Optimization of the Beam Screen for the FCC Injection Kicker Magnets

The Future Circular Collider for hadrons (FCC-hh) is a proposed high-energy frontier particle accelerator, which would be built in a new ~100 km circumference tunnel and would enable proton-proton collisions at a centre-of-mass energy of 100 TeV. The baseline injection energy is 3.3 TeV. The FCC-hh will require a fast injection kicker system that is highly reliable and does not limit accelerator performance. To achieve low ripple and fast rise and fall time field pulses, the injection system will use ferrite loaded transmission line type magnets. The FCC-hh will operate at high beam intensity: hence, the beam coupling impedance will be an issue. The problems are twofold: longitudinal and transverse impedance may drive beam instabilities, while the real part of the longitudinal impedance gives rise to beam induced heating. The power deposition in the kicker magnet can cause a temperature increase of the ferrite yoke above its Curie point: this would affect the ability to inject beam. For a high beam current and a short bunch length, a high power deposition would occur in unshielded FCC-hh injection kicker magnets: hence, a beam screen is a critical feature. Amongst the most challenging requirements for the beam screen are: low broadband beam coupling impedance, fast field rise and fall time, low field ripple during the flat-top field and good high voltage behaviour. In this thesis, we propose and develop a novel concept of a spiral beam screen. The fundamental advantage of the new design, in comparison to the conventional straight screen conductors used for example in the LHC injection kickers, is a significant reduction of the maximum voltage induced on the conductors, thus a greatly reduced probability of an electrical breakdown of the beam screen. Also, the spiral screen design allows a reduction of the maximum transverse beam coupling impedance of the kicker system. Both the conventional and spiral designs are studied from a theoretical, a numerical, and an experimental point of view. Important new contributions brought to the field of kicker magnets include: real-time data analysis of the output waveform of the LHC kicker magnet to ensure the ferrite yoke is below its Curie point; a new CST and PSpice model of the FCC-hh injection kicker magnets; proposal and development of new calculation and measurement methods for determining power loss distribution in the ferrite of a kicker magnet; and novel measurements of the electromagnetic properties of ferrites as a function of frequency and temperature.

Helical beam screen for the Future Circular Collider for hadrons injection kicker magnets

Agnieszka Chmielinska

The Future Circular Collider for hadrons (FCC-hh) is the proposed high-energy frontier particle collider, which is expected to enable proton-proton collisions at a center-of-mass energy of 100 TeV. The specifications for the FCC-hh injection kicker system are very challenging. To provide fast magnetic field rise and fall times, and low ripple during the flat-top, ferrite loaded transmission line type magnets will be used. To limit the beam coupling impedance, a suitable beam screen will be placed in the aperture of each kicker magnet. Due to issues associated with the "conventional" beam screen design, used in the injection kickers magnets of the Large Hadron Collider (LHC), such as high voltage (HV) breakdowns and yoke heating problems, we have developed a novel concept of a helical beam screen. The fundamental advantage of the new design, in comparison to the conventional beam screen, is a significant reduction of the maximum voltage induced on the screen conductors, thus a greatly reduced probability of an electrical breakdown of the beam screen. In addition, the longitudinal beam coupling impedance of the helical beam screen is optimized to reduce power deposition in the kicker magnet. Furthermore, the helical beam screen allows a reduction of the maximum transverse beam coupling impedance of the kicker system. Detailed numerical simulations, theoretical studies and experimental results demonstrate that a new design is a promising solution not only for the FCC-hh, but also for existing machines.

2020

New spiral beam screen design for the FCC-hh injection kicker magnet

Agnieszka Chmielinska

The injection kicker system for the Future Circular Collider (FCC-hh) must satisfy demanding requirements. To achieve low pulse ripple and fast field rise and fall times, the injection system will use ferrite loaded transmission line type magnets. The beam coupling impedance of the kicker magnets is crucial, as this can be a dominant contribution to beam instabilities. In addition, interaction of the high intensity beam with the real part of the longitudinal beam coupling impedance can result in high power deposition in the ferrite yoke. This gives a significant risk that the ferrite yoke will exceed its Curie temperature: hence, a suitable beam screen will be a critical feature. In this paper, we present a novel concept a spiral beam screen. The fundamental advantage of the new design is a significant reduction of the maximum voltage induced on the screen conductors, thus decreased probability of electrical breakdown. In addition, the longitudinal beam coupling impedance is optimized to minimize power deposition in the magnet.

IOP PUBLISHING LTD2019

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

This thesis is dedicated to the study of the magnetization reversal dynamics in compressively strained (Ga,Mn)As and differently functionalized (Ga,Mn)As materials. In the first part the domain wall dynamics of pure (Ga,Mn)As/GaAs materials is in the focus. The changes in the magnetic anisotropy energy landscape occurring as a function of temperature are monitored in detail by different experimental techniques. These measurements provide the necessary information for the identification of the temperature ranges corresponding to different magnetic anisotropy regimes. Knowing the biaxial anisotropy and uniaxial anisotropy dominated regimes the dependence of the domain wall dynamics on the magnetic anisotropy landscape is studied. Critical changes in domain wall alignment observed by Kerr microscopy are found upon changing from biaxial to uniaxial anisotropy. These changes could be partially attributed to the tendency of the system to minimize magnetic free poles at the domain boundaries. To complement the space resolved studies of the magnetization reversal, magnetic time relaxation effects are addressed. In particular the magnetic aftereffect in (Ga,Mn)As/GaAs is studied in detail. The irreversible aftereffect is evidenced as a critical reduction of the domain wall velocity with time under a constant applied magnetic field below coercivity. This time relaxation is tracked as a function of both time and magnetic field. The measurements show that the overall relaxation is composed of two relaxation processes acting in parallel at short and long time scales. By fitting these experimental results the values of the relaxation times of both relaxation processes were obtained. By this modeling also the values of the activation volumes for two independent (Ga,Mn)As materials are estimated. In the second part of this thesis the possibilities of tuning the electrical and magnetic properties of (Ga,Mn)As by volume and surface treatments are explored. As for the volume modification, oxygen species were incorporated into the (Ga,Mn)As films by means of exposure to an oxygen plasma. The incorporation of oxygen was evidenced by depth profile X-ray photoelectron spectroscopy. This treatment is found to weaken the ferromagnetism, visible as a reduction in the Curie temperature, and to hinder the electrical transport evidenced as an increase in the electrical resistance. In agreement with theories accounting for the hole mediated ferromagnetism in (Ga,Mn)As this behaviour was related to a hole compensation mechanism arising from the presence of oxygen impurities sitting in interstitial positions of the (Ga,Mn)As Zinc blende structure. In the last part, the modulation of the electrical and magnetic properties via surface functionalization is presented. The effects produced by the adsorption of molecular species on (Ga,Mn)As are similar to those found after oxygenation, namely a strong weakening of the magnetic properties and an increase in the values of the electrical resistance. However, in this case the distinctive feature is provided by the possibility of regulating the hole quenching effect by exploiting the additional degree of freedom provided by the chemical properties of the adsorbates. The adsorbate chosen for these experiments are dye molecules that absorb light in the visible range, in this way allowing for a light modulated interaction with the substrate. Using this concept is was possible to observe a modulation of the ferromagnetic transition temperature, the coercive field and the electrical resistance upon illumination. These observations provide a proof of principle for the realization of photo-sensitized (Ga,Mn)As devices where the magnetic properties can be regulated by light.

EPFL2011

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

EPFL2011