Proceedings of ITC/ISHW2007

The experimental effect of encouraging transparent-cathode in
relativistic magnetron with "ETIGO-IV"
Hoshiyuki Yamazaki and Weihua Jiang
Extreme Energy-Density Research InstituteNagaoka University of Technology, Nagaoka, Niigata 940-2188, JapanThe Magnetron is one of the efficiency high-power microwave source, although the energy conversion
efficiency decreases with pulsed-power relativistic electron beam. We study the capability to increase energy
conversion efficiency of Relativistic Magnetron by using Transparent Cathode. The Transparent-Cathode consistsof independent cathode strips. Each cathode strip caused azimuthal magnetic field, and the radial drift velocity ofelectrons are accelerated more than that of electron emitted from general cathode. It can make abbreviating start-uptimes. We study the relativistic Magnetron with “ETIGO IV” that is 400kV class repetitive pulsed-power generator.
This pulse length is longer considering pulsed-power. In this work, we appear the effect of Transparent cathodecompared with traditional cathode. The start-up time of magnetron with Transparent cathode is shorter than thatwith traditional cathode. The peak power and oscillation efficiency are unconfirmed. We feel strongly that
Transparent cathode is encouraging method.
Keywords: high-power Microwaves, relativistic Magnetron, Transparent cathode, combined efficiency,
rapid start-up, long pulse

1. Introduction
The Magnetrons are one of the most efficiency
microwave source, so these are studied by for many years.
Magnetrons come into use for radar, electrical powertransmission, microwave oven and etc. The features ofmagnetron are high oscillation efficiency andcomparatively low demand for external magnetic field[1-2]. And this means reduction of the device scale andweight are expected. And these are comparative
excellence efficiency in the region of hi-power
microwaves, although the operation frequency regions arewithin the lower microwave region by comparison otherhigh-frequency generator (e.g. gyrotron). In general theefficiency of high-power microwaves is determined bymomentary output energy and momentary input energy
measured at the time which is fixed the same time at thepeak oscillation. In these years, we consider the total
efficiency: total input energy and total output energy atleast [1]. I.e. enhanced combined efficiency is requested.
Some recipes become a candidate to enhance magnetron
oscillation efficiency: magnetic priming and cathodepriming and anode priming [3-5], among them theTransparent-Cathode is concise and hopeful procedure[6-7]. The Transparent-Cathode consists of independent
cathode strips, as against the conventional cathode which
consists of a solid columnar cathode. Figure 1 (a) shows
the model of Transparent Cathode. Each cathode strip
caused azimuthal magnetic field, and the radial driftauthor’s e-mail: yamazaki@etigo.nagaokaut.ac.jp

velocity of electrons are accelerated more than that ofelectron emitted from general cathode shown in Fig.1 (b).
Additionally azimuthal electric field penetrates thecathode strips. The crossed field devices like Magnetron
generate from the interaction between azimuthal electric
motion and electromagnetic wave. So Transparent
Cathode causes electrons rapid spreading, which means
rapid start-up, and the penetrating electric fieldpotentiates more efficiency interaction. Additionally theshorter start-up increases the interaction term in pulsed
electron beam, and can enhance the combined efficiency.

(a)

Bs

(b)

e-Fig. 1 The models of Transparent cathode(a)The transparent cathode consists ofindependent cathode strips. (b)The magnetic
effect of transparent cathode near the strip

Proceedings of ITC/ISHW2007
It’s hope that shorter start-up times and improve
operation efficiency.
We aspire for the enhanced combined efficiency byamelioration of Magnetrons structure. We approach theenhanced efficiency through the way abbreviating thestart-up time. In this work, we study the effect ofTransparent cathode mainly for rapid start-up. The
experimental setups are fed by ETIGO IV that is 400kV
class repetitive pulsed-power generator.

2. The schematic of Magnetron and Transparent
CathodeThe cross section diagram of Magnetron shows in
Figure 2. The magnetron comprises the cylindrical anodeenclosing coaxial cylindrical cathode. The broken lines
within the conventional cylindrical cathode line, show theTransparent cathode. The anode has the spaces as
resonator vans. The electrons are emitted by the central
cathode are subjected axial magnetic field Bz and drift byLorentz force. The drifting electrons interact in the space
between the two electrodes with the characteristic mode
from the structure of resonator vans. In the case ofMagnetron with Transparent Cathode, the current flowing
each cathode strips cause azimuthal magnetic field Bs .

