Higt-frequency electron guns – current status
The high-frequency guns with thermoionic cathode are effective sources of electrons for linear accelerators. The work on perfecting these sources of electrons actively proceeds now in many laboratories of the world. The direction of these researches is connected first of all with increase of the ave...
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| citation_txt | Higt-frequency electron guns – current status / V.A. Kushnir // Вопросы атомной науки и техники. — 1999. — № 3. — С. 3-5. — Бібліогр.: 41 назв. — англ. |
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| description | The high-frequency guns with thermoionic cathode are effective sources of electrons for linear accelerators. The work on perfecting these sources of electrons actively proceeds now in many laboratories of the world. The direction of these researches is connected first of all with increase of the average beam intensity and reaching the homogeneity of performances of a beam during pulse. Together with traditional thermoionic and photocathode RF-guns the new variants of guns with the use of various types of cathodes are offered and investigated.
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HIGH-FREQUENCY ELECTRON GUNS – CURRENT STATUS
V.A. Kushnir
NSC KIPT, Kharkov, Ukraine
INTRODUCTION
During the past decades the development of
physics and technique of linear electron accelerators
(linacs) has been marked by increased requirements to
the quality of electron beam. For carrying out scientific
investigations and solving the numerous applied
problems the bright electron beams with small particle
energy spread are needed.
Now injector systems, based on using high-
frequency sources of electrons (RF-guns) have obtained
the great prevalence [1]. The RF-gun generally is a
microwave-cavity with electric modes (for example,
E010) turned on accelerator frequency or its sub-
harmonic. The cathode emitting surface is arranged in
the cavity, where the electric field strength is about
(107-108) V/m. This creates the necessary condition to
form the high-energy (105-106 eV) electron bunches and
to obtain the small beam emittance. The typical pulse
current length at the exit of RF-guns with thermoionic
cathode [2 - 6] is 1.5-8.0 µs and the pulse current of 0.5-
1.5 A. When using the photocathode the pulse length is
determined by the pulse length of laser radiation. It can
average the several picoseconds and even hundreds of
femtoseconds, and the current in the bunch amounts to
hundreds of amperes. The frequency range, where the
high-frequency electron sources are used, extends from
108 to 1010 Hz.
Many scientific publications deal with the
problems related to the development and investigation
of RF-guns. The researches concerned with design of
photocathode RF-guns are described in the most of
them. A number of detailed reviews (see, for example,
[9, 10]) have been devoted to the current status in this
sphere. In the present work we analyze the state of
investigations in the field of engineering the high-
frequency electron sources with the thermoionic
cathodes (TC) and give information about one of the
new RF-gun types.
RF GUNS WITH THERMIONIC CATHODE
The first high-frequency gun with thermoionic
cathode was constructed in 1983-1985 years at Stanford
High Energy Physics Laboratory (HEPL) by
G.A. Westenskow and J.M.Madey [2]. The gun was a
cylindrical E010 cavity with LaB6 cathode placed in it.
This high-frequency gun was designed to use it in the
Mark III accelerator [11, 12]. Just the successful
experiments allowing to design a free electron laser on
the base of this accelerator created the basis for wide
use of RF-guns in linacs. Unlike the photo-RF-guns the
guns with TC do not require the unique and expensive
laser devices and, moreover, they have good
performances. On the other hand, in spite of simple
design the thermoionic RF-guns are quite complex
objects in the context of physical processes which occur
in it. Now we shall consider qualitatively the main
processes.
1. In the RF-guns with TC the emission occurs
during the whole accelerating half-period of the high-
frequency field. This fact leads to the following:
1.1. The portion of electrons (for the most of guns over
the range of initial phase of 90°-180°) has not
enough time to leave the cavity during the
accelerating half-period. These electrons are
braked and, accelerating to the back direction,
bombard the cathode. This phenomenon is the
significant range limitation of RF-guns use in
linacs [3, 13, 14]. The cathode bombardment leads
to heating its surface during the time of pulse
current and to increasing the average temperature.
So the pulse length and repetition rate for the most
of guns designed do not exceed 4µs and 25Hz,
respectively, without using any special equipment.
1.2. Every electron emitted by the cathode surface in
different times will have different values of
longitudinal and transverse pulses at the exit of
gun by virtue of the time dependence of high-
frequency field.
