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Accelerator ¨ generator
Technical field
The invention is related to a technical area of supplying the high frequency
power to an accelerator
of the type that is described in patent US 5440211 (or US 5095486) and the
offered invention
describes a device for providing the above purpose.
Background of the invention
The well-known accelerator Rhodotron TM is the most powerful type of the
electron accelerator
among all microwave electron accelerators. The accelerators of this type cover
80% of the total
power of all accelerators being used in the world. Rhodotrons' radio-frequency
generator usually is
based on the most powerful radio-frequency tubes such as tetrodes or
diacrodes.
Since each tube connects to the cavity of the accelerator by means of an
intermediate cavity (as in
article H. Tanaka -BEAM DYNAMICS IN A CW MICROTRON FOR INDUSTRIAL
APPLICATIONS", Proceedings of EPAC 2000, Vienna, Austria) that does not allow
connection of
two or more tubes using parallel scheme to the accelerator cavity without
substantially complicating
the accelerator. This fact limits the total power of the electron beam at the
output of the accelerator
and, in turn, limits the efficiency of the applications where Rhodotron TM is
used.
A new device was suggested for supplying the Rhodotron TM with the additional
high frequency
power some time ago in the patent WO-2008-138998. This device does not contain
any additional
radio-frequency tubes and it is the closest to the device that will he
described in the offered
invention. More precisely, these two devices are similar only by their
structure intended for
transferring the kinetic energy of an high current electron beam, that is
accelerated outside the
accelerator cavity, into the energy of the electromagnetic field inside this
cavity; however, the
methods of their operations are different and there are several distinctive
features of their
constructions.
As shown in the Fig. 1 (copy of the patent WO-2008-138998), this structure
comprises a DC
HV power supply (0,5-1,0 MV and 1-3 A) - (86), a cathode of the injector -
(66) with a grid to
control the beam current - (68), an accelerating tube for the distribution of
the potential of electric
accelerating field - (70), a high frequency power supply - (80) for delivering
the electromagnetic
power into the coaxial cavity - (50, 52) of the accelerator through the loop -
(78) and all of these
devices are placed in the tank - (88).
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This aggregate of elements is executing the method where under the high
potential from the
cathode (66), the electron beam (60) is injected into the accelerating tube
(70) through the grid (68).
The grid is modulating a current of this beam with the resonant frequency of
the accelerator cavity
(50, 52). Then the modulated beam is accelerated by going through the
accelerating tube and then
the maximums of the beam current come into the cavity in phase where the
electrons of the beam
return their kinetic energy to the electromagnetic field during the pass by
electrons through the
cavity. This energy is added to the energy that is delivered by the HF
generator (80) into the cavity
through the loop (78).
In this method the accelerating tube of the straight action accelerator
usually comprises a sequence
of conductive disks interleaved by the isolated rings and has the resistive
divider usually used for
distribution of the potential of the acceleration along the accelerating tube.
Each disk is contacting
with the definite element of this divider for distribution of the potential
according to the preset
gradient of the electric field. The distance between neighboring disks is
usually equal to a few
centimeters and the electric current of the divider is no more than 1-2 mil
liamps in order to decrease
the divider losses.
When a continuous unmodulated beam goes through the accelerating tube, its own
escorted
electromagnetic field is spreading from the beam to the walls of the vacuum
tank without of any
interaction with the accelerating tube since this field has a constant
character and does not depend
on the time. When the modulated beam goes through the accelerating tube its
own escorted
electromagnetic field has the alternative character and it is interacting with
the accelerating tube,
inducing back-current through the resistive elements of the divider because
the distance between the
disks is much less than the length-wave of the beam modulation and also the
field cannot spread
across the accelerating tube. As the result, under the accelerating, the
electron current of the
modulated beam cannot exceed a few percents of the divider current and this
device cannot provide
a lot of additional high-frequency power into the accelerator cavity.
The current about 1-3 ampere in the beam cannot be accelerated by the method
of above patent
WO-2008-138998. But this restriction does not exist if the low-voltage
injector delivers
unmodulated beam with such current into the accelerating tube. In other words
the relativistic
injector of continuous unmodulated electron beam can be formed from mentioned
low-voltage
injector and accelerating tube. The beam with such currents can be accelerated
in the straight action
accelerator when the additional focusing solenoid is placed on the outer side
of the tank (80) to
focus the beam of the relativistic injector.
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The generator on the base of the offered invention provides up to 2-3 MW to
the load in
comparison with the most powerful diacrode TII-628 from Thales used in
Rhodotron's generator
that provides up to 1 MW. The efficiency of the said generator reaches 80% in
comparison to 56%
of TH-628.
