HALHF: A potential future plasma accelerator and e+e- collider

We are have opportunities for master students who are interested in several topics related to the HALHF collider, especially:

• Design and optimization of radio-frequency linear accelerators
• Tests and development of nonlinear active plasma lenses
• Development of the ABEL modelling framework for optimizing particle accelerators

The topics are described in greater detail below. They are possible to combine, especially the ABEL framework is relevant to all. In general, a balance between simulation and experiment can be found according to the interests of the student.

In the particle accelerator group in Oslo, we are heavily involved in the HALF project - both in the overall design and in creating several critical components for it, mainly funded by the SPARTA and NOVELTRACKS projects [TODO sett inn annen lenke enn https://prosjektbanken.forskningsradet.no/en/project/FORISS/313770?Kilde=FORISS&distribution=Ar&chart=bar&calcType=funding&Sprak=en&sortBy=date&sortOrder=desc&resultCount=30&offset=0&Prosjektleder=Erik+Adli ?]. We are also one of the leading groups for development of Active Plasma Lenses, which are also investigated through the SHARP project.

Design and optimization of radio-frequency linear accelerators

The heart of the HALHF collider is the Drive Beam LINAC, which accelerates electron bunches which are then transported to the main colliding-beam LINAC, where the drive beam kinetic energy is extracted and used to power it. This scheme, originally invented for the Compact Linear Collider (CLIC), makes it possible to efficiently power very high gradient accelerators to reach high energy.

The scheme works by first accelerating a moderately intense beam of electrons in a highly efficient Drive Beam LINAC, using accelerating structures such as the one in the drawing below.

These RF structures accelerate the drive beam electrons in the LINAC, converting microwave energy from power sources into kinetic energy of the electrons, at rates on the order of 100 MW per structure, each of which is about 1 m long. The number of structures are determined by the desired final electron energy, and the structure geometry determines the relationship between the microwave power level and the accelerating electric field strength aka the "gradient".

The microwaves are at a frequency of 3 GHz; the electrons must therefore travel in bunches which arrive a similar or lower frequency. In order to increase the power of the drive beam further, a clever scheme of delays and loops are used to stack the bunches in time, increasing the bunch frequency and therefore the beam current. This is illustrated in the video below.

Finally, this drive beam is transported to the main beam accelerator, where it is used to power the main beam accelerator. Here the drive beam bunches are distributed to plasma accelerating stages [CITE DANIEL] to transfer the energy from the high-current drive beam to the high-particle-energy main beam, which collides in the particle detector in the center of the complex. To maximize performance, the properties of the drive beam should be matched to the needs of the plasma accelerator.

The main topics of this project are therefore to:

• Optimize the layout and powering of the HALHF drive beam accelerator, answering questions such as
  • How many drive beam RF structures are needed and how should they be powered?
  • What is the optimum geometry for these structures (within a restricted parameter space)?
  • How does the choices here influence cost, power consumption, and emissions during construction and operation?
• Create simulation models of the drive beam linac using ABEL and CLICopti [5,6] and/or RFTrack [7].
• Model beam dynamics in the HALHF linac - how does the required beam optics look like, what is the influence of e.g. wake fields?

Some studies have started to look into this, such as [Cite Oxford summer students - Emmanuel Tsesmelis said he would put it on CDS: https://cernbox.cern.ch/s/CyM2f3QEKoXczLg ]

A further challenge would be to power the positron arm of HALHF also using the drive beam, similar to CLIC. Positrons are very challenging to accelerate in a plasma, which is why HALHF uses a "standard" linac for this. The main difficulty in driving both from the same drive beam source lies in the timing and frequencies of the drive beam and compatibility between the needs of the plasma linac and a drive-beam driven RF structure linac [8]. To solve this, one would need to do something clever with the delays and combiner ring(s).

The potential reward for solving it is high, as it would make it possible to do away with the complex and expensive RF power sources currently foreseen to be used for the positrons.

Tests and development of nonlinear active plasma lenses

ref. Pierre's work, link to Hektor's thesis, ties to SHARP

Development of the ABEL framework

References

[1] E. Adli et al., "HALHF: a hybrid, asymmetric, linear Higgs factory using plasma- and RF-based acceleration", Input to the European Strategy for Particle Physics - 2026 Update
[2] HALHF project webpage: https://www.adams-institute.ac.uk/halhf
[3] ABEL framework code repository: https://github.com/abel-framework/ABEL
[4] E. Jensen, "CTF3 DRIVE BEAM ACCELERATING STRUCTURES", in Proceedings of LINAC 2002, https://cds.cern.ch/record/581340
[5] CLICopti web page: https://pypi.org/project/CLICopti/
[6] K. Sjobak and A. Grudiev, "The CLICopti RF structure parameter estimator", CLIC NOTE 1031, 2014, https://cds.cern.ch/record/1712948
[7] RFTrack web page: https://pypi.org/project/RF-Track/
[8] B. Foster et al, "Proceedings of the Erice workshop: A new baseline for the hybrid, asymmetric, linear Higgs factory HALHF", in Physics Open vol 23 2025, https://doi.org/10.1016/j.physo.2025.100261