Patent Grant
8598814
This application claims the benefit of DE 10 2011 075 210.2, filed on May 4, 2011.
The present embodiments relate to a method for pulsed operation of a linear accelerator.
DE 10 2009 007 218 A1 discloses an electron accelerator for generating photon radiation. Such an electron accelerator may, for example, be used for radiation therapy or for nondestructive materials testing. The electron accelerator includes an electron source and a vacuum chamber, in which electrons emitted by the electron source are accelerated. Nothing is stated in DE 10 2009 007 218 A1 about a possible time structure of the electron beam generated.
EP 0 037 051 A1 discloses an accelerator for charged particles (e.g., electrons) that is provided for the emission of a particle beam. The particle beam may be used either directly as an electron beam or for generating X-ray radiation.
Another electron source is, for example, known from DE 10 2004 055 256 B4. In this case, a resonator of the electron source (e.g., a high-frequency electron source) is formed from superconducting material.
In medical engineering, for accelerators that are operated in pulse mode, a distinction is made between micropulses and macropulses. The micropulses are determined by the physical properties of the accelerator tube and have a duration of, for example, a few 10-100 picoseconds. A macropulse may be composed of several thousands or tens of thousands of micropulses and have a duration of a few microseconds. The time interval between two macropulses may be a few milliseconds, so that the pulse frequency of the accelerator is a few hundred Hz.
The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, a pulsed particle beam is generated by a linear accelerator.
The embodiments explained below apply for a device (e.g., the linear accelerator), a method, with which the linear accelerator is operated, and software, with which the method may be realized in interaction with the device.
The method for pulsed operation of a linear accelerator includes the following features. Pulses of charged particles are generated, in that particles are emitted by a particle source and are accelerated in an accelerator device that includes several linked cavity resonators. The accelerator device is supplied with energy by a high-frequency energy supply.
With the high-frequency power fed to the accelerator device being kept completely or at least approximately constant, the particle energy (e.g., the energy per particle after passing through the accelerator device) is changed solely by varying the number of particles emitted by the particle source per macropulse.
The number of particles emitted by the particle source is also referred to as the beam loading or beam current.
The present embodiments are based on the consideration that high-frequency power fed to a particle accelerator made up of linked cavity resonators may be approximately constant during operation of the accelerator, or at least is not subject to significant changes from one particle pulse to another. Assuming a constant high-frequency power, an acceleration voltage, with which the particles are accelerated to an energy of, for example, several MeV when passing through the cavity resonators, is a function of the beam current. The following ratio may apply:
Pin=U*I+U2/Rv
where
Pin=injected high-frequency power
U=acceleration voltage
I=beam current
Rv=loss resistance
For the acceleration voltage, this produces:
U=(Pin*Rv+0.25*I2*Rv2)1/2−0.5*I*Rv
An increase in the beam loading (e.g., the particles emitted per time unit and accelerated by the cavity resonators) accordingly results in a diminution in the acceleration voltage and thus to a reduction in the kinetic energy that the particles have after passing through the accelerator. A change in energy of the accelerated particles is thus achieved by a change in the loading.
In addition to the described effect of the change in loading, another effect (e.g., adjustment of the impedance) plays a role in the desired change in particle energy by changing the beam current.
By changing the beam current, the load resistance (impedance) of the particle accelerator changes, whereupon the adjustment of the impedance of the accelerator to the high-frequency source also changes. Such a change in the adjustment of the impedance provides a change in the reflection factor of the accelerator. The power coupled into the accelerator depends on the adjustment of the impedance and thus on the beam current.
This dependency may be used to control the particle energy if the linear accelerator is suitably configured, in that the power coupled into the accelerator diminishes as the beam current increases. The effect of the impedance maladjustment thereby increases the effect of the change in loading. In order to achieve an optimal interaction of the two effects (e.g., change in loading and adjustment of the impedance), the linear accelerator may be configured such that the impedance of the accelerator device is adjusted to the particle source at a minimum particle stream (e.g., theoretically, at zero beam current). This provides that the high-frequency power coupled into the accelerator device is maximum at the lowest beam current and continuously decreases as the beam current increases.
Due to the mutually reinforcing effects of change of loading and adjustment of the impedance, a change in the energy of the accelerated particles of more than 1 MeV (e.g., of more than 2 MeV) may be achieved.
In one embodiment, the linear accelerator is configured to accelerate the particles to an energy between 0.5 MeV and 20 MeV.