2. In connection with the fact that field intensity on
the cathode surface is about (107-108) V/m, the current
density of emission varies according to the Richardson-
Deshmen low and Schottky effect [15]. This condition
plays an important part in forming the beam in the gun
cavity system. The degree of the current variation as a
function of the electric field intensity depends on
characteristics of the gun cathode material under other
equal conditions. First of all, it depends on the value of
work function. The work function becomes less as the
current density increases during the half-period of high-
frequency field. As follows from the simulation of
particles dynamics (see, for example, [16]) for the
monocavity guns, the electrons leaving the cathode at
the moment of field maximum (phase of 90°) have a
low energy and considerable spread at the exit of the
gun. So, in the context of energy homogeneity beam
development with the small emittance the Schottky
effect leads to the negative consequences in this case. It
should be noted, that now the physics of thermoionic
emission from the surface of a cathode placed in the
high microwave fields is not investigated in detail. In
particular, the consideration of this problem under
condition of comparability between the RF period of
oscillation and the representative time of electron-
phonon interaction is of interest.
3. When considerable emission current occurs, the
large part of electromagnetic energy stored in the cavity
is transmitted to the beam particles. In this case,
reduction of the electric field strength and variation of
the field phase during the pulse by beam loading effect
have a great influence on the dynamics of electrons.
It is evident that the amount, energy and angle
distribution of electrons returning to the cathode is
determined by the electric field distribution. On the
other hand, the same field defines the beam
characteristics at the exit of the gun. So, the forming of
optimum field distribution, which would satisfy the
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3
various and sometime antilogy requirements, is a
complex problem. For instance [17] is shown that it is
impossible to achieve the considerable lowering the
power of the back electron current without losing the
beam quality and primarily the degradation of the beam
emittance. One of the ways to reduce the effect of back
bombardment is to choose the appropriate cathode
material and its form. In this connection it is important
to study all the factors determining the variation of the
cathode temperature as well as the change of the
emission density depending on the cathode surface. The
investigation results obtained for the non-steady
temperature processes on the cathode surface under the
influence of the back flue electrons are given in [18]. It
is shown, that the temperature variation during the pulse
caused by the electron bombardment is determined by
the depth of electron penetration into the cathode
material and, consequently, by the energy spectrum of
electrons. This condition should be taken into account,
when choosing the cathode. Generally speaking, at
present the problem of the cathode type selection for
thermoionic RF-gun is not yet studied in detail. As the
analysis shows, "the most acceptable cathode" for a RF-
gun with a typical beam characteristic (the pulse current
of 1-2 A, the field strength on the cathode of 30-50
MV/m, the pulse length up to 10 µs) should have:
- the high emission density of 15-20 A/cm2 that allows
to design a cathode of small size (diameter of 1.5-2
mm) and, consequently, to have the potential
possibility for obtaining the low emittance of the
beam. Besides, the high emission density allows
fixing up the effectiveness of using the cross magnetic
field method to reduce the amount of electrons
bombarding the cathode [19];
- the work function not less than 2.5-2.8 eV for reducing
the influence of the Shottky effect;
- rather high operating temperature (not less than
1700 K) to reduce the effect of temperature variation
on the emission density;
- the cathode stability to the sputtering;
- the small sublimation rate of emitting substrate (not
more than 10-7 g/cm2s);
- the possibility of multiple exposure in atmosphere
under the air-pressure.
Now the different types of oxide cathodes (pressed,
impregnated and so on) or LaB6 cathodes, traditionally
used in the accelerator technique, are employed in RF-
guns. The temperature of their surface is 1000-1800 K;
the work function is 1.7-2.8 eV. The application of the
monocrystal LaB6 cathode is the most preferable one in
spite of some difficulties connected with heating.
However, such a choice is not, apparently, the only one.
In particularly, the use in RF-guns of high-temperature
cathodes on the base of alloys of iridium, rhenium and
osmium with the elements of lanthanide's group (IrCe,
IrLa etc.) seems to be rather preferable [20]. By our
means it is advisable to use directly heated metal
cathode, when the RF-guns with a small pulse current
(less than 1mA) and with a large repetition rate are
designed. The use of a special cathode form can reduce
the influence of bombardment on the gun
characteristics. For example, the circular pressed BaNi
cathode was used for this purpose in the one of the
LUE-60 accelerator RF-gun modifications [21, 22]. It is
apparently possible to propose such a cathode form that
the electrons will not reach the emitting surface at all. In
particularly, one of the possible cathode variants is a
cylinder with the emitting layer disposed on its element.
