High-Gradient Low-Beta Accelerating Structure Using The First Negative Spatial Harmonic Of The Fundamental Mode

The development of high-gradient accelerating structures for low-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 MeV/u carbon ions, which sets the requirement of having 35 MV/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 MV/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 beta = v/c > 0.4. However, the simulations show that for lower phase velocities, these structures have either high surface fields (>200 MV/m) or low shunt impedances (<35 M Omega/m). At the same time, a significant (similar to 10%) reduction in the linac length can be achieved by using the 50 MV/m structures starting from beta similar to 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.