Bs =µ0 I
2πrs

And the radial drift velocity of electrons ve are shownve =E0 Bz
Bz2 + Bs2

The electrons are accelerated more than that of electron
emitted from traditional cathode. So Transparent Cathodecauses electrons rapid spreading, and rapid start-up.
The characteristic dimensions are designed fromtype A6 Magnetron, which has studied for relativistic
magnetrons [3]. The parameter of A6 magnetron is theradius of cathode rc=15.8 mm, radius of anode ra=21.1mm, the gap of resonator vanes on the side of anodeψ = 20° , and the depth of resonator vans rv=41.1 mm.
The number of resonator vans is M=6. The external
radius of transparent cathode is same to the radius ofconventional magnetron cathode rc. And the number ofstrips is same to that of resonator vans M. This number isreported developed by numerical simulations [6-8].

3. Experimental Setup
Figure 3 shows the features of Magnetron operations
region. These lines show the Hull cutoff andBuneman-Hartree resonance conditions [9] for A6
Magnetron. The power source is repetitive pulsed-power
generator called “ETIGO IV” [10], which is capable ofdelivering, to a matched load, an output pulse of 400 kV
in voltage, 13 kA in current, and 130 ns in pulse width, atthe repetition rate of 1 Hz. The circle dots and full line
are point out Hull criterion. In general, the Hartree
voltage is less than the Hall cutoff voltage magnetic
insulation. Other dots and lines show Hartree voltage forseveral oscillation modes. We assume the main operation
modes are fundamental π-mode (2.34GHz) or high order
2π-mode (4.6GHz). The Hartree voltage is corresponds tothe breakdown voltage of magnetron in the presence of arotating perturbation field. The Hartree voltage causes theperturbation facilitates the motion of electrons across thecathode-anode gap. The potential energy of the electron
efficiently
interacts
withandprovides
theelectromagnetic wave. On this device, A6 relativistic
magnetron requires external magnetic field about 0.5 T to1.0 T.
The experimental setup consists of stainless steel(SUS304) vacuum chamber encrusted by externalmagnets. The external magnets architected at Helmholtz
coil. These magnets form constant axial magnetic fieldcovering the Magnetron resonator. It can be controlled byouter power source and can form maximal 1.0 T magnetic
filed. Anode blocks made from aluminum, and cathode
500

rv

rc400

ψ

Voltage [kV]raHall criterion
πmode
4/3πmode
2π high order mode
ETIGO IV operation300
200
10000.20.40.60.8

Axial magnetic field [T]Fig. 2 The schematic of magnetronFig. 3 The features of Magnetron operations region

1Proceedings of ITC/ISHW2007
waveforms from the magnetron with transparent cathodedraw full lines, and the waveforms from the magnetronwith traditional cathode draw dashed lines. And heavylines indicate diode voltage, normal lines indicate diodecurrent. The top lines shows output microwave pulses
measured at the point, which distance 2.5 m from output
window on central axis. The operation regions are
assumed about S-band to C-band: mainly 2 GHz to 5GHz, we use horn antenna admitting nearly S-band: it
assuring calibration coverage 2.45 GHz to 4.6 GHz. The
microwave pulse lengths of the magnetron with “ETIGO

Our magnetrons ware run at constant diode input
power, which is maximum voltage not to discharge.
Figure 4 shows the voltage and current characteristics ofour magnetron as a function of external axial magnetic
field Bz. Full dots show voltage values as a function ofmagnetic field, and hollow dots show current values as afunction of magnetic field. The diamond dots indicatemeasured value with traditional cathode, and the triangle
dots indicate measured value with transparent cathode.
The voltage rises slightly and the current falls fairly
monotonically, results which are typical of conventional
magnetrons [3]. The transparent cathode shows highresistance than traditional cathode. And in the lower
magnetic filed the voltage falls andante, so current notincrease rapidly near the Hall cutoff magnetic field. On
the other hand, the current with transparent cathodeincrease by the plasma forming might happen at the highmagnetic field. This is caused by pulse time increase as
high impedance. Figure 5 shows diode voltage, diodecurrent and microwave pulse from oscilloscope. The20350

