While the year-end technical stop (YETS) is still in full swing in the LHC and progressing as planned, the injector complex is gradually being restarted. A week ago, Linac4, the PS Booster (PSB), the PS and their transfer lines were closed again.
Before powering the equipment or even considering injecting a beam, the operations team carries out mandatory patrols to ensure that no one is present in the tunnels. Closing the machine means that the access control and safety systems are fully operational, preventing any unauthorised entry when the beam is active and ensuring that no beam can be injected when access is granted.
As mentioned in my first report of 2025, the final step in this process is the DSO tests, which are designed to confirm that the system is functioning as it should. These tests were successfully completed on 10 February for Linac4, the PSB, the PS and their transfer lines. With this milestone achieved, everything is now in place for the restart of the machines.
The Linac4 source was restarted over a week ago. It is currently pulsing every 1.2 seconds, producing a beam current of 40 mA. Until 19 February, the extracted particles will be absorbed by a beam dump located just downstream of the source. This allows measurements of the beam current to be taken while preventing further acceleration within Linac4.
During this period, the operations team, together with equipment experts, is gradually bringing all of Linac4's subsystems back online, including the RF accelerating structures, the magnet systems and the beam diagnostic systems, in preparation for injecting and accelerating the beam. This will ensure that the beam is ready to be received by the PSB for commissioning on 27 February.
By that date, the PSB's subsystems will also have been fully restarted and will be operational. The PSB is then expected to provide its first beams for the PS beam commissioning, which is scheduled to begin on 5 March.
Some of you may recall that, in previous years, the Linac4 source provided a beam current of 35 mA for all operational beams. This year, following successful tests at up to 45 mA, we have decided to start and operate with a beam current of 40 mA.
The total number of protons injected into the PSB is determined by the combination of the Linac4 beam current and the duration of its pulse. By increasing the beam current by 15%, the pulse length and thus the duration of injection into the PSB, can be reduced by the same percentage. While this adjustment is not strictly necessary for any of the currently required operational beams, gaining experience with this mode of operation could be valuable for future beam configurations and help to improve performance.
Of course, our primary objective is still the delivery of stable, high-quality beams to all the experiments. If we observe any degradation of beam performance due to this change, the Linac4 beam current can be readjusted to 35 mA. With a few hours of fine-tuning, all beams could then be restored to the exact same conditions as last year. If that is the case, it will be essential to study and understand the cause of any performance variations. However, at this stage, there is no indication that the change of the Linac4 source current will negatively impact the beam delivery to the experiments.
| A few words on the Linac4 source Although the accelerator complex downstream of Linac4 primarily operates with proton beams, the Linac4 source initially produces H- ions (negatively charged hydrogen atoms). These ions are formed by adding an extra electron to hydrogen atoms, which originate from a pressurised gas bottle and consist of a single proton and a single electron. To generate these ions, a small amount of hydrogen gas is injected into the vacuum chamber - also called the plasma chamber - of the Linac4 ion source. Inside the chamber, powerful microwaves break up the hydrogen molecules into their fundamental components, creating a hot, ionised gas in which hydrogen atoms are separated into protons and free electrons. This hot gas is called plasma. To create H- ions, the protons must capture two electrons. This process takes place in two ways:
Once the negatively charged H- ions are formed, they are sensitive to electric and magnetic fields and can be extracted from the source and guided into Linac4 to be further accelerated. The extraction is done using a strong (45 kV) electrostatic field that pulls the negatively charged H- ions together with electrons out of the plasma. While the electrons are bent by a magnetic field and disposed of in a dedicated dump, the extracted H⁻ ions come out of the source with an energy of 45 keV before they are captured and accelerated up to 160 MeV in Linac4. Stay tuned as in a future report I will explain how we go from an H- beam in Linac4 to proton beams in the downstream accelerator complex! |