The particle source may be an electron source. The present embodiments may also be implemented with accelerators that accelerate any other charged particles (e.g., protons or ions). Even though in the following an electron source is cited as a particle source, a corresponding technical function may likewise be achieved with accelerators for other electrically charged particles.
In the case of an electron source, the beam current and thereby the energy of the accelerated electrons may be varied by changing, for example, the grid voltage of the electron gun (e.g., of the particle source). In a configuration, this variation is possible in a matter of milliseconds. A selective change in the electron energy from pulse to pulse is thereby possible. Other changes in the control of the particle source or of the accelerator downstream thereof, supplied with power by a high-voltage source, are not provided in order to change the electron energy. The clock frequency of the electron pulses lies in the range from 1 to 1000 Hz. In one embodiment, the clock frequency of the electron pulses may be above 100 Hz. These are macropulses, which are distinguished from micropulses, as explained in the introduction.
According to one embodiment, a control device provided for controlling the particle source is configured to generate a particular dose rate per pulse of emitted particles while keeping the high-frequency power fed to the accelerator device absolutely or at least largely constant (e.g., optionally, in the case of a first lower particle energy or in the case of a second higher particle energy). The provision of a particular, constant dose rate is achieved by two effects simultaneously working in opposite directions: as the beam current increases, the number of particles per time unit increases, but the energy per particle drops. The operating unit provided for operation of the linear accelerator (e.g., software) offers the user who sets a desired dose rate a choice between two particle energies, with which this dose rate is achieved.
The advantage of the present embodiments may be, for example, in that the energy of the individual particles emitted by a linear accelerator (e.g., an electron accelerator) may be varied easily and with a high rate of change. Only the beam current may be changed, while all other operating parameters may be kept.
A linear accelerator characterized overall by reference character 1 includes an electron source 2 (e.g., designated a particle source) and an accelerator device 3 operable for accelerating emitted electrons. The accelerator device 3 has several linked cavity resonators 4. Regarding the function of the linear accelerator 1 (e.g., the electron accelerator), reference is made to the prior art cited in the introduction.
The accelerator device 3 is supplied with high-frequency power by an energy supply unit 5 supplying high-frequency power. A control device 6 is provided for controlling the electron source 2. The control device 6 permits a pulsed operation of the electron source 2 and a variation in the pulses (e.g., a change in the number of electrons emitted per pulse). The pulsed emission of electrons produces a beam current, a quantity of which is designated as a beam current strength. The electron beam emitted by the electron source 2 and raised to an increased energy level by the accelerator device 3 hits an exit window 7 lying opposite the electron source 2 and closing the accelerator device 3, in order to be used either directly as an electron beam or for generating electromagnetic radiation (photons).
An interval between two consecutive pulses of the electron source 2 (e.g., between two macropulses) is a few milliseconds, corresponding to a pulse frequency of a few hundred Hz. The linear accelerator 1 is configured to change the beam current selectively from one pulse to the next in order to vary the energy per electron accelerated by the accelerator device 3 per macropulse, as required. The variation of the electron energy from pulse to pulse is effected solely by the control device 6 controlling the electron source 2. No active change is thereby made at the high-frequency supply supplying the accelerator device 3 with energy (e.g., at the energy supply unit 5).
The electron source 2 and the accelerator device 3 are aligned with one another such that the adjustment of an impedance during no-load running (e.g., zero beam current) is optimal. As the beam current increases, the adjustment of the impedance deteriorates, as desired, in order to selectively reduce the electron energy. The effect of change of loading as the beam current increases (e.g., as the number of electrons emitted by the electron source 2 per pulse increases) is added to the effect of the adjustment of the impedance. This helps to reduce the electron energy.
The relationship between an energy E of the electrons emitted by the linear accelerator 1 (e.g., nominal energy in MeV) and the beam current I (e.g., “beam” in mA) is illustrated in
The program startup designated by S1 is followed by act S2, in which the operator of the linear accelerator 1 inputs parameters. For example, an operator inputs the desired dose rate. Act S3 includes a query, in which the program checks whether the dose rate input may be realized with different energy settings, related to the energy of the electrons on leaving the accelerator device 3. If the dose rate input may be realized with different energy settings, the program offers the operator the corresponding selection and accordingly effects either a first lower energy setting E1 of, for example, 8 MeV or a second higher energy setting E2 of, for example, 10 MeV. A switchover between the two possible energy settings E1, E2 is effected, where appropriate, as described above, by a change in the beam current emitted by the electron source 2.
While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.
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