1816250

V :conventional cathode14200

V :Transparent_Cathode12

I :conventional cathode150

10

I :Transparent-Cathode

100

850

60

4
0.4

0.5

0.6

0.7

0.8

0.9

10-50

-1-100-2-150-3-200-4-250-5-300-6-350-7-50-8
050

100

150

200

250300350

T ime [ns]Fig.5 Voltage, current and microwave pulse shapes at0.76 T. The full lines show the operation withtransparent cathode. The dashed lines show theoperation with traditional cathode. The heavy lines
indicate diode voltage. The normal lines indicate
diode current. The top lines indicate microwave
pulse shapes.
25Current [kA]

Voltage [kV]

300

0-400

1.1

External Magnetic Field [T]Fig.4 The voltage-current characteristics of themagnetron as a function of magnetic field. The
diamond dots denote measured values withtraditional cathode. The triangle dots denotemeasured values with Transparent Cathode.

Excitation Time :10-90% [ns]400Voltage [kV]

4. Experimental result

Current [kA]

made from carbon graphite. Graphite as cathode has theadvantage of forming good filed emission characteristics
and small excitation delays [11] and having resistance todischarge more than aluminum. The axial length of body
is 125 mm, which is decided by the wave length of theoperation electromagnetic wave 2.34 GHz, and avert
discharge from converging electric filed. The stick ofTransparent cathode made from graphite shapes likerounded trapezium by manufactural problem. Graphite isbreakable.

Traditiona Cathodel
Transparent Cathode201510500.50.60.70.80.91

External Magnet [T ]Fig.6 The Excitation time until first peak: 10% to 90%time. The diamond dots denote the time withtraditional cathode. The triangle dots denote thetime with Transparent Cathode.

Proceedings of ITC/ISHW2007
IV” are more than 100 ns. And the pulse width (half tohalf) of voltage and current are respectively about 150 nsand 160 ns. These times go way beyond the timerequiring static state magnetron operation. Our
experimental setups may research steady oscillation
region for relativistic magnetron operation, although thepower source is pulsed power device. The peak voltage isless than rated capacity, because the plasma sheath isformed at circumferential cathode, which can not beanalyzed at particle in cell simulation. The current withtransparent cathode instantaneously increases, as theplasma and electron sheath forming decrease diode
impedance. This problem stands out in proportion asincreasing external magnetic field, as the electrons areconstrained strongly and electron sheath thicken. These
problems will come to a settlement by re-designingcathode. It would increase diode voltage and stabilize
pulse forms and enhanced pulse width that the cathode isre-designed considering the plasma sheath. The
comparing the output microwave form of Transparent
cathode to that of traditional cathode, the start-up times
are rapidly on an average. The peak value itself isdetermined by mode stability, which is dependent on
situation of electron emission. The oscillation modes areapproximate shown in Fig.3, and the diode voltage
fluctuates. So the oscillation mode fluctuates too asaffairs stand. The excitation time until first peak: 10% to
90% time are shown in Figure 6. The diamond dots
denote the time with traditional cathode. The triangle dots
denote the time with Transparent Cathode. The oscillation
mode turn as external magnetic filed, consequently thestart-up time turn as external magnetic field. On theaverage, the start-up times of oscillation with Transparent
cathode are rapidly than that with traditional cathode.
Whereas the rapid start-up bind the oscillation mode
when the voltage is rising. This mode is not most efficientresonance mode not infrequently. The peak power andenhanced efficiency need high accuracy mode control.

5. Summary and Discussion
The transparent cathode makes high impedance
diode at the same external diameter cathode. Especially inthe lower magnetic filed nearly Hull cutoff the magnetronwith transparent cathode is stabilized. The relativistic
magnetron with Transparent cathode may rapid start-up
than that with traditional cathode. Our experimental
setups may research on over to steady oscillation region
for relativistic magnetron operation, using the pulsed
power source. By the calibration and assortment of output
microwave will discover the occasional oscillation modes,
the momentary maximum output powers, total outputpowers, conventional (momentary) efficiency and total
efficiency too.

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