Compact dynatron modulator
It is proposed to use a dynatron effect to modulate the voltage across the vacuum inverted coaxial diode with magnetic insulation supplied by an external pulsed high-voltage source connected to the modulator via the RL-circuit. Oscillations of the voltage due to oscillating regime of diode charging...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2001
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| Назва видання: | Вопросы атомной науки и техники |
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| Цитувати: | Compact dynatron modulator / A.V. Agafonov // Вопросы атомной науки и техники. — 2001. — № 5. — С. 169-171. — Бібліогр.: 6 назв. — англ. |
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irk-123456789-790112015-03-25T03:02:39Z Compact dynatron modulator Agafonov, A.V. It is proposed to use a dynatron effect to modulate the voltage across the vacuum inverted coaxial diode with magnetic insulation supplied by an external pulsed high-voltage source connected to the modulator via the RL-circuit. Oscillations of the voltage due to oscillating regime of diode charging and/or azimuthal instability of a rotating electron flow stimulates back-bombardment electron flow to the cathode and leads to power spikes of secondary emission current exceeding the primary one. As a result, the amplitude of oscillations grows and the system can turn to self-supporting regime. Results of computer simulations are given to illustrate the main physical processes inside the modulator and its possible applications. 2001 Article Compact dynatron modulator / A.V. Agafonov // Вопросы атомной науки и техники. — 2001. — № 5. — С. 169-171. — Бібліогр.: 6 назв. — англ. 1562-6016 PACS number: 29.17.+w http://dspace.nbuv.gov.ua/handle/123456789/79011 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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It is proposed to use a dynatron effect to modulate the voltage across the vacuum inverted coaxial diode with magnetic insulation supplied by an external pulsed high-voltage source connected to the modulator via the RL-circuit. Oscillations of the voltage due to oscillating regime of diode charging and/or azimuthal instability of a rotating electron flow stimulates back-bombardment electron flow to the cathode and leads to power spikes of secondary emission current exceeding the primary one. As a result, the amplitude of oscillations grows and the system can turn to self-supporting regime. Results of computer simulations are given to illustrate the main physical processes inside the modulator and its possible applications. |
| format |
Article |
| author |
Agafonov, A.V. |
| spellingShingle |
Agafonov, A.V. Compact dynatron modulator Вопросы атомной науки и техники |
| author_facet |
Agafonov, A.V. |
| author_sort |
Agafonov, A.V. |
| title |
Compact dynatron modulator |
| title_short |
Compact dynatron modulator |
| title_full |
Compact dynatron modulator |
| title_fullStr |
Compact dynatron modulator |
| title_full_unstemmed |
Compact dynatron modulator |
| title_sort |
compact dynatron modulator |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2001 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/79011 |
| citation_txt |
Compact dynatron modulator / A.V. Agafonov // Вопросы атомной науки и техники. — 2001. — № 5. — С. 169-171. — Бібліогр.: 6 назв. — англ. |
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Вопросы атомной науки и техники |
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AT agafonovav compactdynatronmodulator |
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2025-07-06T03:08:18Z |
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2025-07-06T03:08:18Z |
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1836865342045945856 |
| fulltext |
COMPACT DYNATRON MODULATOR
A.V. Agafonov
Lebedev Physical Institute
Leninsky Pr. 53, Moscow, Russia, 119991
agafonov@sci.lebedev.ru
It is proposed to use a dynatron effect to modulate the voltage across the vacuum inverted coaxial diode with mag-
netic insulation supplied by an external pulsed high-voltage source connected to the modulator via the RL-circuit.
Oscillations of the voltage due to oscillating regime of diode charging and/or azimuthal instability of a rotating elec-
tron flow stimulates back-bombardment electron flow to the cathode and leads to power spikes of secondary emis-
sion current exceeding the primary one. As a result, the amplitude of oscillations grows and the system can turn to
self-supporting regime. Results of computer simulations are given to illustrate the main physical processes inside the
modulator and its possible applications.
PACS number: 29.17.+w
1 INTRODUCTION
In investigating regimes of self-sustaining secondary
emission of electrons in devices with crossed ExB fields
main attention was paid to analysing the dynamics of in-
tense electron beam formation with a dominant influ-
ence of the space charge in geometrically simple mag-
netron guns. These can be considered as a coaxial diode
in an external magnetic field where the inner electrode
serves as a cathode [1 – 5]. In papers [4, 5] it was pro-
posed that the secondary emission of electrons be used
to capture and store particles in a magnetron diode as an
intermediate stage in forming a short pulse beam of a
large charge. To control (initiation and quenching) the
secondary emission, its threshold characteristics, and
dependence on angle of incidence and energy of
particles were used. After a certain transition process,
such integral characteristics as voltage on the diode,
current in the external circuit and average number of
particles in the accelerating gap reached a constant
level. At the same time, there arose within the gap regu-
lar structures of electron flows in dynamic equilibrium
with large (comparable to the external) variations of
electric fields.
