Synchrotron apparatus

A synchrotron apparatus comprising an RF accelerating cavity, a pair of bending magnets, a pair of focusing magnets, and a pair of defocusing magnets, respectively, for accelerating, bending, focusing, and defocusing the particle beam to accelerate and/or store the particle beam, and a beam-cooling high-frequency accelerating cavity for generating a high-frequency electromagnetic field of an even TM-mode number in relation to direction transverse to the particle beam to decrease energy dispersion of the particle beam. Further, the RF accelerating cavity in the synchrotron apparatus provides a fundamental-mode exciting unit for exciting a fundamental-mode electromagnetic field, a detector for detecting phase and strength of respective higher-mode electromagnetic fields other than the fundamental-mode electromagnetic field, and exciting means for exciting a higher-mode electromagnetic field which is in antiphase with and has the same strength as the detected higher-mode electromagnetic field in accordance with the result of the detection by the detector so as to weaken the strength of the detected higher-mode electromagnetic field.

BACKGROUND OF THE INVENTION 
The present invention relates to a synchrotron apparatus for accelerating 
or storing particle beams. 
The basic arrangement of a conventional synchrotron apparatus which has 
been disclosed, for example, in "Superconducting Racetrack Electron 
Storage Ring and Coexistent Injector Microtron for Synchrotron Radiation" 
of TECHNICAL REPORT of ISSP Ser. B No. 21, September 1984 by Yoshikazu 
Miyahara et al. is shown in FIG. 1. This synchrotron apparatus is composed 
of a loop-shaped vacuum chamber 7 through which a beam of charged 
particles passes, an RF accelerating cavity 2 for accelerating the 
electron beam, a pair of focusing magnets 3a for focusing the electron 
beam, a pair of defocusing magnets 3b for defocusing the electron beam, 
and a pair of bending magnets 4 for bending the electron beam. These 
components together form an electron storage ring. The electron beam 
accelerates along a balanced orbit 1 which is a closed orbit determined by 
the energy of the electron beam and the magnetic field intensities of the 
focusing magnets 3a, the defocusing magnets 3b, and the bending magnets 4. 
In the electron storage ring indicated by the balanced orbit 1, energy 
loss, which occurs from generation of synchrotron radiation at the moment 
the electrons are being bent, is replenished by the RF accelerating cavity 
2 to continuously store electrons having a certain energy level. However, 
the energy levels of each electron disperse in an energy band having a 
certain width (called energy dispersion hereafter). How this energy 
dispersion is determined will be explained below. 
The above energy dispersion can be thought of by converting the time of 
arrival of the electrons at RF accelerating cavity 2 to a phase of RF 
voltage. The phase at which radiation energy or the energy loss per one 
round of the electron is equivalent to an RF voltage or an acceleration of 
the electrons resulting from replenishment by the RF accelerating cavity 
2, is represented by .phi..sub.0. If the energy of an electron is higher 
than a standard level for some reason, the electron circles around an 
orbit outside of the balanced orbit 1. In this case, when the electron 
arrives at the RF accelerating cavity 2, it is in a slight phase lag 
condition, that is the phase angle is delayed more or less in regard to 
the phase .phi..sub.0, so that the acceleration voltage becomes less than 
the energy loss from radiation. Accordingly, the energy of the electron 
gradually decreases every circulation. On the other hand, in case of an 
electron having less energy than the standard level, the inverse 
phenomenon occurs, whereby the energy of the electron is increased. 
Therefore in relation to the high-frequency phase, the electrons oscillate 
(synchrotron oscillation) around the standard phase .phi..sub.0. 
Practically, however, since the radiation energy of the particle per 
circuit is in proportion to the square of the energy of the particle, a 
kind of damping is added to the above oscillation (synchrotron damping). 
Accordingly, the energy dispersion of the electrons in the ring is 
determined by the balance between the energy fluctuation of each electron 
from the synchrotron radiation and the synchrotron damping. As a result, 
the energy dispersion is in inverse proportion to the square root of the 
radius of curvature of the bending magnets 4. 
As noted, in the synchrotron apparatus it is often necessary to make the 
energy dispersion as small (narrow) as possible. If the energy dispersion 
is large (wide) due to a small square root of the radius of curvature of 
the bending magnet 4 according to the prior art arrangement, the electron 
beam orbit expands to bring a diminution (decrease) in particle density 
because the beam path broadens, the beam cross section increases, and the 
beam length lengthens. Accordingly, this brings a decrease in collision 
frequency between particles in particle beam collision experiments. In 
order to overcome this drawback, it is necessary to store a very large 
current, and problems such as instability of the particle beam occurs. 