Summary of the Invention
Accelerator-generator comprises at least one high-frequency cavity and a
relativistic electron
injector having an exit for transmitting the continuous unmodulated electron
beam from this injector
into this cavity for action upon all electrons of this beam to change their
energies. The cavity
provides accretion of energy in one part of electrons and reduction of energy
of other part of
electrons of this beam under going through of this cavity, providing
concentration of all initial
energy of electron beam in that part of electron beam that corresponds to the
accelerating phase of
field in the cavity and which next energizes the same or another cavity,
transforming the energy of
electron beam into the energy of the electromagnetic field in the cavity with
great effectiveness.
Brief Description of the Drawings
The invention contains following drawings:
FIG. 1- is copied Fig. 4 from patent WO-2008-138998
FIG. 2- demonstrates scheme of the first part of the offered device.
FIG. 3- demonstrates the dependence between the integral of the beam's kinetic
energy from the
increment of impulse of electrons in cavity after their accelerating or
decelerating.
FIG. 4- demonstrates the scheme of the second part of the offered device for
dividing the initial flow
into two parts.
FIG. 5- shows the scheme of the accelerator-generator as a powerful I-IF
generator.
FIG. 6- shows the scheme of the accelerator-generator as a powerful HF
generator or as a fIV
injector for the accelerator with several cavities.
Detailed Description
The continuous unmodulatcd flow of electrons accelerated previously up to the
intended initial
energy is acted on by the electric component of high frequency electromagnetic
field of the cavity
which is energized by the external generator. This generator compensates the
power losses in the
cavity's walls and supports the preset definite level of electromagnetic field
in the cavity when there
are different changes of the initial electron beam's current and the cavity's
load. Under this action the
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said field in the cavity accelerates the first part of beam electrons that
meet the accelerating phase of
the electromagnetic field in the cavity and decelerates the other part of beam
electrons that meet
decelerating phase of the electromagnetic field in the cavity. And then this
continuous beam with
electrons that have the different and periodic modulated energy is transported
to the next area where
the flow is acted by the force of transverse magnetic field which deflects
electrons of these parts of
the beam variously. The said flow after the crossing of this area filled up by
magnetic field becomes
divided up into two flows each of which already has the density of the
modulated current.
After that the first flow with increased kinetic energy of electrons, is
transported to the next area for
further using and the second flow with minimum kinetic energy of electrons, is
transported to the
dump for elimination.
The first part of the offered device is shown in Fig.2. It comprises the outer
and inner cylinders of
the accelerator's coaxial cavity - (1), a high frequency generator ¨(2) for
energizing the said cavity
through the loop ¨(3), and a relativistic injector (10) that comprises a DC HV
power supply - (7)
connected to the injector's cathode ¨(4), a DC power supply ¨(6) for varying
the current of the
injected unmodulated electron beam by means of the grid ¨ (5), an accelerating
tube ¨ (8) to
accelerate the electron beam from the cathode (4) and to transport this
relativistic unmodulated
continuous uniform beam of electrons ¨(9) into the coaxial cavity (1). All
these devices are placed
in a tank ¨ (11a) and the coil of the focusing solenoid ¨ (11) is positioned
outside of this tank.
This part of device works as follows. The injector's cathode (4) is connected
to the high-voltage
output of the power supply (7). The current of the injector is controlled by
the output voltage of
power supply (6) connected to the cathode (4) and the grid (5). After that the
continuous electron
beam from the injector goes through the accelerating tube (8) and electrons
acquire kinetic energy
that is equal to the voltage of the power supply (7), before entering the
coaxial cavity (1) that is
energized by the high-frequency generator (2).
All electrons in the beam while crossing the cavity acquire additional impulse
in accordance to the
phase of each electron when entering the cavity and in accordance to the
amplitude of the
electromagnetic field in the cavity. This increment of the impulse can be
calculated by the following
formula (1):
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A p = e0fEp(p, t, yO)dt = 2(e0/c)Asin (90+ RoVci(1/Q) cosQdQ (1)
Where:
c - velocity of the light and u ¨velocity of the electron
eo, mo ¨ charge, mass of the electron
p = moyu, = (1- pz) , p = uic
r, R ¨ radius inner and outer conductors of the coaxial cavity
= 2nf, f = resonant frequency, To ¨ input electron phase
Ep (p, t. (p0) = (1/p) Asin (ot + (p0) ¨ radial component of electric field in
the cavity.
As the total impulse of electrons after crossing the cavity is equal to the
sum po (initial impulse
at the entrance to the cavity) and Ap (the increment of impulse at the exit
from the cavity), the
integral of the total impulse along the phase 9 = 90+Rw/c from 0 up to 2n is
equal to 2np0.
The right part of the formula (1) was obtained under the assumption that all
electrons of the
beam have velocities which are approximately equal to the velocity of light.