In inverse coaxial diode, where the external elec-
trode is the cathode, would appear to be quite attractive.
For the same external dimensions, one could expect to
obtain large currents from the cathode. Investigation of
non-stationary regime of operation of inverted magnet-
ron diodes showed that in a certain range of parameters
such a device with a secondary-emission cathode and
external circuit is capable of operating in a self-generat-
or regime at rather large frequencies.
2 PHYSICAL PROCESSES IN AN INVER-
TED MAGNETRON DIODE
As a rule, when analysing the non-stationary dynam-
ics of intense beams in such devices, the external circuit
is not considered. The regime of operation, for that or
other reasons, is chosen from the condition of aperiodic
charging of a capacitance, which the diod represents. At
the same time, inclusion of an external RLC-circuit with
a source of voltage V0(t) in the scheme of calculation is
necessary. This is particularly so when modelling non-
stationary processes.
When there is no emission of particles in a magnet-
ron diode, it can be represented by a condenser C
charged by a voltage V0(t) from a source of voltage via
an additional RL-circuit. In the general case the charge
can occur in an oscillating regime with natural fre-
quency of damping oscillations ω = (ω0
2 - δ2)1/2, where
ω0 = 1/(LC) 1/2 and δ = R/2L - is damping constant. If in
the diode there is a constant emission of primary beam
and a cathode with secondary emission is used, then
these voltage oscillations on the diode represent "seed-
ing" for the growth of secondary emission. In the usual
magnetron diode, where the inner electrode serves as
the cathode, these oscillations promote rapid growth of
the secondary-emission process. However, the charac-
teristics of the beam insignificantly differ from the case
of aperiodic charging. In an inverted magnetron diode,
where the outer electrode serves as the cathode, these
oscillations can increase and develop into a self-sustain-
ing regime (self-generator) in a certain range of para-
meters.
From the physical standpoint, the pattern of the pro-
cesses is quite simple and, generally speaking, consti-
tutes simply manifestation of the dynatron effect used
earlier in electric bulbs. Suppose that from the external
electrode of the diode electrons are continuously emit-
ted (for example, as a result of thermal emission) with
noticeable current. With increasing voltage on the di-
ode, the emitted electrons cannot return to the cathode
and accumulate in the gap. A drop in voltage on the di-
ode as a result of oscillations leads to an increase in the
back-bombardment of the cathode. This causes an ab-
rupt spike of secondary emission current, which to an
even greater extent "pulsed" voltage on the diode. As a
result, the accelerating gap may be completely cleared
of all electrons in it if the voltage changes sign or some
of them if the voltage sign does not change.
The difference in the behaviour of direct and inver-
ted-polarity diodes is that in a certain range of paramet-
ers the beam in an inverted diode is strongly unstable
with respect small voltage variations and the large cath-
ode surface permits briefly drawing from it large sec-
ondary emission currents, thereby securing deeper mod-
ulation of the voltage on the diode. After withdrawing
the charge in the gap, the diode again begins to charge
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 169-171.
169
and if the emission of the primary beam is continuos the
process is repeated.
3 RESULTS OF COMPUTER SIMULA-
TIONS
The dynamics of voltage on an inverted magnetron
diode was investigated by means of KARAT code [6] in
two two-dimensional geometries. In the first case, it was
assumed that the beam is homogeneous in the axial (z)
direction. In the second case - homogeneous in the azi-
muthal (θ) direction. Calculations performed for the
model of a circuit with lumped parameters can also be
generalised for the case of systems with distributed
parameters.
3.1 r-θ-Geometry
Below are presented calculation results for a mag-
netron diode with anode radius ra = 0.66 cm and cathode
radius rc = 1.06 cm. The diode is immersed in a magnet-
ic field B0 = 3 kGs. By way of example, here was
chosen a trapezoidal form of external voltage pulse. The
rise time and fall of V0(t) was 8 ns and the flat top had a
duration of 8 ns. The chosen coefficient of secondary
emission was the standard for a metal [1]. The voltage
amplitude at the external source was 50 kV. For the giv-
en variant of calculations illustrating the possibility of
operation an inverted magnetron diode in a self-generat-
or regime, we chose a constant primary beam emission
current from the cathode of 30 A from 1-cm unit length
in the z-direction. A reduction of primary beam emis-
sion current leads to a change in the modulation of
voltage.
