Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Mu ltirhodotron
Technical Field.
This invention related to electron accelerators that are usually used for
irradiation of various
substances, such as food products, either directly by electrons or by X-rays
emitted using conversion
of electrons on a heavy metal target or are used for transforming the energy
of electrons into FEL
radiation.
An electron accelerator comprises a resonant cavity energized by a high
frequency electromagnetic
field source and an electron source able to inject electrons into the cavity.
The electrons are
accelerated by means of the electric field during their passes through the
cavity if input phase of the
electrons and their velocity satisfy conditions for acceleration.
Background of the Invention.
In accordance with this principle, in some accelerator types, the electron
beam crosses the cavity
several times. In this case the accelerator, in addition, comprises a magnetic
deflecting device
receiving the beam that has been already accelerated once, deflects the beain
at approximately 180
and injects the beam into the cavity for further acceleration. A second
deflector deflects the beam
that has undergone two accelerations. This is done to make the beam pass
through the cavity several
times to obtain several beam accelerations.
TM
Typical examples of this accelerator type are race-track microtron /1 / and
Rhodotron /2 /. The
first of them usually produces the electron beams with energy of a few tens of
MeV, however the
average power of the beam in this accelerator is lower than in the second one.
In the RhodotronTM
the electrons do not reach so high maximum energy after acceleration as in the
microtron; the energy
of the electrons usually doesn't exceed 10-12 MeV. These characteristics limit
some applications of
these types of accelerators where high average power and high energy of the
beam are needed
simultaneously. The transformation of the electron beam energy into the energy
of the FEL radiation
and the transportation of this energy at long distances is an application for
accelerators that have
high energy and high average power at the output.
In fact, both microtron and RhodOtronTM have practical limit for number of
recirculation of the
electron beam in these devices. This limit is about 10 - 30 passes for
microtron and 7 - 12 passes for
RhodotronTM. In the race-track microtron the electron beam is usually
accelerated using acceleration
structure based on standing or traveling microwaves with acceleration gradient
about 5 - 20 MeV/m.
It provides the increase of the electron energy by 10 - 20 MeV during a pass
through the acceleration
structure. The RhodotronTM increases the electron energy by 1 - 2 MeV during
one pass and has a
few passes of electron beam for the total accelerating. The beam current is
limited by the rigidity of
the focusing channel of the accelerator. This limit is higher for the
RhodotronTM so it has higher
average power of the electron beam in the CW mode.
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Summary of the Invention.
This invention has a goal to increase the average power of an accelerator such
as RhodotronTM by
using two new embodiments of accelerating devices. The first provides the
increase of the output
electrons energy of the accelerator up to 40 - 50 MeV or more. The second
provides the increase of
the total electrons current in accelerator up to 200 - 250 mA. For comparison
the current of the most
powerful RhodotronTM has approximately 10 MeV and 50 - 60 mA respectively
which means that
the average power of RhodotronTM does not exceed 600 kW; however, using
devices of this
invention, the average power can be increased up to 2 -3 MW in the beam. The
increase of the
energy of the electrons at the output of the accelerator expands the area of
application for this
accelerator, for example, for production of the radioactive isotopes or for
use in FEL. The increase
of the full current at the output of the accelerator increases the
productivity of sterilization devices,
for example by means of simultaneous multilateral irradiation of the object.
The invention increases
the upper limit of the electron energy or the full electron current for the
electron accelerator of
Rhodotron'STM type by means of the coaxial cavity of the special form with the
new shape of the
trajectory of the electron beam that passes through the cavity many times for
repeated acceleration.
Brief Description of the Drawings.
The characteristics of the invention are described in more details in the
description hereunder. This
description is based on attached drawings:
FIG. 1 displays distribution of E and H fields of the fundamental mode of the
resonant
electromagnetic field in the coaxial cavity of the RhodotronTM
FIG. 2 illustrates a feature of the coaxial cavity that provides absence of
the magnetic field in the
median plane of the cavity,
FIG. 3 displays a transversal sectional view of the coaxial cavity in the
Rhodotronl",
FIG. 4 displays the resonant electromagnetic field in the coaxial cavity in
the case when the field has
longitudinal variations with n = 4,
FIG. 5 displays locations of the inlets and outlet ports for the electron beam
on the surface of the
accelerating cavity according to the invention,
FIG. 6 displays the electron trajectory for transferring the beam from one
accelerating plane to
another,
FIG. 7 displays a transversal sectional view of the electron accelerator
according to the invention,
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FIG. 8 displays the electron trajectory in the accelerator with increased
electron energy at the output,
FIG. 9 displays electron trajectories in the accelerator with two electron
sources,
FIG. 10 displays a variation in the form of the inner conductor of the cavity
to attenuate high
frequency power losses of the conductor.