It is met if Ap<<po and
the beam's kinetic energy matching po is equal to 0.7-1.0 Mev before the
entrance to the cavity. The
results of the precise simulation for the actual Rhodotron sizes (r and R)
give similar magnitude of
the impulse compared with the formula (1) even for Ap¨po. The shift of phase
changes a slightly for
the electrons which are decelerated by the electromagnetic field in the cavity
in comparison to
(Rifle). Hence this expression (])can he used for analysis of the behavior of
the electrons in the
beam after crossing the cavity.
All electrons disposed in each transverse section of the beam have the same
the input phase 90
before the cavity and also they have the same the output phase after the
cavity. The transverse
motion was not taken into account since all electrons had relativistic
velocities. In this case the
integration along the phase is the same as the integration along the length of
the beam. The kinetic
energy of the electrons corresponds to their impulses by means of the
relativistic invariant and hence
this energy can be calculated by means of the invariant using the total
impulse of electrons after
crossing the cavity, if Ap¨po. The integral of the beam's kinetic energy along
the length of the beam
for the interval 0 < 9 < 22 after the crossing of the cavity is not equal to
the same integral of the
kinetic energy when the beam has not yet crossed the accelerated cavity. The
calculation of this
integral can be done using numerical integrating. The character of dependence
of this integral from
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Ap is shown in Fig. 3 by the line ¨ a, which claims that the HF generator,
energizing the cavity,
supplies nonzero electromagnetic power during the process of the crossing of
the cavity by the
continuous electron beam and increases the total kinetic energy of the beam,
if Ap approaches to po.
Furthermore the part of the beam that is decelerated (for TE <(p <21t) returns
back its energy to
the electromagnetic field and hence the remaining part of the beam (for 0 <p
it) it) accumulates the
power from the HF generator and from the first part of the beam too. The parts
of the said integral of
the beam's kinetic energy for the intervals 0 < < it and it <p < In are shown
in Fig.3 by the line ¨
b and by the line ¨ c accordingly. The energy from the HF generator is given
by the line - d. If
Ap<po, the similar results (lines a, b, c and d) can be reached using the
simulation of the electrons
dynamics when the difference of the velocity of electrons for the phases it <
< 2rc from the
velocity of the relativistic electrons is taken into account. And these
results will not differ from lines
in Fig. 3 more than on 5-10%. However, it is not important because this part
of the electron beam
for it < < 27c is not used in the subsequent processes. The part of beam for 0
< < it has just
accrued already the all that might be possible. The results of these integrals
are given in Fig.3 in
relative forms, assuming WO is equal to the integral of the kinetic energy of
the initial beam for the
interval 0 < < 231 before crossing of the cavity.
FIG.3 demonstrates that line b exceeds the total kinetic energy of the initial
beam if
0.95<Ap/po<1 though decelerated electrons (line c) still keep positive
velocities when they leave the
cavity. It means that all decelerated electrons can be removed of the beam
without significant losses
for deal. This function can be achieved by the device that works similarly to
a magnetic mass-
spectrometer and is shown in FIG.4. This device comprises: vacuum chamber (15)
joined to the first
exit hole and the next input hole of the coaxial cavity, two deflecting
magnets (12a, 12b, 14) and the
dump-box (13) for the waste electrons. All these elements work as follows:
accelerated and
decelerated electrons of the beam that crossed the coaxial cavity for the
first time are transported
through the exit hole of the cavity into the vacuum chamber (15) and go
through the first deflecting
magnet (12 a, 12 b). The magnetic field of this magnet is chosen in such way
that it takes out the
decelerated electrons of the beam and transports them into the dump-box. It is
possible if the core of
the magnet consists of two parts (12a, 12b) placed next to each other. The
first part of the core (12a)
has a form close to a rectangle and its width is equal to the maximum radius
of the electrons orbit
(16) that leads decelerated electrons to dump-box. For instance, this radius
can correspond to the
kinetic energy of the initial beam before the cavity so all electrons after
crossing the cavity that have
the phase TC <
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<27t will be diverted from the beam to the dump-box (13). Because the
remaining electrons in the
beam that have the phase 0 < < it have the energy higher than the energy of
the initial beam, they
will go through the first part of the core of magnet (12a) and then will go
further along the orbits
(17) through the second part of the core of magnet (12b) that has a form close
to an asymmetric
trapeze.
These electrons have a big dispersion at the exit from the first deflecting
magnet and the second
deflecting magnet (14) compensates this dispersion, directing these electrons
along the definite line
before the next entry of the electrons into the cavity. As mentioned above, if
0.95<dp/p0<1, these
electrons have the total kinetic energy that is more than the initial beam
before the first crossing of
the cavity and they have the structure of the periodically modulated electron
beam because all
electrons with phases between it and 271 have been disposed into the dump-box.
After that the
modulated electron beam is transported through the cavity for the second time.
