The search for a low α is one of the lines of research being pursued by the Source Division of SOLEIL. What does that mean? The purpose is to reduce the length of the electron packets circulating in the storage ring. This length, σs, is dependent on several parameters (see boxed section).
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σs : Length of electron packets circulating in the ring
E : Energy of electrons
VRF : Accelerating voltage supplied by the RF cavities
ωs : Synchrotron frequency
α : Compression factor of moments
The factor α is an optical characteristic of the storage ring. It makes it possible to calculate the trajectory of the electrons on an orbit (L) according to their deviation from the nominal energy (E) : (ΔL/L) = αx ΔE/E. When α is small on machines such as SOLEIL, the second-order terms must also be examined: (∆L/L) = α1 x ΔE/E + α2 x (ΔE/E)2 |
Packet length is thus proportional to the square root of α, but only for packets not containing too many electrons (when there are many electrons in a packet, the interactions between electrons cause an extension of packet length and the equation shown in the box is no longer valid).
For experiments using the temporal structure of synchrotron radiation, there is a great advantage to obtaining shorter electron packets. For experiments of this type, which provide data for kinetic and dynamic studies, the duration of the light pulses (related to the length of the electron packets) is crucial. 'It can be compared to the pause time in photography', explains Laurent Nadolski, machine physicist at SOLEIL; 'the more this time is shortened, the easier it is to observe brief phenomena'. This demonstrates the interest of a low α for the acquisition of more precise temporal information, and in particular, for the observation of very fast chemical reactions.
With the ring at its standard setting, α1= 4.4 10-4, and α2= 4.5 10-3, and the length of a packet of electrons circulating in the SOLEIL ring is 20 ps rms for an RF cavity voltage of 2 MV (i.e. a length of 6 mm rms). In order to reduce the compression factor of the moments, α1, and therefore the packet length, the parameters of some of the magnets through which the electrons circulate must be modified; these magnets are the quadrupoles and sextupoles. These 'magnetic lenses', which are responsible for focusing the beam, number 280 in the ring (160 quadrupoles and 120 sextupoles), and are divided into several families. The Machine Physics group therefore aims to find the combination of parameters that will reduce the value of α1 without adversely affecting the other beam characteristics (e.g. dynamic aperture, which affects the injection efficiency and beam lifetime whilst maintaining low emittance).
This puzzle was solved by Maher Attal in the context of his doctoral research, and was tried out on the storage ring on 6 November last year. Maher is a physicist working on the SESAME synchrotron project in Jordan whilst preparing a doctoral thesis at the University of Orsay under the direction of Amor Nadji. The modifications suggested by Maher – which would involve disconnecting certain magnets to reverse the direction of the supply current – made it possible to obtain a new value of α1 which was reduced by a factor of 20, whilst maintaining good beam dynamics. The length of the low-current packet is thus reduced from 20 to 4.8 ps rms.
Figure 1: The camera records the impact of photon beams due to the curvature of the trajectory of the three electron beams at the output of one of the storage ring dipoles.
Beyond this result, which is very interesting in its own right, an exceptional phenomenon was observed during testing on 6 November, which was 'a world first, to the best of my knowledge' according to Laurent.
Three electron beams were stored at the same time in a single radiofrequency period. Figure 1 clearly shows three beams with distinct horizontal positions. The cumulative current for the three beams reached 32 mA with a lifetime of 15 h and 1/4 of the ring (containing 312 packets of electrons instead of the usual 104 packets).
These beams do not have the same energy; one had the nominal energy, the second had a higher energy (approximately +4%) and the third had a lower energy (approximately -1.8%). These different energy values were able to coexist in the ring thanks to another characteristic of the machine: energy acceptance, defined as the maximum acceptable energy difference for an electron to remain stored in the ring. This parameter is very high in SOLEIL (approximately 6%) compared to other synchrotrons. Laurent adds: 'A high energy acceptance reduces electron loss due to the Touschek Effect'.* The designers of the machine made this choice in order to increase the lifetime of the electron beam, to provide more stability for users of the beamlines.
To summarise (and simplify): this world first is a technological feat, made possible by the characteristics of the machine—particularly its great energy acceptance.
During this trial phase, various tests were performed and it was possible to reproduce the three beams during several consecutive reinjections into the ring; it was therefore not just an isolated occurrence! The machine physicists were not able to play with the three beams as long as they would have liked, however, because other machine adjustment tasks were scheduled for that day, and they had to return to the initial quadrupole and sextupole parameters to resume 'classic' operation.
Following this trial run, the Machine Physics group is planning to repeat and refine the experiment in January during one or more eight-hour beam sessions. All these experiments were made possible by the involvement of the operations, power supply, and diagnostic groups, as well as the streak camera team.
And the effort to further reduce electron packet length goes on, with the goal of reaching a packet length in the region of 1 ps rms (0.4 mm rms).
* Touschek Effect: collisions between electrons which induce electron energy changes.
