For this magnetron and in contrast to the WoAC magnetron

increasing le eliminates power pulse shortening.

• In most magnetron studies it is customary to attempt to reduce

the axial current; this generalization may not always be correct.

IT=IM+Iax

Axial current significant!

WAC WoAC

Impedances vs. time

WoAC: Decreasing le reduces the emitted current and

increases the magnetron impedance so that matching is

achieved

WAC: Increasing le causes an increase in the axial

current which seems to drain sufficient current from the

magnetron for the impedances to match

le = 35 mm le = 62 mm

WoAC

WAC

• In both magnetrons, two characteristic modes may be sustained – one

corresponding to low radiated power, the other to high radiated power.

• By changing the emission length, le, one can move between the two

modes through a phase of mode competition.

• Mode competition is not the cause for pulse shortening.

beating period ~3 ns

beating period ~10 ns

Slow oscillations persist long times ;

diminish when one mode outlasts the other

WoAC WAC

Electrodynamical observables: Eθ

Eθ is bound between the anode caps

which leaves the electrons at the edge

of the emission region free to run away.

As the emission region gets closer to the edge of

the interaction region, more current can escape

as axial current allowing for better balance.

Eθ spreads far out from the edges of

the emission region.

Increasing the emission region increases the

emitted current without allowing current to leak

out. This increases the magnetron current.

Er

Bθ

Ez

|Qe|

Eθ

WoAC WAC

le = 18 mm le = 35 mm

SUMMARY

Radiated power pulse-shortening in the absence of plasma expansion

for two magnetron configurations was demonstrated.

By studying the electrical observables (currents, voltages and

impedances) we have shown that an under-matched magnetron

impedance relative to the power generator attached to it is responsible

for pulse shortening, for both WoAC and WAC magnetrons.

The physics responsible for the difference between the two

magnetrons, is revealed by studying the behavior of the electro-

dynamic observables.

Our study was enabled by the advances in the abilities of PIC codes

empowered by modern computing which were very limited when these

systems were first looked at.

Though we have revisited particular cases we have shown new aspects

in the physics of magnetrons which question some of the beliefs rooted

in this field.

100 ns 150ns