If electrons with
phases 0 < 9 < TE were accelerated by the electromagnetic field during their
first pass through the
cavity, they have to return their kinetic energy into the energy of the field
during the second pass.
This condition can be met, if the length of electrons trajectory from the
first input hole of the cavity
to the second input hole of the cavity is equal to odd number of halves of
wavelength of the
electromagnetic field in the cavity. Since the level of the electromagnetic
field in the cavity is the
same as in the first crossing of the cavity by these electrons and the phase
of the field is shifted
by1800, these electrons of the beam will lose the part of their energy and
will have their initial
energy at the injection moment. As differences in the lengths of trajectories
of electrons with
different phases are small in the vacuum chamber (15) percentagcwise to the
wave-length, they have
negligible effects on the decelerating that can be counted during the next
crossing of the cavity by
these electrons. When the electrons complete the second pass (18) through the
cavity (1) they are
transported through the vacuum chamber (19) as shown in FIG. 5.
As all electrons of the beam here have the same impulse of motion that is
equal to the initial impulse
at the injection moment, they all move through the deflecting magnet (20)
along the same trajectory
(21). After that the beam is transported into the cavity the third time. By
choosing the magnetic field
magnitude in magnet (20) under the condition that the total time of electrons
travelling from the
second input hole of the cavity to the third input hole of the cavity is equal
to an whole number of
the periods of the high-frequency field in the cavity the second act of the
decrease of the kinetic
energy of electrons can be met and hence all electrons will be decelerated one
more time along the
trajectory (22).This portion of electrons precisely corresponds to the part of
the initial beam that was
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decelerated during the first crossing of the cavity by the beam because this
part has the same
impulse p=p0 before the last crossing of the cavity and has the same magnitude
of integral of the
kinetic energy in interval (x, 2n). Therefore the beam is also transported
into the second dump-box
(23) for eliminating. This means that the sum of the losses of the beam's
energy corresponds to the
double magnitude that is shown by the line ¨ c in FIG.3 and it also means that
the transforming
efficiency of the kinetic energy of the initial beam into the energy of the
electromagnetic field is
very high and achieves 75-80% when 0.95<Ap/po<1.
The coaxial cavity of the Rhodotron is not unique type of the cavity that
provides reusability of the
cavity for beam accelerating. There are several other types of cavities
designed for the same
purpose, for instance, like cavities for FANTRON or RIDGETRON that were
described in patent
CA 1306075 or patent US 5107221 and patent US 5376893.
In more common events of several cavities, connected in series, or a single
cavity of any type that
are crossed by the beam (9) only once can be used as the above cavity (1) for
the purpose of
periodical modulation of the electrons' energy in the beam. Also the exit of
these cavities can be
connected to the entry of the magnetic inverter directly or through other
devices (such as focusing
quadrupole magnets etc.) and the exit of magnetic inverter can be connected to
another cavity,
different from the cavity (1), where modulated electron beam will be
accelerated or decelerated as
shown in the Fig. 6.
In the Fig 6 items from (1) up to (18) and (23) have the same meaning as in
Fig.5, but items from
(19) up to (22) arc absent. The item (24) designates the second cavity of the
generator for converting
kinetic energy of the modulated electron beam into the energy of
electromagnetic field which goes
to the load (26) through an exit loop (25). This device functions similarly to
a klystron hut where the
drift tube is changed to the magnetic inverter. The cavities (1) and (24) can
be independent or
dependent. In latter case the feedback will decrease the requirement for power
of generator (2), but
in both cases the requirements for the time synchronization of the electrons
exiting from cavity (1)
and travelling to the cavity (24) are weaker or are absent completely if the
device works as a
generator. The cavities (1) and (24) may comprise sequences of several
cavities connected in series
and, if the device is used as high-voltage injector and accelerator, the item
(26) means the second
external generator for the cavity (24). Requirements of the time
synchronization must be kept in
accordance to the synchronization between external generators (2) and (26) in
this case.
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The invention is characterized by two main features. Firstly, the cavity (1)
provides the increase of
the energy for a portion of the electrons in the beam (9) that goes through
the said cavity during an
acceleration phase of the electromagnetic field in said cavity, and also
provides the decrease of the
energy for the rest of electrons in the beam (9) that goes through the said
cavity during the
deceleration phase of the electromagnetic field in said cavity, and has an
electron beam exit
connected to the magnetic inverter.
Secondly, this magnetic inverter has a volumetric area, filled by the magnetic
field dividing the
said beam (9) at least in two flows, separating low-energy electrons from high-
energy electrons of
the said beam (9) under the transportation of the said beam (9) through said
area. The first flow,
containing only the part with low-energy electrons of the said beam (9), will
be dispatched further to
a dump (13) located inside or outside of the said inverter, and, at the same
time, the accelerated part
of the said beam (9) will be converted into the periodically modulated
electron beam (18).
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