Detailed Description.
The RhodotronTM accelerator has a coaxial cavity that is energized by a high
frequency SHF source
that is connected to the cavity via the loop. The cavity has a longitudinal
axis of symmetry (A) and a
median plane (Pm) perpendicularly to this axis. Among all possible resonance
modes of this cavity,
there is one, called the fundamental mode (TEM) that has the transverse
electric and magnetic type
for which the electric field (E) has only radial character in the cavity.
Magnetic field (H) of this
mode is purely azimuthal. The electric field has maximum in the said median
plane and decreases on
both sides of the said plane down to zero at the end flanges. In contrast, the
magnetic field has
maximums along the flanges and changes own direction along the opposite sides
of the median
plane. The electromagnetic field in such cavity for TEM mode is shown on the
Fig.l.
According to the invention /2 /, the electron beam is injected into the
coaxial cavity in the median
plane (Pm). In this plane there is no parasitic electromagnetic field that can
deflect the beam. How it
is shown in the part "a" of FIG. 2 (the cross section of the cavity), the
electric fields (El) and (E2)
are equal along two distinct radiuses in the median plane of the cavity. A
contour (17) is defined by
these two radiuses and by two circular arcs and the electric field is radial
along them. The circulation
of the electric field (i.e. the integral of this electric field) is zero along
the said contour. Thus, the
flux of the magnetic induction through the surface defined by the said contour
is also zero. In other
words, there is no magnetic component of the electromagnetic field along the
direction that is
perpendicularly to the median plane.
As shown in the part "b" of FIG. 2 (the longitudinal sectional view of the
cavity), the electric field is
symmetrical with respect to the median plane. The fields (E3) and (E4) along
two infinitely close
radiuses on either side of said plane are equal. The circulation of the
electric field is zero along a
contour (18) defined by these two radiuses and two longitudinal branches.
Thus, the magnetic
induction flux across a surface defined by the said contour is also zero. In
other words, there is no
magnetic component of the electromagnetic field along the direction that is
parallel with the median
plane.
Since there is no magnetic component in the median plane (Pm), the median
plane of the cavity is
purely capacitive zone. Thus, the electron beam is not exposed to any
deflecting force.
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FIG. 3 diagrammatically shows the complete accelerator according to the
invention of the
Rhodotron. The apparatus comprises an electron source(S), a coaxial cavity
(CC), formed by an
external cylindrical conductor (10) and an internal cylindrical conductor
(20), as well as two
electron deflectors (D1) and (D2) and a high frequency source (SHF).
The apparatus functions as follows. An electron source (S) emits an electron
beam (Fe) directed in
the median plane of the coaxial cavity (CC) shown in the transverse cross-
section view. The beam
enters the cavity through a hole (11) (inlet port) and then it passes through
the cavity along a first
diameter (dl) of the external conductor (10). The internal conductor (20) has
two diametrically
opposite holes (21), (22) for passing the beam. The electric field accelerates
the electron beam if the
phase and frequency conditions are satisfactory (i.e. the vector of the
electric field must remain
parallel to the vector of the velocity of the electrons but oppositely
directed because electrons have
negative charges). The accelerated beam leaves the cavity through a hole (12)
(outlet port) which is
placed diametrically opposite to the hole (11) and then a deflector (D1)
deflects the beam for
repeated entering into the cavity.
The beam reenters the coaxial cavity through the hole (13) (another inlet
port), moves along the
second diameter (d2) and undergoes a second acceleration in the cavity.
Subsequently, the beam
passes through a hole (14) (another outlet port) and, being deflected again by
a deflector (D2),
reenters the cavity through a hole (15), moves along a third diameter (d3),
undergoes a third
acceleration and exits via a hole (16). All passes of the electron beam lay in
the said median plane
(Pm) for RhodotronTM and this limits the total number of passes of the beam
through the coaxial
cavity of RhodotronTM.
The goal of this application is achieved by a special coaxial resonant cavity
used for accelerating
beam. The new cavity also comprises an outer conductor (10), an inner
conductor (20) that have
cylindrical form and have the same axis of rotation A and two flange covers
(31,32) which are
joined the ends of the cylindrical conductors.
