Abstract
The development of high-gradient accelerating structures for low-$\ensuremath{\beta}$ particles is the key for compact hadron linear accelerators. A particular example of such a machine is a hadron therapy linac, which is a promising alternative to cyclic machines, traditionally used for cancer treatment. Currently, the practical utilization of linear accelerators in radiation therapy is limited by the requirement to be under 50 m in length. A usable device for cancer therapy should produce 200--250 MeV protons and/or $400--450\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ carbon ions, which sets the requirement of having $35\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ average ``real-estate gradient'' or gradient per unit of actual accelerator length, including different accelerating sections, focusing elements and beam transport lines, and at least $50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ accelerating gradients in the high-energy section of the linac. Such high accelerating gradients for ion linacs have recently become feasible for operations at S-band frequencies. However, the reasonable application of traditional S-band structures is practically limited to $\ensuremath{\beta}=\mathrm{v}/\mathrm{c}g0.4$. However, the simulations show that for lower phase velocities, these structures have either high surface fields ($g200\text{ }\text{ }\mathrm{MV}/\mathrm{m}$) or low shunt impedances ($l35\text{ }\text{ }\mathrm{M}\mathrm{\ensuremath{\Omega}}/\mathrm{m}$). At the same time, a significant ($\ensuremath{\sim}10%$) reduction in the linac length can be achieved by using the $50\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ structures starting from $\ensuremath{\beta}\ensuremath{\sim}0.3$. To address this issue, we have designed a novel radio frequency structure where the beam is synchronous with the higher spatial harmonic of the electromagnetic field. In this paper, we discuss the principles of this approach, the related beam dynamics and especially the electromagnetic and thermomechanical designs of this novel structure. Besides the application to ion therapy, the technology described in this paper can be applied to future high gradient normal conducting ion linacs and high energy physics machines, such as a compact hadron collider. This approach preserves linac compactness in settings with limited space availability.
Highlights
Radiation therapy is a large segment of clinical oncology, where the treatment is performed by delivering ionizing radiation beams directly to the tumor
To compensate the reduced energy gain due to the smaller amplitude, we have introduced aperture noses similar to those shown in Fig. 7 for the NHS, which will concentrate the electric field near the beam axis and improve the shunt impedance
We have described the novel concept of a high gradient accelerating structure, operating at the first negative spatial harmonic, and suitable for the acceleration of low β (
Summary
Radiation therapy is a large segment of clinical oncology, where the treatment is performed by delivering ionizing radiation beams directly to the tumor. Ion types Minimum β Frequency, MHz Structure type Spatial harmonic Accelerating gradient, MV=m Section length, сm Number of cells Shunt impedance, MΩ=m Peak electric field, MV=m Modified Poynting vector, MW=mm Beam pulse width, μs Filling time, μs Repetition rate, pps Input rf power, MW. The plot of the effective shunt impedance of the conventional pillbox cavity with the fixed size of the aperture as a function of phase velocity, computed numerically for different frequencies and shown in Fig. 3 demonstrates that for β range from 0.3 to 0.4, S-band provides the highest power efficiency compared to L- and C-bands. Operating frequency Phase velocity, β Accelerating gradient (constant along the section) Length of the section Aperture diameter Beam pulse width Filling time Repetition rate
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