High-gradient low- β accelerating structure using the first negative spatial harmonic of the fundamental mode

Abstract

The development of high-gradient accelerating structures for low-β 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 β ¼ 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 MΩ=m). At the same time, a significant (∼10%) reduction in the linac length can be achieved by using the 50 MV=m structures starting from β ∼ 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.

Transformative Technology for FLASH Radiation Therapy

“…A shorter and more compact cell increases the rate of electric breakdowns and makes dissipating the power required for operation at such high gradients challenging. R&D in this field is being pursued at CERN [37], other European institutions, and, more recently, in the US by RadiaBeam and Argonne [38]. In this collaboration, we have developed a β ~0.3 traveling-wave S-band structure (NHS) and demonstrated the 50 MV/m accelerating gradient required for ACCIL [39]; see Figure 4.…”

Design Criteria for High-Gradient Radio-Frequency Linacs

“…Advances in understanding limitations on accelerating gradients go beyond linear colliders. Accelerators are used in applications such as inverse Compton scattering gamma ray sources [5,6], compact free-electron lasers (FELs) [34][35][36], and compact medical linacs for hadron therapy [37][38][39][40][41][42].…”

Electron bunchers for industrial RF linear accelerators: theory and design guide

“…Other considerations for minimum phase velocity include fabrication feasibility: low-β cells have larger radii, and much larger radius-to-length ratio [58], which can be challenging to machine (for example, in a split structure DLS [42,43]), or might not have space to place frequency tuners [17]. The practical limit of β ph is~0.3 for S-band structures but is usually set at 0.5-0.6 for higher-frequency or compact industrial accelerators [21,44].…”

“…The harmonic with m 0 is called the "fundamental harmonic" and has the highest amplitude [15]. Because of this fact, the beam is usually synchronized and interacts with the fundamental harmonic; however, in some cases, operation on higher spatial harmonics can be beneficial due to lower phase velocities [16][17][18]. All further discussions are valid for any harmonic chosen for acceleration, which we will call the "accelerating harmonic.…”