The accelerator tube is the key component of the accelerator. It accelerates the electrons injected from the electron gun to high energy under the action of the microwave electric field, and finally hits the target to produce high energy X-ray.
Chinese name
Accelerator tube
Foreign name
accelerating tube
Meaning
Key components of accelerator
Classification
Traveling wave accelerating tube and standing wave acceleration
Run
From electron acceleration injected by electron gun to high energy
Accelerator tube isHigh pressure acceleratorKey component of, modern largeElectrostatic acceleratorThe increase of terminal voltage is mainly limited by the voltage withstand level of the accelerator tubeion sourceorelectron gunAfter being launched, it enters the accelerator tube for acceleration.The accelerator tube shall meet the following main requirements:
1. Maintain a good vacuum to reduce the scattering between gas molecules and accelerating particles, and improve the pressure resistance of the accelerating tube.The general requirement is 10sixMmHgWorking under left and right vacuum.
2. It has sufficient mechanical strength. In the horizontal electrostatic accelerator, there are relatively high requirements in this respect.
3. Have a good focusing effect on accelerating particles to obtain a well focusedParticle Flow, reduce the loss of particles in the acceleration process, and improve the acceleration efficiency.It is also good for improving the pressure resistance of the accelerator tube.
4. It has good withstand voltage performance. The potential difference at both ends of the accelerator tube is equal to the highest voltage of the electrostatic booster, so it must be able to work stably at the highest voltage to accelerate particles to higher energy.
Basic structure and optical characteristics
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Similar to the insulating pillar, in order to improve the uniformity of the electric field along the axial direction, the whole accelerating tube is formed by overlapping and sealing sections of insulating rings and metal sheets.These pieces of metal are called acceleration electrodes and are connected to the voltage divider on the insulating pillar.In large accelerators, the accelerator tube is generally connected with an independent partial pressure system.
Accelerators operating at atmospheric pressure, such as most pressure doubling accelerators, usually have long accelerator tubes and low average potential gradients.This kind of accelerating tube is often divided into fewer sections, each of which is ten to dozens of centimeters long.The accelerating electrode is long cylindrical, and the electrode length is much larger than the electrode gap, as shown in Figure 1 below. In this accelerating tube, the electric field is concentrated near the gap, there is almost no electric field inside the cylindrical electrode, and particles drift at a constant speed when passing through. Therefore, this accelerating tube is called the accelerating tube with drift tube. Two adjacent cylindrical electrodes form a double cylindrical electrostatic lens,It has a certain focusing effect on the beam.Of this type of accelerator tubePotential gradientGenerally not more than 1 MV/m.
Figure 1 Several Basic Structures of Accelerator Tube
Since the mid-1960s, the atmospheric pressure doubler accelerator has started to develop towards the direction of strong current. At this time, the nonlinear space charge effect becomes very strong, especially at low energy, which worsens the optical properties of the beam and brings difficulties to the beam transmission.The solution is to accelerate the particles to a higher energy as soon as possible in a shorter distance.For this reason, the atmospheric type high gradient accelerating tube has been developed. In this accelerating tube, the potential gradient of the tube wall is still 0.4~0.7 MV/m due to the use of the extended electrode structure, but the acceleration gradient between the accelerating electrodes can be as high as 3~5 MV/m..
In the high pressure accelerator, it is very significant to increase the average potential gradient of the accelerator tube for reducing the size of the steel barrel and reducing the cost.In order to further improve the uniformity of the electric field distribution, the sections of the high gradient accelerating tube are very thin, generally no more than 25 mm.The specific shape of the acceleration electrode has many designs, and in recent years, it is commonly used as a flat electrode.As long as the ratio of the beam aperture to the electrode spacing is large enough, an ideal shielding can be obtained between the particle beam and the insulation wall.At this time, the distribution of the accelerating electric field along the axis of the accelerating tube is basically uniform, so this accelerating tube is called a uniform field accelerating tube or an equal gradient accelerating tube.When charged particles enter the uniform field, they are only accelerated by the axial electric field, and there is no radial directionElectric field force, so there is no focus.However, there is a sudden change in the field strength at the entrance and exit of the accelerator tube, which is equivalent to two diaphragm lenses.If the length of the accelerating tube is l, the normalized potential of the accelerated particle at the population is V1, and the normalized potential at the outlet is V2, then the field strength in the uniform field area is:
E= (Vtwo-Vone)/l≈Vtwo/l
Focal length of inlet diaphragm lens
fone=4ζVone/E≈4ζl/ntwo
Where ζ is the aperturecorrection factorIn general, 1<ζ<2.5, n2=Vtwo/VoneIs the normalized potential ratio of the accelerator tube.Focal length of outlet diaphragm lens
ftwo= 4Vtwo/(-E)≈-4l
It can be seen that the entrance diaphragm lens has a strong focusing effect, while the exit diaphragm lens has only a weak defocusing effect.The whole uniform field accelerator tube can be regarded as a combination system of inlet diaphragm lens, uniform field zone and outlet diaphragm lens, of which the inlet diaphragm lens plays the most important role, making the whole accelerator tube focus.However, if the entrance lens is too strong, it will also produce a phenomenon of convergence, which will make the image waist of the beam fall in the accelerator tube, and become a divergent beam outside the exit.[1]
Several kinds of high gradient accelerating tubes
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To reduceElectronic loadTo overcome the full voltage effect, effective measures must be taken to weaken thecouplingSo that the micro particles and the ions causing the micro discharge can not fly over in a long distance. There are two common ways to weaken the coupling effect of the long tube: one is to use a small aperture, and the other is to introduce a suppression field.Small aperture can limit the movement range of secondary particles and reduce the impact energy of micro particle events.The most widely used inclined field accelerator tube, spiral inclined field accelerator tube andNECThe acceleration tubes of the company use radial electric field as the suppression field, and the effect is very good.