On the other hand, the coming back electrons can be
used for heating the cathode surface. In this context the
results of experiments carried out in 1992 on the LUE-
60 accelerator are very interesting. The possibility of
stable operation of the RF-gun with BaNi-cathode was
shown in this experiment, when the source of cathode
heating was turned off. The results of this experiment
are important since they allow to study the problem
connected with the design of a new RF-gun type with
the thermoionic cathodes.
The method of transverse magnetic field proposed
in [19] is the most common way to reduce the power of
the back electrons in the monocavity RF-guns. The idea
of this method is that the transverse magnetic field is set
up near the cathode by means of a special magnetic
system. The intensity of the transverse field decreases
rapidly along the longitudinal coordinate. The
maximum field is only hundreds of oersteds at the
cathode plane in first experiments. However, the
possibility arises to increase the current pulse length of
the Mark III accelerator RF-gun from 1.2 µs to 6 µs
[19]. According to the calculation given in [6], the use
of this method makes it to reduce the quantity of back
electrons to 75% and to increase the pulse length to 10 µ
s with the optimum magnetic field distribution. The
transverse magnetic field method has been used in the
RF-gun of Beijing FEL accelerator-injector [13, 23].
The increase of the current pulse length to 5.0 µs has
allowed obtaining the saturation regime of FEL
generation [14, 24]. The main drawback of this
technique is the influence of the magnetic field on the
track of the "useful" particles escaping the cavity.
Therefore, the special correcting magnet elements are
set up at the exit of the RF-gun. However, generally, in
these devices the beam emittance increases because of
the large energy spread of particles.
The output beam characteristics of the RF gun are
changing during the pulse because of the back-
bombardment effect. Besides, the particles have the
considerable energy and phase spread even at the fixed
time moment. The typical amount of the energy and
phase spread is 60 -70% and 60°, respectively, for 80%
of particles. So, for application of monocavity RF-gun
in the precise accelerator the beam is additionally
formed at the special magnetic systems (α-magnet)
[25, 26, 27]. Owing to these magnet systems the energy
spread does not exceed 1% and the current in the bunch
achieves the tens of amperes at the accelerator exit of
the injector system containing RF-gun with TC and α-
magnets. Such accelerators are successfully used as
injectors of electrons in FEL [12, 24] and as injectors
for sources of synchrotron radiation [22].
The application of the more complex, specially
optimized resonance systems consisting, in particularly,
of the several cavities is the alternative of using the
outside forming devices. As a simple variant such
resonance systems consist of two cylindrical cavities
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. №3.
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4
with electric and magnetic couplings [4, 28, 29] and π -
type mode. The optimum field distribution is
determined by means of particle dynamics simulation
and calculation of beam characteristics at the exit of the
gun (see, for example, [30]). The design of some guns
gives the possibility to change the frequency of each
cavities and thereby to vary quickly the accelerator field
distribution along the gun axis. Such gun [4] is used
now as an electron source of the LIC facility [31,32].
The diameter of BaNi cathode is 5mm. The beam has
the following characteristics at the exit of the RF-gun:
particle energy of 0.7-0.9 MeV, pulse current of 1.5 A,
phase bunch length equal or less than 50°, current pulse
length of 0.7 - 1.5µs, normalized emittance not more
than 12π⋅mm⋅mrad.
The multicavity S-band RF-guns, designed in the
Beijing University and IHEP, China [33] and in NSC
KIPT, Ukraine [34,35] can serve as an example of using
the most complex resonance systems. These guns
operate as injector systems with beam characteristics
allowing to use them both in precise accelerators, and in
technological accelerators. The RF-gun of NSC KIPT
serves for operation in the regime of high average beam
intensity. The gun resonance system consists of three
cavities connected by means of coupling cavities. The
mode of oscillations is π/2. The LaB6 circular cathode
has been used in the gun. The pulse power of the back
flow electrons is 21kW, when the beam pulse current at
the exit of the gun is 0.5A and the energy of particles is
0.8 MeV. This power is five times less, than in the case
of a usual monocavity gun with similar output beam
characteristics. The phase bunch length does not exceed
43°, and the normalized beam emittance is 11 π⋅mm⋅
mrad. The high average beam intensity at the exit of the
gun (up to 0.5 mA) allows to use it in technological
linacs.
