# Characterization of a Modified Clinical Linear Accelerator for Ultra-High Dose Rate Beam Delivery
## 2 Materials and Methods
### Clinical Accelerator Upgrade  
The upgrade of the LINAC aiming to generate a UHDR electron beam was supervised by Elekta technicians, who implemented both hardware and software modifications to the machine, following the work of [PERSON] et al. [10]. In the control console, the default parameters of the 10 MV photon mode were adjusted to deliver an electron beam, preventing the X-ray target from interfering with the beam path. All the filters, including the wedge and the shutter foil, were removed, leaving a free spot of approximately 100 cm\({}^{3}\) on the wedge tray between the ICs and the light mirror. This spot corresponds to the irradiation setting iv, listed above. The electron gun filament current increased from the conventional 5.6 A to 7.3 A. The bending magnet intensity was tuned to optimize the beam transport and maximize the accelerated beam current under the new conditions. The power drawn from the magnetron was increased to approximately 6 kW, which remained below the specified maximum power of 7 kW. The gun current and magnetron settings were changed from the control panel using LINAC's proprietary software and following the instructions of the company's technicians. Throughout the process, the magnetron's power was monitored using an oscilloscope connected to one of the accelerator's test points. As the gun current was increased, the voltage and magnetron current were controlled, always trying to stay within the safety range of the datasheet specifications. Each time the LINAC is started, a standard warm-up procedure is performed by delivering about 1000 Monitor Units (MU) of a 15 MV X-ray beam and about 1000 MU of the 10 MeV electron clinical beam to reach the optimal working conditions. Once the regime is switched to UHDR mode, the parameters related to the electron gun current, gun aim, gun standby and tuner control are manually adjusted to the optimal values. The parameters generally experience small variations during the optimization, and the operation usually requires a few beam deliveries for prior adjustments. During LINAC operation, the water temperature is maintained between 26 \({}^{\circ}\)C and 28 \({}^{\circ}\)C, a range in which the best performance can be achieved. Specific interlocks must be overridden to operate the LINAC in UHDR conditions [10; 11; 13]. To prevent the internal ICs interlock from being triggered, a custom attenuator circuit provided by Elekta was connected to the LINAC, as similarly documented by [PERSON] et al. [11], who obtained comparable UHDR dose rates to those found in this study. This circuit was installed between the ICs and the dosimetry system of the accelerator to reduce the current signal from the ICs, thus avoiding the activation of the interlocks. Figure 1: The LINAC Elekta SL 18 MV with three (out of the four) irradiation settings available: isocenter (i), crosshair foil (iii) and wedge (iv). The attenuator circuit makes the ICs' readings unreliable, and so they cannot be used to monitor the dose delivered. Therefore, a pulse counter circuit (PCC) was developed to control the LINAC delivery in UHDR mode by counting the number of beam pulses [10; 12]. The circuit converts the current signal generated in an unbiased silicon diode sensor into voltage signals using a transimpedance amplifier. The voltage signals are then filtered through a Sallen-Key filter and amplified to produce an acceptable input signal for a Schmitt Trigger. The resulting 5 V amplitude square pulses are counted using an Arduino NANO board. The PCC accuracy was tested with the pulsed beam of the LINAC by checking the match between the PCC output and the number of oscilloscope readings, and under these circumstances, no pulse loss was observed. All measurements reported in this study are referred to the High Power (HP) mode of the RF injection cycle of the magnetron, where two charging cycles occur before the thyratron is triggered by the Pulse-Forming Network (PFN). HP mode is needed for reaching the highest possible dose rates.