Further if the beam diameter increases, it is necessary to enlarge vacuum 
vessels through which the beam passes and to expand the effective magnetic 
field areas, causing increases in size of the total apparatus and creating 
problems in relation to cost and area used by the apparatus. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention to overcome the 
above problems by providing a synchrotron apparatus in which the energy 
dispersion of the particle beam can be very small. 
In the synchrotron apparatus of the present invention, a high-frequency 
accelerating cavity for beam cooling which forms a high-frequency 
electromagnetic field of an even transverse magnetic (TM) mode number in 
relation to the transverse direction of the particle beam, is disposed on 
the closed orbit of the particle beam within the electron storage ring 
thereby enlarging the nearby dispersion function .eta. of the storage 
ring. The beam-cooling high-frequency accelerating cavity is rectangular 
having a longer side in a direction transverse of the particle beam. 
In the synchrotron apparatus of the present invention, the beam-cooling 
high-frequency accelerating cavity decelerates high-energy electrons and 
accelerates low-energy electrons by exciting (generating) a high-frequency 
transverse electromagnetic (TEM) field of an even mode number to make it 
possible to reduce the energy dispersion. In the electron storage ring of 
the prior art, beam energy dispersion was determined by the balance 
between the synchrotron radiation damping and the synchrotron radiation 
excitation, and beam energy dispersion was about 0.1%. Here, the present 
invention, by noticing the characteristic that a beam orbit shifts 
slightly from the central orbit (the balanced orbit 1,) depending on the 
differences of beam energy thereof, it is arranged so that a decelerating 
action is imported to high-energy particles and an accelerating action to 
the low-energy particles, by means of passing the electron beam through a 
standing wave of the even TM mode number, e.g. TM.sub.210 -mode, where 2 
is the number of half-period variations of the magnetic field along the 
longer transverse dimension, 1 is the number of half-period variations of 
the magnetic field along the shorter transverse dimension, and 0 is the 
number of half-period field variations along the axis. The beam cooling 
high-frequency acclerating cavity of the present invention is effective 
even in very high-frequency (1 GHz) applications and accordingly the whole 
apparatus can be reduced in size, and it also becomes possible to decrease 
the beam energy dispersion to about 0.01%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 2, the synchrotron apparatus according to a preferred 
embodiment of the present invention comprises a loop-shaped vacuum chamber 
7 through which a beam of charged particles 1 passes, an RF accelerating 
cavity 2, a pair of focusing magnets 3a, a pair of defocusing magnets 3b 
and a pair of bending magnets 4 for accelerating, focusing, defocusing and 
bending the beam, respectively, and further comprises a beam-cooling 
high-frequency accelerating cavity 5. Further, FIG. 3 is a sectional view 
of the beam-cooling high-frequency accelerating cavity 5 in the vertical 
direction of the beam, wherein the direction of progression of the beam is 
indicated at 1a, and directions of the high-frequency electric field of 
the TM.sub.210 -mode which is generated by the beam-cooling high-frequency 
accelerating cavity 5 are indicated at 6a and 6b, where 6a is at the inner 
side of the storage ring. Exciting antennas for exciting the TM-mode of 
the electromagnetic field of the RF accelerating cavity 2 and the 
beam-cooling high-frequency accelerating cavity 5 have been shown in the 
report, "RF System for Slac Storage Ring" on P.253-254 of IEEE Trans. 
NS-18, published in 1971. 
Now the TM-mode and transverse electric mode will be briefly explained by 
considering a X, Y, Z three-dimensional space where the Z axis direction 
is defined as the progressive direction of the electromagnetic wave. In 
the present case, the progressive direction of the electromagnetic wave 
and the progressive direction of the electron beam are in the same 
direction. When E indicates the electric field and H indicates the 
magnetic field, Ez.noteq.0, Hz=0 in a TM-mode (e.g., an electromagnetic 
wave of which a Z axis direction component Hz of the magnetic field is 
zero), and in a TE-mode, Ex=0, Hz.noteq.0, (e.g., an electromagnetic wave 
of which a Z axis direction component Ez of the electrical field is zero). 
