Longitudinal dispersion in the second bunch compressor of the MAX IV linear accelerator

Abstract

Bunch compressors in linear accelerators (linac) are magnetic devices used to shorten electron bunch length, and therefore, increase the peak current. MAX IV, located in Lund, Sweden, is a synchrotron light source facility that includes a linac containing two double-achromat bunch compressors, BC1 and BC2. Each bunch compressor consists of two identical arcs bent in opposite directions. As electrons pass through the bunch compressors, higher-energy electrons travel longer paths while lower-energy electrons travel shorter paths, resulting in longitudinal compression if an energy chirp is applied with the correct phase. This relationship between energy and path length, which causes longitudinal compression, can be described by two parameters, the first- and second-order longitudinal dispersion, referred to as R56 and T566, respectively. To control the higher-order effects of the bunch compression, i.e. T566, a sextupole magnet is implemented in each arc of the bunch compressors.

This thesis aimed to characterise BC2, the second bunch compressor at MAX IV, by determining R56 and T566 as well as studying the effect of the sextupole field strength on these parameters. This characterisation was of specific interest as BC2 is located right before the Short Pulse Facility (SPF), which requires short electron bunches. The parameters R56 and T566 were determined through both simulations and experimental measurements; however, the experimental measurements were only conducted for the first arc of BC2. The simulations were performed using the accelerator simulation program elegant.

The simulation results indicated that the sextupole field strength had little to no effect on R56, whereas T566 exhibited a strong inverse relation to it, decreasing as the sextupole field strength increased.
The experimental results also demonstrated a strong relation between T566 and the sextupole field strength, showing a similar but steeper decreasing trend. However, for the experimental results, a slight decreasing trend was also observed for R56.

This thesis aimed to characterise BC2, the second bunch compressor at MAX IV, by determining R56 and T566 as well as studying the effect of the sextupole field strength on these parameters. This characterisation was of specific interest as BC2 is located right before the Short Pulse Facility (SPF), which requires short electron bunches.

The parameters R56 and T566 were determined through both simulations and experimental measurements; however, the experimental measurements were only conducted for the first arc of BC2. The simulations were performed using the accelerator simulation program elegant.

The simulation results indicated that the sextupole field strength had little to no effect on R56, whereas T566 exhibited a strong inverse relation to it, decreasing as the sextupole field strength increased.

The experimental results also demonstrated a strong relation between T566 and the sextupole field strength, showing a similar but steeper decreasing trend. However, for the experimental results, a slight decreasing trend was also observed for R56.

Popular Abstract

Lund is one of Sweden's oldest cities, but it's also home to MAX IV, one of the world's brightest synchrotron facilities, known for its innovative design. It can be compared to a giant microscope, allowing materials to be studied at an atomic scale using X-rays produced by accelerating electron bunches close to the speed of light. X-rays are photons, just like visible light, but with much higher energy. Experiments using X-rays are conducted in many fields, including medicine, material science, and nanotechnology.

Although synchrotrons often appear as large circular buildings above ground, much more is hidden underground. MAX IV consists of two electron guns, a linear accelerator, two storage rings, and a short pulse facility, as well as experimental stations. The focus of this project lies in the 300-meter-long linear accelerator (linac) underground, the purpose of which is to accelerate the electrons to very high energies and inject them into the storage rings or the Short Pulse Facility (SPF).

In the SPF, X-ray pulses with a duration of femtoseconds are used to study fast processes in samples such as chemical reactions (one femtosecond is a billionth of a millionth of a second!). To produce pulses this short, the electrons in each bunch must be packed very close together. This is because the duration of a bunch, or the time it takes for the entire bunch to pass a certain point, is directly related to the duration of the resulting X-ray pulse. In simple terms, the shorter the bunch, the shorter the X-ray pulse. Hence, the electron bunches need to be compressed. This is done using bunch compressors.

The MAX IV linac has two bunch compressors, both of a type referred to as double-achromats, consisting of two identical arcs bending in opposite directions. The arcs contain magnets that steer the electrons along the curved path. Electrons of different energies will take different paths through the compressor, as the effect of the magnetic fields on the electrons depends on their energy. Imagine you're in a race, if you enter a curve at a higher speed, you will "take out" the curve more, making you travel a longer distance. Similarly, higher-energy electrons will travel a longer distance by "taking out the curves", while lower-energy electrons will take shorter, straighter paths.

This effect is utilised by making the electrons at the front of a bunch have higher energies while electrons in the back have lower energies. As a result, the electrons will catch up to each other, reducing the time spread of the bunch.

The main advantage of the MAX IV bunch compressors, compared to other types, is that a sextupole magnet is placed in each arc. A sextupole is a six-sided magnet, and these are tunable; they can be turned on or off, and their strength can be adjusted depending on the goal. This is a big advantage, as it makes it possible to fine-tune the compression and make the bunches shorter.

In this project, electron path length and energy will be measured for the second bunch compressor at MAX IV to determine the relationship between electron energy and bunch compression. The effect of the sextupole field strength on compression will also be studied. The insights from this could lead to improved compression, which would result in even shorter X-ray pulses, increasing both precision and time resolution for experiments. This could unlock new possibilities in many fields, such as medicine.

• author: Tolonen, Sofia
• supervisor: Francesca Curbis; Johan Lundquist
• organization: Department of Physics; Synchrotron Radiation Research
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: MAX IV, linear accelerator, electron bunch compressor, longitudinal dispersion
• language: English
• id: 9211258
• date added to LUP: 2025-09-05 15:30:46
• date last changed: 2025-09-05 15:30:46