The said new coaxial resonant cavity has inner volume length that is equal to
sum of any whole
(integer) number (more than one) of halves of the wavelength of the resonant
frequency of the said
cavity. A high frequency source energizes the coaxial cavity in TEM mode that
has only radial
component of the electric field and only azimuthal component of the magnetic
field. The electric
field and the magnetic field in this TEM mode has "n" variations along the
cavity axis where "n" is
the number of halves of wavelength of resonant frequency that can be stacked
along the inner
volume length of cavity.
Therefore the said cavity has exactly "n" planes that are positioned
perpendicularly to the cavity
axis. These planes are placed at the points of the cavity axis where the
azimuthal component of the
magnetic field is zero and the radial component of the electric field has
maximum. The first and the
last of such planes are located in a quarter of the resonant frequency
wavelength distance from
CA 02787794 2015-09-08
flange covers (31, 32). The distance between the others neighboring planes of
cavity is equal to the
half wavelength of the resonant frequency. According to this invention, the
accelerator is
characterized by the said coaxial cavity having several beam inlet and beam
outlet ports positioned
along the lines where the planes, that are located perpendicularly to the
cavity axis in such points of
the axis where the radial component of resonant electric field has maximums,
intersect with the said
outer conductor. The electromagnetic field in such cavity for TEM mode with n
4 is shown in
F ig.4.
Each cavity inlet and corresponding outlet port are located oppositely to each
other along the cavity
diameter and each diameter lays in one of the said "n" planes (PI, P2, P3,
P4).. According to the
invention the number of possible passes of the electron beam through the said
cavity for the
acceleration is increased because these passes do not have to lay only in one
plane as in
RhodotronTM but they may lay in all of these said planes. See Fig. 5.
Several deflective means, located outside of the said cavity, transport
electron beam from one cavity
outlet port to the next inlet port of the cavity.
When said outlets and inlet ports are located in the same plane, the electron
beam may be deflected
by the deflection mean in the same plane similar to RhodotronTM (see Fig. 3).
In this case, in order to
achieve electron acceleration, the synchronization requirements mean that the
sum of the time
interval when electrons cross the cavity along cavity diameter and of the time
interval when
electrons are transported from cavity outlet port to next cavity inlet via the
deflection mean, must be
equal to a whole number (integer) of periods of the resonant frequency of the
said cavity.
If the cavity outlet port and the subsequent cavity inlet are located in the
neighboring planes (see
Fig 5), the new synchronization requirement must be met. The total interval of
time for transporting
electrons of the beam from the previous cavity inlet port in one said plane to
the next cavity inlet
port in the neighboring said plane must be equal to (1/2+k) xTr, where Tr is
the period of the
resonant frequency and k is an integer greater or equal to 1. This requirement
stipulated by the fact
that the phase of the electromagnetic field in the first said plane differs
from the phase of the
electromagnetic field in the next neighboring plane by 180 . (See Fig. 4)
In the case where inlet and outlet ports are located in neighboring planes the
deflection mean is
different from the case where the inlet and outlet ports are located in the
same plane.
For example this mean may comprise two deflection magnets and a rectilinear
electron pipe, where
each of the magnets deflects the beam approximately at an angle 90 , and the
pipe joins to the outlet
port of the first magnet and to the inlet of the second magnet. The pipe
length approximately must
be equal to half of the wavelength of the resonant frequency. (see Fig. 6). In
the most general case
when two sequential passes through the cavity for accelerating the beam don't
lay in the same plane
(the first pass lays along the line (AB) and the second pass lays along the
line (EF)), the first magnet
deflects electrons from the line (AB) to the line (CE) that lays in one plane
for the lines (AB) and
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(CE) and the second magnet deflects electrons from the line (CE) to the line
(DF) that lays in one
plane for lines (CE) and (DF), but this plane differs from the first plane for
lines (AB) and (CE).
Both of synchronization requirements for electron beam transportation in the
same plane and
between different planes (P1, P2, P3, P4) are met, if the diameter of the
outer conductor (10) of the
cavity is slightly less than the length of the wave of the resonant frequency,
the distance between the
neighboring said planes of the cavity is equal to the half of the wavelength
of the resonant frequency
and the velocity of the electrons in the beam close to the velocity of light.