Regarding the TM-mode, the equation for Ez is: 
##EQU1## 
Now a TM-mode wave in a rectangular solid as shown in FIG. 4 will be 
considered. The boundary conditions of the Z axis direction component Ez 
of the electrical field are as follows: 
##EQU2## 
accordingly, by expanding HQ. (1), the Z axis direction component Ez is 
shown as follows: 
##EQU3## 
In EQ. (2), l, m, n are called mode numbers and an electromagnetic wave 
generated in the pertinent mode is called TM l, m, n-mode. FIGS. 5A, 5B, 
5C and 5D show conditions of component Ez in the Z axis direction of the 
TM.sub.110 -mode, the TM.sub.210 -mode of the preferred embodiment, the 
TM.sub.120 -mode and the TM.sub.111 -mode, respectively. The mode numbers 
l, m, n are related to the X, Y, Z directions, respectively, and they 
indicate the number of maximum and minimum value points (peaks) of 
strength of the electromagnetic wave between "0" and "a", "0" and "b", "0" 
and "c" respectively. Accordingly, a TM.sub.310 -mode, for example, would 
have an Ez component with three peaks between "0" and "a" in relation to 
the X direction and one peak between "0" and "b" in relation to the Y 
direction. 
Now the action of the electrons in the storage ring will be considered. In 
the beam-cooling high-frequency accelerating cavity 5, the high-energy 
electrons (particles) pass through on orbits outside of the central orbit 
1, and the low-energy electrons pass through on orbits inside of central 
orbit 1. The beam-cooling high-frequency accelerating cavity 5 excites the 
TM.sub.210 -mode which is shown in FIG. 5B. This TM.sub.210 -mode has a 
phase relationship with the decelerated particles (electrons) passing on 
the orbits outside of the central orbit 1 and the accelerated particles 
passing on the orbits inside of the central orbit 1 as shown in FIG. 3. 
Accordingly, since the high-energy particles are decelerated and the 
low-energy particles are accelerated, respectively by the RF voltage, the 
energy dispersion of the particles is therefore reduced to a lower level, 
making it possible to reduce the synchrotron apparatus in size. For 
instance, in the TM.sub.210 -mode, when the maximum electrical field 
strength is set up at 1 KV/cm, the energy dispersion becomes one-tenth of 
previous synchrotrons. 
Further, though the above embodiment is for employment in an electron 
storage ring, the present invention is however not limited, however, to 
the above embodiment. The present invention is also practical for a free 
electron laser and an ion storage ring respectively, whereby the same 
effects may also be achieved. Furthermore, though the TM.sub.210 -mode was 
used in the above embodiment, the same result also can be obtained in the 
cases of using a TM.sub.410 -mode and TM.sub.610 -mode respectively. 
Further, FIG. 6 illustrates a horizontal sectional view of a conventional 
RF accelerating cavity 2, which is shown in Proceedings of the First 
Course of the International School of Particle Accelerators of the "Ettore 
Majorana" Center for Scientific Culture, Erice 10-22 November 1976 (CERN 
77-13, 19 July 1977). In FIG. 6, the particle beam which is illustrated 
as the central orbit 1 passes through the center portion of the RF 
accelerating cavity 200. The RF accelerating cavity 200 provides a 
TM.sub.110 -mode absorbing antenna 201. In the RF accelerating cavity 200, 
the TM.sub.110 -mode occurs from the passage of the particle beam 1 in 
addition to the fundamental-mode. 
This TM.sub.110 -mode is different from the above TM.sub.110 -mode shown in 
FIG. 5A because of the method of expression. That is, this TM.sub.110 
-mode is expressed based on a cylindrical coordinate. When the TM .theta., 
r, z-mode is used, the respective mode numbers .theta., r, z relate to the 
circumferential direction in the vertical-plane of the cylindrical 
coordinate (in the transverse-plane of the beam direction), the radial 
direction of the cylindrical coordinate, and the longitudinal direction of 
the cylindrical coordinate (the beam direction), respectively. They also 
indicate the respective numbers of peaks of the electromagnetic wave 
strength in the corresponding directions. 
The directions of the electric-field vectors and the magnetic-field vectors 
of the TM.sub.110 -mode are indicated by numerals 20 and 21, respectively, 
in FIG. 6. In this moment, the particles receive a deflecting force (orbit 
deflection) in the X axis direction by the interaction against the 
magnetic field 21. In the case of a ring-form synchrotron apparatus, since 
the deflecting force in the X axis direction is imparted to the particles 
at every circulation thereof, the particles soon pass on orbits which are 
quite apart from the central orbit 1, strike against the inside wall 
surface, and then disappear. 
To avoid the deflection of the particles, the TM.sub.110 -mode absorbing 
antenna 201 has been used in the prior art. The TM.sub.110 -mode absorbing 
antenna 201 has characteristics for weakening the TM.sub.110 -mode by 
converting the electromagnetic energy of the TM.sub.110 -mode into heat to 
stabilize the beam. The theory thereof being that, since the absorbing 
antenna 201 is disposed so that one part of the magnetic field of the 
TM.sub.110 -mode passes therethrough, the alteration of the magnetic field 
in this state produces an eddy current in the absorbing antenna 201 to 
produce heat by reason of the impedance of the absorbing antenna 201. That 
is the energy of the TM.sub.110 -mode is converted to heat. 
