CA2787794C - Multirhodotron - Google Patents

Abstract

The invention increases the upper limit of the electron energy or the full electron current for the electron accelerator of Rhodotron type by means of the coaxial cavity of the special new form with the new shape of the trajectory of the electron beam that passes through the cavity multiple times for acceleration.

Description

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.

2 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,

3 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.

4 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 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 (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 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 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).

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 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.

Claims (3)

We claim:

1. An electron accelerator that is using a coaxial cavity for multiple accelerations of an electron beam being injected into the accelerator lengthways of different diameters of the coaxial cavity, and is comprising a plurality of deflection devices redirecting the electron beam from one diameter to another diameter after the crossing the cavity, and said accelerator is characterized by event where different cavity diameters, lengthways of which electrons of said beam are obtaining acceleration, are disposed in two or more cavity planes being disposed perpendicularly to an axis of said cavity in axis points, where a radial component of the resonant electric field in said planes has maximum, and said diameters are connected by said deflection devices into a common unified consequent chain for providing acceleration of beam electrons along each out of said diameters in said chain.

2. Accelerator, according to claim 1, is characterized by event in which the common unified consequent chain intended for accelerating one electron beam is reformed into two or more independent subchains intended for accelerating two or more electron beams each of which have own electron sources, providing acceleration of said electron beams in result in one accelerator.

3. Accelerator according to claim 1 or 2 characterized by event in which an inner conductor of the coaxial cavity for multiple accelerations of the electron beam is formed by a sequence of been alternating cylindrical cuts of pipes, which have two different diameters, and said 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) and at the same time pipes midpoints with minimum diameter are placed in the cavity axis's points where the resonant radial component of electric field in said cavity has maximum and pipes midpoints with maximum diameter are placed in the cavity axis's points where said resonant radial component of electric field in said cavity is equal to zero.