Conceptual Design of Superconducting Combined-Function Magnets for the Next Generation of Beam Cancer Therapy Gantry

An increasing number of proton therapy facilities are being planned and built at hospital based centers. Many facilities use rotatable gantry beamlines to direct the proton or ion-beam at the patient from different angles. A key issue is the need to make future gantries lighter and more compact with the use of cryogen-free superconducting magnets, in particular for the final bending section which can be of large aperture. Benefits of using the superconducting technology are: (1) the possibility to have a large momentum acceptance, hence reducing the need to ramp the magnet and enabling new treatment techniques, (2) the size reduction due to a lower bend radius and (3) the weight reduction up to a factor ten. The latter will also significantly reduce the costs of the supporting structure. We present a conceptual design based on Nb3Sn superconducting combined function magnets (dipole, quadrupole, sextupole). The geometry using racetracks, the superconducting strand and cable parameters and the results of the thermal and the mechanical studies are reported. These magnets will work at a temperature of about 4.2 K cooled with cryocoolers. INTRODUCTION The number of the centres offering proton therapy has grown significantly over the past years and the number of hospitals and research institutions delivering protons or carbon ions for tumour treatment is following also an increasing trend. For the next generation of these machines, the superconducting technology applied to magnet development will play a key role as it will enable developping compact and light gantries. A gantry is the final section of a proton therapy facility, which consists of beamline magnets, beam diagnostics elements and the mechanical support structure. The gantry rotates around the patient and irradiates the tumour from different directions. The increased field strengths using superconducting magnets will decrease the bending radius, decrease the overall weight of the system and reduce the demands on the mechanical structure. Moreover superconducting magnets allow increasing the momentum acceptance, hence reducing the need to ramp the magnet and enabling new treatment techniques [1]. The present concept is based on an isocentric gantry design with the transverse scanning performed downstream of the final bending magnet (Fig.1). A transverse scanning field of 30 cm x 40 cm with a beam spot size of 2σr ≈ 5 mm at the isocenter is required. The gantry should also allow a beam energy modulation between 70 MeV and 230 MeV (corresponding to a magnetic rigidity Bρ of 1.2 Tm and 2.3 Tm, respectively). In our gantry layout, the last bending section aims at deflecting the proton beam by 135°. An achromatic layout is chosen with a very large momentum acceptance (p/p ~ ±12%). Energy change between two layers will be performed in less than 100 ms, within the momentum acceptance window, keeping a ramping speed of magnetic field between these windows below 0.1T/s. Figure 1: Gantry based on achromatic superconducting combined function magnets for the bending section. The bending section consists of a series of superconducting combined function magnets described in this work, resulting from the conclusions of a preliminary study based on an upstream design [2]. The magnet geometry, the field maps, the conductor characteristics and the results of the thermo-mechanical calculations are discussed. Each dipole is cooled using two stage cryocoolers working at 4.2 K. To enable a sufficient temperature margin avoiding quenches after four consecutive current cycles (the treatment for the maximal target size), Nb3Sn cables are used in the coils. LAYOUT AND MAGNET DESIGN Bending Section layout The transport section is a curved, compact and locally achromatic, to minimize the proton beam dispersion. The section consists of three types of combined function magnets: (1) two superconducting combined dipolesquadrupole and sextupole magnets (SDC1, SDC2), (2) a superconducting combined quadrupole-sextupole magnet (SCQ), (3) two tuneable normal conducting quadrupoles (Q1&2) at each side to meet with the beam optic conditions [1]. All the geometries are based on racetrack coils to keep the manufacturing as easy as possible. The design is optimized in different steps. From the magnets 3D field maps, the field harmonics are calculated and compared with the ones required by theoretical first order THCDMH01 Proceedings of RuPAC2016, St. Petersburg, Russia ISBN 978-3-95450-181-6 138 C op yr ig ht © 20 17 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s Superconducting accelerators and cryogenics calculations. Tracking simulations including all orders are then performed and the beam parameters are analysed. Magnet Specifications Table 1 describes the specifications of the three types of magnets (x and y directions are shown in Fig.2). Table 1: Magnet specifications Type Q1&2 SDC1&2 SCQ Length (cm) /bending angle (°) 10 67.5 35 Bending radius (m) 0.8 Half-aperture (cm) (Half good field region) 25 10 (x) 4 (y) 12.5(x) 2 (y) Dipole field (T) 0 2.57 0 Quadrupole (T/m) 25.7 -5.3 21.4 Sextupole (T/m) 0 -9.8 21.9 Operating current in superconducting magnets (A) 170