These requirements of synchronization can be generalized to the case when the
electron beam is
transported between two said planes that are not neighboring (don't follow one
after another). If the
distance between the planes equals to even number of the halves of the
wavelengths of the resonant
frequency, the synchronization requirement is identical to the one where the
electron beam
transportation happens in the same plane, because phases of the
electromagnetic fields of these
planes are equal. If the distance between the planes is equal to odd number of
the halves of the
wavelength of the resonant frequency, the synchronization requirement is
identical to the one when
the electron beam is transported between neighboring said planes, because
phases of
electromagnetic fields of these planes are in opposite phase.
Starting from the second pass of the electrons through the cavity the velocity
of the electrons in the
beam close to the velocity of light, therefore after the first each of all
passes adds the same
increment to energy of the electrons. This increment can be evaluated using
the expression, if both
synchronization conditions are met.
2Rw/c Rw/c
p=e0 E
i
Ap(pJA)o)dt=2(eo/c)AcoS(04)0+Rw/c) (1/Q)singdp
0
rw/c
Where: c - velocity of the light and u - velocity of the electron
eo, mo - charge, mass of the electron
,
p = mom, v = (1_ [32)-1/2 13 _ - u/c
r, R - radius inner and outer conductors of the coaxial cavity
,
w = 2nf, f = resonant frequency, clk - input electron phase
Ep(p, t, 4:10) = (1/p)Asin(wt + .43,0) - radial component of electric field
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This expression links main accelerator parameters such as radiuses of the
inner and outer conductors
of the resonant cavity, amplitude of the electromagnetic field in the cavity
and the increment of the
electron energy during single pass of the electron beam through the cavity.
The ability to accelerate the electron beam with finite electron current in
the cavity as well as the
ability to transport the beam through the deflection devices in the structure
of the accelerator is
provided by rigidity of the transverse focusing forces in the electron channel
of the accelerator. The
company IBA (Belgium) /3/ showed that in practice the current in the electron
beam can reach 50-60
mA while being accelerated up to 7-10 MeV.
The rigidity of the transverse focusing forces can be increased by using the
focusing method like the
method used in the betatron magnet or by establishing some additional magnetic
focusing
quadrupoles between the resonant cavity and the deflection means.
The electromagnetic field in the cavity has focusing character proportional to
its amplitude. In the
direction perpendicularly to the cavity axis this effect exists due to the
radial convergence of
electrical field in the coaxial cavity. In the direction that is parallel to
the cavity axis the focusing
effect exists due to the gradient of the magnetic field in the cavity.
The accelerator proposed in this application functions as follows. A source of
a high frequency
electromagnetic power (SHF) energizes an electromagnetic field by the loop
(34) in the coaxial
cavity (CC). The cavity is formed by cylindrical inner (20) and outer (10)
conductors (see FIG.4).
The coaxial cavity is energized by the source (SHF) at the resonant frequency
of the TEM mode that
has several longitudinal variations. The length of the inner volume of the
cavity is chosen equal to a
whole number (integer) of the halves of the wavelength of the frequency of the
source (SHF). There
are several planes in this case which are perpendicular to the cavity axis at
those points of the axis,
where the radial component of electric field of resonant oscillations has
maximum and the magnetic
component equals to zero.
The first and the last of such planes are positioned at a distance from flange
covers (31, 32) that is
equal to the quarter of the wavelength of the resonant frequency. The distance
between the
remaining neighbor planes of the cavity is equal to the half of the wavelength
of the resonant
frequency. In the example (see FIG.4) there are four said planes (P1, P2, P3,
P4).
The source of electrons (S) injects the electron beam into the cavity (CC) in
the first plane (P1)
through the cavity inlet port (11) in outer conductor (10). The beam is
transported along the
diameter (dl) of cavity (see Fig. 7). The view of the transverse section of
the cavity in the plane (P1)
(see Fig. 7) is identical to the view of the transverse section of the
RhodotronSTM middle plane (see
Fig. 3).
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The electrons in the beam are accelerated by the radial component of electric
field in both parts of
the diameter (d1) between the outer and inner conductors of the said cavity
when the beam passes
through the holes (11, 21) and (22, 12) fully crossing the cavity.
Then the beam goes to a device that deflects electrons from the end of the
diameter (d1) to the
beginning of the diameter (d2). If the diameters (d1) and (d2) lay in the same
plane similar to the
Rhodotron, the total aggregate time for transporting electrons from the hole
(11) to the hole (13)
through the cavity and the deflection device must be equal to a whole number
(integer) of periods of
the resonant frequency.