Inclined field accelerator tube
In the inclined field accelerating tube, the normal of the accelerating electrode forms a certain angle with the axis of the accelerating tube.Because the initial energy of the secondary particles generated on the electrode surface is very low, they will move along the normal direction of the electrode under the action of the accelerating electric field, so they will hit other electrodes after a short distance.In this way, the maximum energy of secondary particles is limited,X-rayThe background is greatly reduced. As a result, the coupling between segments is weakened, and the electronic load is greatly reducedFull voltage effect。The main disadvantage of the inclined field accelerator tube is that the inclined electric field also has an effect on the accelerated ion beam.The radial component of the electric field also makes the beam deviate from the axis.Therefore, in this kind of accelerator tube, the tilt direction is changed alternately every other short segment, and as a result, the trajectory of the accelerated particles will have a small oscillation around the axis of the accelerator tube.The trajectory of different charged ions will also produce segregation.If the design is reasonable, it will not cause large beam loss, but will cause aberration, increase beam divergence, and increase the pulse width of the pulse beam.The slope field acceleration tube of HVEC uses stainless steel as electrode material and insulation ringBorosilicate glassPVA glue is used for sealing, and the upper limit of the actual operating gradient is about 2MV/m.
Helical inclined field accelerator
Helical inclined field accelerator
In the spiral inclined field accelerator tube, the shape of the electrode is similar to that of the ordinary inclined field accelerator tube, but the projection direction of the normal lines of the adjacent electrodes in the cross section is staggered by a certain angle and arranged in a spiral shape, which makes the direction of the radial component of the electric field continuously change, rather than abrupt changes in segments as in the ordinary inclined field accelerator tube.Therefore, the oscillation amplitude of the accelerated particle trajectory in the spiral field is small, and the maximum energy of the secondary electron is also lower than that in the ordinary oblique field.If the helical oblique field rotates in only one direction (such as clockwise direction), the accelerated particles will have radial displacement and off axis at the projection position of its cross section after passing through a section of accelerator tube.Therefore, in the actual helical inclined field accelerator tube, the rotation direction changes alternately clockwise and counterclockwise.
Cermet accelerating tube
This is a kind of accelerator tube produced by NEC Company, which has many unique features.In material, it uses titanium as electrode and ceramic as insulation ring.Aluminum ring is padded between titanium electrode and ceramic and sealed by pressure diffusion welding. Because PVA glue is not used, it can not only avoid organic contamination, but also can be baked at high temperature for degassing.With oil-free vacuum system, hydrocarbon contamination can be completely eliminated.The vacuum degree of accelerator tube can reach 4 × 10-6Pa. The NEC acceleration tube is equipped with a "dead zone" at the connection of each small section, and a small hole that can be heated in the middleDiaphragm。The electrodes in the dead zone are arranged to form a cylindrical lens, and the radial component of the electric field has defocus effect on the secondary particles generated from the diaphragm, but has no effect on the particles moving along the axis.The electric field of NEC accelerator tube is completely axisymmetric, and there is no side effect such as oblique field for ion beam transmission. Annular spark gap is installed outside the accelerator tube to provide overvoltage protection.Generally, three sections form a standard section, and the working voltage is 1MV.If the dead zone is deducted, the working gradient of the accelerator tube is 2.3 MV/m based on the effective length.