However, in the RF accelerating cavity 200 of the prior art, for 
controlling the TM.sub.110 -mode, the absorbing antenna 201 must be 
inserted fairly deep in the cavity 200. Consequently there is a problem in 
that the absorbing antenna 201 influences the fundamental-mode. 
A RF accelerating cavity of the present invention comprises a detecting 
means for detecting the phase and strength of higher-mode electromagnetic 
fields other than the fundamental-mode electromagnetic fields by detecting 
an electromagnetic field in the RF accelerating cavity, and excitation 
means for exciting a higher-mode electromagnetic field in the accelerating 
cavity, having an antiphase and the same strength in relation to the 
detected higher-mode electromagnetic field, in accordance with the result 
of the detection by the detecting means, whereby the strength of the above 
higher-mode electromagnetic field is weakened. 
FIG. 7 illustrates a detailed constructional view of a preferred embodiment 
of the RF accelerating cavity in the present invention, as indicated by 
the numeral 2 in FIG. 2. In FIG. 7, the particle beam which is illustrated 
as the central orbit 1 passes through the center portion of the RF 
accelerating cavity 210. The RF accelerating cavity 210 comprises a 
fundamental-mode exciting antenna 211 for accelerating the beam (the RF 
accelerating cavity 200 in the prior art also being provided therewith but 
omitted in FIG. 6), an antiphase TM.sub.110 -mode exciting antenna 212 and 
search coil 213 for the high-frequency wave. A filter 214 for cutting the 
fundamental-mode electromagnetic fields, a phase detector 215 and a 
strength detector 216 are serially connected to the search coil 213. On 
the other hand, a generator 217 for exciting the antiphase TM.sub.110 
-mode, an attenuator 218, a phase shifter 219 and a terminating resistor 
circuit 220 having an infinite impedance against the fundamental wave are 
connected serially, and the terminating resistor circuit 220 is further 
connected to the antiphase TM.sub.110 -mode exciting antenna 212. Further 
the phase detector 215 is connected to the phase shifter 219, and the 
strength detector 216 is connected to the attenuator 218. Also, a 
generating means (not shown) for driving the fundamental-mode exciting 
antenna 211 is connected thereto. 
In operation, first, the TM.sub.110 -mode which is provided by reason of 
the beam current is sensed by the search coil 213, and the 
fundamental-mode component in a detected signal is cut off by the filter 
214, and the phase and strength of the detected signal are further 
detected by the phase detector 215 and the strength detector 216, 
respectively. The phase shifter 219 regulates the phase of output of the 
generator 217 based on output of the phase detector 215 so that the 
exciting antenna 212 excites an electromagnetic wave having an antiphase 
to that of the TM.sub.110 -mode resulting from the beam current. Further 
the attenuator 218 regulates the output strength of the generator 217 so 
that it equals the output strength of the strength detector 216. 
Accordingly, since the antiphase TM.sub.110 -mode exciting antenna 212 
excites an electromagnetic field having the same strength and in antiphase 
to the TM.sub.110 -mode in the cavity 210, it therefore becomes possible 
to eliminate the TM.sub.110 -mode in the cavity 210 positively, whereby 
the particle beam can be stabilized without affecting the 
fundamental-mode. Furthermore since the terminating resistor circuit 220 
having infinite impedance against the fundamental wave is connected 
between the antiphase TM.sub.110 -mode exciting antenna 212 and the phase 
shifter 219, the generator 217 is not affected by the fundamental wave 
mode. 
Further, although in the above preferred embodiment, the generator 217 for 
exciting the antiphase TM.sub.110 -mode and the generator (not shown) for 
exciting the fundamental-mode are provided separately, one generator may 
also be used for exciting the fundamental mode and the antiphase 
TM.sub.110 -mode in the present invention. In this case, the antiphase 
TM.sub.110 -mode is generated by way of modulating the fundamental mode. 
Also, even though the above preferred embodiment was explained for a case 
in which the higher-mode other than the fundamental mode was the 
TM.sub.110 -mode, the present invention can also be practiced in cases 
where the higher mode is some other mode, with the same resulting effects. 
Further it is possible to provide a plurality of the above systems to 
stabilize a plurality of modes. 
Moreover, in the above preferred embodiment, a terminating resistor circuit 
having infinite impedance against the fundamental mode is inserted in 
series for keeping out the connection between the power supplies, however, 
it is possible to use a directional coupler or a circulator in place of 
the terminating resistor circuit.