SECONDARY EMISSION RF-GUNS
The first works on design and research of RF-guns
with secondary emission cathodes were executed by
F.M. Mako and W. Peter [36, 37] in USA. In these guns
the high-current pulses are generated by multipacting
the electrons from the walls of the microwave cavity.
The experiments were carried out both in S- and L-
bands. The resonant system consists of one cavity
(cylindrical Е010-mode cavity in L-band and rectangular
H101-mode cavity in S-band). On one of the walls of the
cavity is the emitting surface (cathode), on the contrary
wall the metallic grid is mounted. This grid has a high
transmittance and as a cathode it is the source of
secondary electrons. In essence, in these guns the well-
known two-electrode high frequency resonance
discharge is used (see, for example [38]). The
possibility to obtain a high-current beam during long
time is shown experimentally. The macropulse current
of 18 A, e.g. in experiments at S-band (2.85 GHz), was
obtained. The corresponding peak current is 360 A. This
gun operates at a repetition rate of 50-300 Hz with a
macropulse length of 2.25 µs. The analysis of beam
dynamic and experimental data shows the possibility of
obtaining the short electron bunches (about 5 % of the
RF period). Unfortunately, in the literature there is no
information about any experimental researches on a
beam emittance and energy spread behavior. This
problem is a key for determining the application area of
these guns.
The similar operational mode of a gun was
observed in KIPT on the two-cavity universal RF-gun of
the accelerator LIC [4]. In this case the source of
secondary electrons was thermoionic BaNi cathode
bombarded by "return" electrons. The temperature of
the cathode was below the threshold for the significant
thermoionic emission. The gun was operating at a pulse
duration of 2.8µs with a pulse current of about 1.5 A.
The researches on this operationing mode are continued.
There is one more type of electron sources, which
application in RF-guns on our opinion can be
promising. It is a so-called magnetron secondary
emission gun with the cold cathode [39 - 41]. This
electron source is a magnetron diode with smooth
cylindrical electrodes. The basic mechanism of diode
operation is the secondary emission multiplication in
crossed electric and magnetic fields. The pulse current
up to hundreds of amperes have been obtained
experimentally. One of advantages of such a source is
the large service life (on some evaluations up to 105
hours).
CONCLUSIONS
The high-frequency guns with thermoionic cathode
are effective sources of electrons for linear accelerators.
The work on perfecting these sources of electrons
actively proceeds now in many laboratories of the
world. The direction of these researches is connected
first of all with increase of the average beam intensity
and reaching the homogeneity of performances of a
beam during pulse. Together with traditional
thermoionic and photocathode RF-guns the new variants
of guns with the use of various types of cathodes are
offered and investigated.
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ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. №3.
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ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 1999. №3.
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6
RF GUNS WITH THERMIONIC CATHODE
CONCLUSIONS
REFERENCES
|
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| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-07T18:06:49Z |
| publishDate | 1999 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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| spelling | Kushnir, V.A. 2015-05-11T17:49:54Z 2015-05-11T17:49:54Z 1999 Higt-frequency electron guns – current status / V.A. Kushnir // Вопросы атомной науки и техники. — 1999. — № 3. — С. 3-5. — Бібліогр.: 41 назв. — англ. 1562-6016 https://nasplib.isofts.kiev.ua/handle/123456789/81142 The high-frequency guns with thermoionic cathode are effective sources of electrons for linear accelerators. The work on perfecting these sources of electrons actively proceeds now in many laboratories of the world. The direction of these researches is connected first of all with increase of the average beam intensity and reaching the homogeneity of performances of a beam during pulse. Together with traditional thermoionic and photocathode RF-guns the new variants of guns with the use of various types of cathodes are offered and investigated. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Higt-frequency electron guns – current status Высокочастотные электронные пушки – современное состояние работ Article published earlier |
| spellingShingle | Higt-frequency electron guns – current status Kushnir, V.A. |
| title | Higt-frequency electron guns – current status |
| title_alt | Высокочастотные электронные пушки – современное состояние работ |
| title_full | Higt-frequency electron guns – current status |
| title_fullStr | Higt-frequency electron guns – current status |
| title_full_unstemmed | Higt-frequency electron guns – current status |
| title_short | Higt-frequency electron guns – current status |
| title_sort | higt-frequency electron guns – current status |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/81142 |
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