# Transformative Technology for FLASH Radiation Therapy
## 3 High-Gradient Ion Linacs for FLASH-RT Developed by Argonne National Laboratory and RadiaBeam
### The ACCIL Ion Linac: General and FLASH Capabilities  
The advanced compact carbon ion linac (ACCIL) is the most compact full-energy carbon ion linac proposed for therapy [34]. In Europe, there are proposals for a combined cyclotron and linac (cyclinac) and an all-linac for carbon beams [35], in addition to the ongoing LIGHT project for a proton therapy linac [35]. ACCIL is designed to deliver a full energy of 450 MeV/u, which exceeds the maximum energy required for carbon ion therapy. It is also capable of accelerating protons and many other ion beams to the same energy per nucleon. Figure 3 presents a schematic layout of the ACCIL design. The system is about 45 m long but could, in principle, be folded into two 25 m-long sections.  
Figure 3: Layout of the proposed ACCIL design.  
The linac comprises an electron cyclotron resonance ion source, followed by a radio-frequency quadrupole accelerating the carbon beam to 3 MeV/u, which is further accelerated in a drift tube linac (DTL), then in a coupled DTL linac up to 45 MeV/u. The essential features to achieve compactness in the ACCIL design are high-gradient structures, each capable of delivering 50 MV/m, that accelerate the beam to the full energy of 450 MeV/u in \(\sim\)25 m.  
The main advantages of ACCIL are the fast pulse-to-pulse beam energy change and ion beam switching capabilities. Different ion sources could be used in the front end to allow fast beam switching between different ion species. The delivered beam intensity could also be controlled by adjusting the pulse length at the source or changing the pulse repetition rate, typically from 100 to 400 Hz, and R&D for accelerating structures capable of operating at 1000 Hz is ongoing. Ultimately, the tuning flexibility of the ACCIL design will allow fast and effective variable-energy and intensity-modulated multi-ion beam therapy.  
ACCIL is capable of accelerating a variety of ion beams from proton to neon, up to a maximum energy of 450 MeV/u. At this energy, ions lighter than carbon, including protons and helium ions, have ranges exceeding the depth of the human body and could therefore be used for imaging such as proton/ion tomography. It is also possible to deliver these beams with lower energies for treatment. Despite having ranges shorter than the human body, ions heavier than carbon, such as oxygen and neon, could still be used for treatment at adjustable energies up to the full linac energy.  
As for FLASH, ACCIL's capability is comparable to other existing proton and ion machines [36]. For example, for a proton beam of 230 MeV, losing about half of its energy in the last 10 cm, the energy deposited at 10\({}^{10}\) protons/s is \(\sim\)0.16 J/s. The corresponding dose delivered to a spot size of \(\sim\)5 \(\times\) 5 mm\({}^{2}\) (2.5 cm\({}^{3}\) beam-stopping volume) is 64 Gy/s, which is well above the FLASH dose requirement of 40 Gy/s. For a carbon ion beam of 430 MeV/u, losing about half of its energy in the last 10 cm, the energy deposited at 10\({}^{9}\) carbon ions/s is \(\sim\)0.38 J/s, and the corresponding dose delivered to the same stopping volume is 152 Gy/s, which exceeds the FLASH dose requirement and calls for a larger beam spot size. However, to satisfy all cases, for all tumor sizes and beam energies, we would need at least 10 times more particles per second (10\({}^{11}\) protons/s and 10\({}^{10}\) carbon ions/s), which is feasible with the ACCIL linac design. In addition, higher repetition rates may be required for faster beam scanning and more flexibility in beam delivery.