In addition, some people use the transverse magnetic field as the suppression field.The weak transverse magnetic field can effectively deflect electrons, but cannot change the threshold voltage of micro discharge, so it is usually only used as an auxiliary means.There is a magnetic suppression acceleration tube that uses a strong transverse magnetic field to deflect the lower energy secondary particles, thereby weakening the long tubecoupling effect To avoid the deflection of the main beam by the magnetic field, the polarity of the magnets placed at a certain distance in the accelerator tube should be changed alternately.[2]
Exercise of acceleration tube
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The purpose of accelerating tube training is to reduce the pre breakdown current as far as possible and eliminate the source of initial microparticle events.The intensity of exercise should be properly controlled. Excessive exercise and large sparking can lead to the decline of the pressure resistance of the accelerator tube, which is called retreat exercise.The training methods of acceleration tube mainly include the following
Electric current exercise
Current training is most effective for new electrodes. This method slowly increases the voltage to control the pre breakdown current.The pre breakdown current can come from field emission, micro discharge or micro particle events.With the current training, the continuous pre breakdown current gradually decreases, and the frequency of random current spike pulse also gradually decreases.This means that the micro protrusions, loosely attached micro particles and adsorbed gas on the electrode surface have been removed.Because this process is carried out at a lower voltage, it is not enough to causebreakdown。The goal of current training is usually to make the applied voltage finally reach 125% of the planned operating voltage.
Sparking exercise
The sparking exercise is usually used to break through the accelerator tube of backward exercise, and sometimes it is also used for the new accelerator tube. This method achieves the purpose of destroying local hidden dangers through repeated sparking.
Arc discharge exercise
In the early 1980s, Isoya and others developed the arc discharge training technology for accelerator tubes.This technology relies on relatively high power plasma bombardmentelectrodeSurface to raise the electrode temperature to 400~500 ℃, so as to remove the gas and hydrocarbon contamination adsorbed on the surface.In order to reduce the sputtering phenomenon, the technology uses hydrogen as the discharge gas, and uses a relatively high gas pressure (10~40 Pa). During the discharge process, hydrogen maintains a high flow rate to take away the gas released from the electrode.The micro discharge phenomenon of the accelerating tube tempered by arc discharge is greatly weakened, and the X-ray intensity under high voltage is also greatly reduced, with the highest voltage gradient up to 3 MV/m.The accelerating tube bonded with PVA adhesive cannot bear high temperature, so arc discharge exercise method is not suitable.[3]
Traveling wave accelerator tube
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The accelerating tube uses traveling wave mode to accelerate electrons. It is brazed by a disk loaded waveguide made of oxygen free copper. The 2 π/3 mode is used to make the accelerating tube have high beam collapse threshold current and high shunt impedance.
traveling waveElectron linear acceleratorThe accelerator tube is a section of disk loaded waveguide, which is composed of a smooth circular waveguide with a circular diaphragm periodically placed with a central hole. It can be seen as loading the circular waveguide with a disk, so it is called a disk loaded waveguide, which is essentially a slow wave structure.The microwave power emitted from the microwave power source (klystron or magnetron) is sent to the accelerator tube through the microwave power transmission system and input coupler. In this slow wave structure, a traveling wave "synchronized" with the electron speed is established, and this traveling wave continuously accelerates the electron.The residual power of traveling wave is fed out through the output coupler and lost in the absorption load.The central circular hole of the disc loaded diaphragm is availableelectromagnetic wavePass, also for the electron beam to pass through.There is a strong accelerating electric field on the axis of the central hole, generally up to 60-100 kV/cm.In order to ensure that the electrons can be effectively accelerated through the accelerator tube, the disk loaded waveguide is required to have strict machining accuracy (± 5 μ) and good smoothness. In the container inspection system, the accelerator tube accelerates the electrons injected from the electron gun to an energy of 9 MeV under the effect of the microwave electric field, and then hits the target to generate X-ray.The accelerating tube uses traveling wave mode to accelerate electrons, and its operating frequency is 2856MHz. The accelerating tube is brazed from a disk loaded waveguide made of oxygen free copper. The 2/3 π mode enables the accelerating tube to have a high beam collapse threshold current and a high shunt impedance.[4]
Standing wave accelerating tube
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The standing wave accelerator (tube) structure plays an important role in the standing wave electron linac. It is the core of the standing wave accelerator, and its performance largely determines the performance of the whole machine.The accelerating tube uses standing wave mode to accelerate electrons. The standing wave accelerating structure has high shunt impedance, which can stimulate higher acceleration field strength under a given microwave power, which is conducive to the miniaturization of the accelerator.
In the course of thirty years' development, there have been various standing wave accelerating (tube) structures.According to different characteristics, they have different classifications.
One is divided into π mode, 2 π/3 mode and 0 mode according to the average phase shift of each cavity.
One is divided into single period, double period and three periods according to the number of periods included in the structure.
One is divided into axial coupling, side coupling, ring cavity coupling and coaxial coupling according to the position of coupling holes.
One is divided into electric coupling and magnetic coupling according to the electromagnetic field coupling mode.
Compared with other structures, the axial coupling structure is symmetrical, with good processability and small radial size, which is convenient for ray shielding and reducing the weight of the chassis.