Fig. 1. Behaviour of the voltage on the inverted diode.
Fig. 1 shows the behaviour of voltage on the diode.
The dynamics of changing the number of primary ("b")
and secondary ("e") electrons in the accelerating gap
and the spiking behaviour of secondary-emission cur-
rent (in amperes) can be seen in Fig. 2.
The frequency of voltage oscillations on the diode is
approximately equal to the natural frequency of the cir-
cuit and correspondingly at least in a certain range, can
be regulated by external parameters. The amplitude of
oscillations and average voltage also can be regulated
by a choice of the existing parameters, in particular,
magnetic field and primary beam emission current.
Fig. 2. The dynamics of the change in the number of
primary and secondary electrons in the accelerating
gap and behaviour of secondary emission current.
3.2 r-z-Geometry
Approximately analogous results are also obtained
in r-z-geometry. Fig. 3 shows the geometry of the gun.
Fig. 3. Gun geometry.
The primary beam of 50 A is emitted from a flat part
of cathode of radius rc = 2.12 cm and 2-cm long in the
axial direction. The anode radius ra = 1.32 cm and rise
time of the external voltage V0(t) to the maximum value
of 25 kV is 8 ns, after which the voltage holds constant.
The magnetic field in the given case is 1 kGs. The beha-
viour of the voltage on the diode and the frequency
spectrum of oscillations are presented in Fig. 4. In this
case, the coefficient of secondary emission is increased
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 169-171.
170
1.5 times relative the results of calculations for the
r-θ-geometry.
Fig. 4. Behaviour the voltage on magnetron gun and
frequency spectrum of oscillations.
It should be noted that the values of primary current
used in the given calculations are quite large and for
technical reasons are difficult to realise for diodes of in-
dicated dimensions. For experiment, it is desirable to
use cathode material of large coefficient of secondary
emission, which permits to decrease the value of
primary beam current and also of external injection of
primary beam.
To conduct experimental investigations of coaxial
diodes with magnetic insulation and secondary emission
cathodes, a special setup was developed on which vari-
ous methods of initiating secondary emission could be
realised and also controlled, including by means of a
primary beam of electrons from a thermionic cathode.
In experiments on the generation of RF-oscillation, a
quarter-wave coaxial resonator will be connected to the
inverted diode (see Fig. 5).
The central electrode of the coaxial resonator, which
one side connected to the diode anode and the other on a
passing-through ceramic insulator, is mounted on the
resonator flange. For high frequency, the insulator is
shorted by shunting condenser. The rate of rise of the
voltage applied to the diode via the resonator central
electrode, insulated for direct current, is varied bay
changing the value of the condenser. To sharpen the
pulse of the voltage on the diode, an additional spark
gap is provided between the central electrode and shunt-
ing condenser.
Fig. 5. Schematic diagram of the experimental
setup. 1 - Capacitance-loaded quarter-wave reson-
ator; 2 - solenoid; 3 - tungsten-wire thermionic
cathode; 4 - feed input; 5 - central resonator elec-
trode - diode anode; 6 - working area of secondary
emission cathode; 7 - excitation loop/diagnostics; 8
- pulsed high-voltage generator; C1 - blocking con-
denser.
4 CONCLUSION
An inverted magnetron diode with an external circuit
has been proposed and shown to be promising as a com-
pact modulator for various applications. Results of com-
puter simulation of the modulator operation, the prin-
ciple of which is based on the dynatron effect, are
presented. Calculations performed on a model of the ex-
ternal circuit with lumped parameters can be generalised
for the case of a system with distributed parameters.
Modulators of proposed construction could be easily in-
tegrated into construction of guns to obtain modulated
intense beams and also can be used for other applica-
tions.
Work supported by RFFI under grant 00-02-16182.
REFERENCES
1.V.A.Agafonov, V.M.Fedorov, V.P.Tarakanov // Proc.
of 1997 Particle Accelerator Conf., Vancouver, Canada.
1997, v. 2, p. 1299 - 1301.
2.V.A.Agafonov, V.M.Fedorov, V.P.Tarakanov //
Problems of Atomic Science and Technology. Issue:
Nuclear-Physics Research (29-30). 1997, v. 2-3, p. 137–
139.
3.V.A.Agafonov, V.M.Fedorov, V.P.Tarakanov //
Problems of Atomic Science and Technology. Issue:
Nuclear-Physics Research (35). 1999, v. 4, p. 11-13.
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ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.
Серия: Ядерно-физические исследования (39), с. 169-171.
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