The choice of the magnitude of magnetic field in deflective magnet allows
meeting this requirement
as it changes the radius of the orbit of the electrons in the magnet and
controls the length of the
electron trajectory.
Similarly the electrons are accelerated along the diameters (d2), (d3), etc.
of the first plane (P1) of
the coaxial cavity.
After being accelerated along the last diameter in the first plane (P1), the
beam goes to an input of
the device that deflects electrons from the end of the last diameter in the
first plane to the beginning
of the initial diameter in the neighboring plane (P2) where the radial
component of the electric field
of the cavity has maximum too.
The process of the electron beam acceleration is repeated in the second,
third, etc. planes until the
cavity will be crossed by the beam along all diameters on all said planes of
the cavity. For instance
the electron trajectory is shown (see Fig. 8) for two neighboring planes (P1)
and (P2).
When electrons are transferred from the one plane to the neighboring plane,
the total aggregate time
for transporting electrons from the last cavity inlet in the previous plane to
the initial cavity inlet of
subsequent plane (through the cavity and the deflection device) must be equal
to a semi-integer
number of periods of the resonant frequency greater than one.
This requirement can be satisfied by the choice of magnetic field in magnets
and by the moving of
the deflective magnets along lines (AB) and (CD) where the electron trajectory
conforms to the case
shown on Fig.6.
The described above implementation of the accelerator provides the increase
the energy of electrons
at the output of accelerator in several times in comparison with the original
RhodotronTM because
the accelerator has the special form of the coaxial cavity with the new form
of the trajectory of the
accelerating electron beam, that penetrates the cavity along all diameters in
all planes (P1, P2 etc.).
The accelerator may have another embodiment in which the total current of
accelerated electrons is
increased. In this case the view of electrons trajectory for two nearest-
neighbor planes (P1) and (P2)
and for two electron sources (S1) and (S2) is imaged in Fig. 9 where (S1)
injects the first beam in
the plane (P1) and (S2) injects the second beam in the plane (P2) for instance
and both beams are
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accelerating in opposite phases of electromagnetic field in the cavity. This
can easy be found
analyzing the picture of field in the cavity in Fig. 4. In other words the all
plurality of cavity's
diameters, lengthways each of which electrons of beam might be accelerated in
the coaxial cavity,
are jointed into two independent consequent chains with the help of deflection
devices. The
synchronization conditions for these chains are carried out by way of choosing
of parameters of
deflection devices for providing acceleration of beam electrons along each out
of these diameters for
both chains.
This implementation provides the increase of the full electron current of the
accelerator at the output
in several times compared to the original RhodotronTM.
The coaxial cavity (CC) has internal losses of 1-IF power that occur in the
skin-layer on the inner
surface of the cavity. The part of the inner surface of cavity that lays on
the inner conductor (20) has
the highest losses that exceed losses on the remaining surface of the cavity
in approximately four -
five times because the radius of the inner conductor (20) is always less than
the radius of the outer
conductor (10) in four-five times. Therefore the resistance of the inner
conductor for surface
currents is higher in the same number of times.
The parts of the inner conductor, located closely to axis's points, where
magnetic field of resonant
oscillation has maximums, are loaded more than the rest of the inner conductor
because surface
currents in the outer wall of the inner conductor have maximums in those same
areas. This effect
will be significantly attenuated, if the radius of the inner conductor in
those areas is made increased.
This embodiment of accelerator can be performed, if to note that TEM mode of
oscillation and
resonant feature of the coaxial cavity weakly depends on form of inner
conductor of cavity under
small local varying of the inner conductor's radius. Therefore the form of the
inner conductor can
little differ from cylindrical surface without the damage of the cavity's
resonant feature under the
using of such coaxial cavity for multipass accelerator and the inner conductor
can be constructed as
the sequence of the alternating cylindrical cuts of pipes, which have two
different diameters, and
these pipes are jointed to each other by means of smooth gradual transitions
from minimum
diameter to maximum diameter (33) and then alternatively back from maximum
diameter to
minimum diameter (35). Besides, pipes midpoints with minimum diameter are
placed in the cavity
axis's points where the radial component of electric field has maximum and
pipes midpoints with
maximum diameter are placed in the cavity axis's points where the radial
component of electric field
is equal to zero, as in Fig. 10.
