Variable energy linear accelerator

Details Variable energy linear accelerator Variable energy linear accelerator

2001-02-01

2002-06-18

Anderson, Bruce (Department: 2881)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Combined cathode ray tube and circuit element structure

C315S005420, C315S005460, C315S505000

Reexamination Certificate

active

06407505

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a charged particle acceleration device, and more particularly to a variable energy standing wave linear accelerator.
BACKGROUND OF THE INVENTION
Many different types of devices may be used to accelerate charged particles. All rely on either electric fields or rapidly changing magnetic fields to impart energy to charged particles. Circular accelerators are generally driven by RF (Radio Frequency) signals (e.g., cyclotrons, synchrotrons, microtrons) but may also be driven by pulsed magnetic fields (e.g., betatrons). Linear accelerators (linacs) may be DC, electrostatic devices (e.g., VandeGraaf or tandem accelerators, including pelletrons and dynamitrons), pulsed magnetic field devices (e.g., induction linacs), or RF devices (e.g., drift tube linacs, standing wave linacs, traveling wave linacs, RF quadrupole accelerators).
For the parameters desired in conventional radiation therapy (e.g., acceleration of electrons to multi-MeV energies at average current of below about 500 &mgr;A in compact structure), standing wave or traveling wave accelerators are a preferred choice. Currently, most electron accelerators available for medical radiation therapy applications are standing wave linear accelerator structures, with occasional use of traveling wave structures, betatrons, or microtrons for specific applications.
A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. An electron linear accelerator is located within the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam may be an electron beam or photon (x-ray) beam, for example. During treatment, the radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation.
Linear accelerators may be used in the medical environment for a variety of applications. A beam of charged particles (e.g., electrons) from a linear accelerator may be directed at a target which is made of a material having a high atomic number, so that an x-ray beam is produced for radiation therapy. Alternatively, the beam of charged particles may be applied directly to a patient during a radiosurgical procedure. Such radiosurgery has become a well-established therapy in the treatment of brain tumors. A high-energy beam may be directed at a localized region to cause a breakdown of one or both strands of the DNA molecule inside cancer cells, with the goal of at least retarding further growth and preferably providing curative cancer treatment.
A conventional RF linear accelerator includes a series of accelerating cavities that are aligned along a beam axis. A particle source, which for an electron accelerator is typically an electron gun, directs charged particles into the first accelerating cavity. As the charged particles travel through the succession of accelerating cavities, the particles are accelerated by means of an electromagnetic field. A RF source is coupled to the accelerator to generate the necessary field to operate the linear accelerator. The accelerated particles from a clinical linear accelerator have a high energy (e.g., up to 25 MeV). The output beam is often directed to a magnetic bending system that functions as an energy filter. The beam is typically bent by approximately 270 degrees. Then either the output beam of high energy particles or an x-ray beam generated by impinging a target with the output beam is employed for radiation treatment of a patient.
As discussed above, the most common accelerator type for radiation therapy is the standing wave accelerator. Standing wave accelerators are often used for other applications as well, such as basic nuclear and subatomic research, positron production, industrial x-raying, food irradiation, product sterilization, plastic and rubber polymerization, and oil and gas logging.
A standing wave linear accelerator is comprised of a series of high-Q resonant cavities, each weakly coupled to its two nearest neighbors. RF energy is coupled into the structure, typically from a rectangular waveguide through a coupling iris into one of the cavities. This sets up a standing wave along the chain of cavities, causing the cavities to resonate at high voltages. If the cavities are designed with holes along their axes, and with the appropriate dimensions, many electrons can be accelerated along the axis of the cavities.
A series of N such identical cavities will resonate at N different collective resonant modes and frequencies. The RF voltage in any cavity i (where i cavities are numbered from ) through N−1) is proportional to cos(m&pgr;i/(N−1))cos(&ohgr;t), where mode number m may take the values) through (N−1). Sometimes m is referred to as the mode of the structure, but more often the mode of the structure is referred to as m&pgr;/(N−1).
For example, assume the structure is resonating in the zero mode (m=0). Then each cavity will have an identical excitation and all will resonate in phase. If the structure is resonating in &pgr; mode (m=N−1), each cavity will have an identical RF voltage amplitude, but the phase will reverse from cavity to cavity (i.e., there will be a phase shift of &pgr; from each cavity to the next). If there are an odd number of cavities and m=(N−1)/2, the structure will be in &pgr;/2 mode. The first cavity will have a strong field excitation, the second will be unexcited, the third will have a strong field with an inverted phase, the fourth will be unexcited, etc.
The &pgr;/2 mode has significant practical advantages, as it is much more tolerant to mistuning than the other collective resonant modes. However, only roughly half of the cavities have strong fields and are useful for acceleration, the others are unexcited. The unexcited cavities may be designed to be smaller than the accelerating cavities (since they do not have high fields) and may be moved to the side of the structure (so that they do not take up space along the axis which can be used for acceleration). This yields the side coupled cavity &pgr;/2 mode standing wave structure.
FIG. 1
illustrates a conventional side coupled standing wave linear accelerator. For additional information on the design and operation of standing wave structures, see “Principles of Charged Particle Acceleration” by Stanley Humphries, Jr., published by John Wiley and Sons, 1999.
It is often desirable to operate a coupled cavity standing wave linear accelerator at different energies. This may be accomplished by changing the excitation of the accelerating cavities. However, this tends to cause particles to slide in phase, thus adversely affecting the beam dynamics and reducing the efficiency of the accelerator. Another option is to operate a nominally &pgr;/2 mode accelerator in a different mode. However, this puts large fields in side cavities and it is difficult to optimize performance for two different modes.
Another option is to make an accelerator with two independent sections, each section having independent phase or amplitude adjustments. This technique is commonly used in large research accelerators, and has also been used for commercial x-band accelerators. However, this configuration is costly and complex, and requires careful frequency matching of the two accelerator sections.
Another conventional approach is to change the coupling at a fixed point along the accelerator. This can be accomplished by various methods. For example, two side cavities may be used to couple two main cavities, with each cavity having different coupling ratios and with one cavity shorted, as disclosed in U.S. Pat. No. 4,746,839. Also, a single side cavity may be mechanically distorted to change the coupling ratio as disclosed in U.S. Pat. Nos. 4,382,208 and 4,400,650. A side cavity may also be mechanically modified to change the resonant mode as disclosed in U.S. Pat. No. 4,286,192. These methods all have drawbacks. For example, the change of coupling from the main cavity to side ca

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