Structured Laser Beam  (SLB) for alignment of particle accelerators

Introduction to SLB

Among many particle physics institutes in the world, Conseil Européen pour la Recherche Nucléaire (CERN) has a long and fruitful history reaching 1954 [1]. The projects performed at CERN are challenging not only for theoretical physics knowledge but also for engineers. The survey engineers working in such a specific environment must adapt their tools to meet high requirements imposed by a technical specification. The specification becomes even more demanding when it comes to new ambitious plans concerning the Future Circular Collider (FCC). The FCC is a research infrastructure to host the next generation of particle accelerators. The accelerator will be the biggest circular collider in the world measuring about 91 km in circumference. This study covers a high-energy lepton collider (FCC-ee) as the first operational mode of the new facility [2]. Innovative geometric measurement techniques should follow the pioneering accelerator layout of FCC.

The measurement techniques for accelerators vary from the ones used in land surveying and topography [3]. However, they include basic approaches such as the polar method and geometrical levelling, which remain useful in some cases. The principal limitation is a length of a facility and expected accuracy that exceeds a typical surveying scenario. The limited space and high radiation introduce additional constraints [4]. The dedicated systems perform better, reducing the demanded workforce, limiting the total ionizing dose and speeding up data acquisition [5]. Generally, the techniques used for accelerator adjustment refer either to gravity or to a straight line.

Gravity is a common reference for measurements whether it is an inclination measurement with respect to a gravity vector or commuting vessels referring to an equipotential gravity surface. The measurement of inclination is often a part of more complex systems [6]. At CERN, inclinometers were planned as a part of the metrological network for the Compact Linear Collider (CLIC) [7]. Nevertheless, the most important system exploiting the gravitational field of Earth at CERN is Hydrostatic Leveling System (HLS) [8]. The HLS constitutes the main vertical reference in the interaction points for the High Luminosity Large Hadron Collider (HL-LHC) [9] and is used at many laboratories around the world such as ESRF [10], ANL [11]and SLAC [12]. The repeatability of hydrostatic levelling systems using the capacitive technology reaches 2 μm and the accuracy of 10 μm. Although successful use of hydrostatic levelling, the accuracy is limited by the local geoid model [13] and can be susceptible to earth crust movements [14]. In addition, the HLS is strongly dependent on its medium, for example, water which is the subject of waving and evaporation.

Another group of systems are these which refer to a line in a horizontal or vertical plane. The line can be established in space by a physical object or an optical axis. The measurement of offsets to a reference line is one of the oldest measurement techniques and was already known in ancient Egypt [15]. Two points at the extremities define unambiguously the reference line whereas measured points between them remain out of the line. By combining multiple lines, it is possible to define an entire survey network, where estimates and uncertainties are defined based on the least square model.

In the past, a physical reference line was often established by rope. Nowadays, stretched wires are used. The wire-based manual method of ecartometry was used to measure the radial offset of magnets at CERN for many years [16]. For some specific areas of modern accelerators, permanently stretched wires were introduced. The Wire Positioning System (WPS) exploits the capacitive sensors for automatic offset measurement with respect to the stretched wire [17]. Another example of using stretched wire for alignment can be found at PAL-XFEL. The resolution of the conductive wire measurement is 0.1 μm and the drift of the sensor is not worse than 0.4μm±10μm per month [5]. The resistance and weight of the wire allow to stretch it over 140 m. WPS sensors allow measurements in two directions horizontal and vertical thanks to the catenary reconstruction [18]. Mechanical wire reference systems are the preferred solution at CERN, but they are susceptible to the air condition swinging a wire and thermal expansion. For non-metallic wires, expansion due to humidity occurs. The wire is also the subject of a sag and has a certain weight that limits its maximal length to 500 m and reduces the accuracy [19].

The optical reference line has been used for surveying for centuries. It is possible to distinguish passive and active optical systems. Passive systems based on reflected environmental light have been the main surveying technique and remain in use today. Although, in the context of accelerator alignment, the active methods are very popular. The active methods use any kind of focused light, but usually, it is a laser beam. Besides atmospheric refraction, which is the most significant problem for optical systems, the divergence of light is the largest obstacle to long-distance propagation. To overcome divergence, numerous laser systems were proposed.

The principal way to mitigate the influence of the divergence is to use a lens focusing the laser light on a specific focal point. Since a lens with a long focal distance reaches significant sizes and prices, diffraction plates or Fresnel plates have been used to focus the pattern at many institutes. The precursor of laser methods is the SLAC laboratory where the Fresnel plates were used for the alignment of the accelerator in 1966 [20]. Similar solutions were proposed at KEK [21], CSNS [22] and CERN [23]. The inconvenience of focusing a beam on a specific point is that a Fresnel plate must be dedicated to that specific target. The displacement of the target in the longitudinal direction causes an unsharp beam spot on a detector and a decrease in accuracy.
Another method exploiting the diffraction phenomenon is the Poisson pattern [24]. Poisson pattern systems consist of measuring light diffracted by the physical sphere attached to the object of monitoring. The pattern observed at the end of a measured line is a narrow central spot. The solution was introduced at DESY [25], SLAC [26] and ANL [27]. The Poisson patterns allow the monitoring of multiple objects simultaneously by one detector. Unfortunately, the number of observable points is limited to a few Poisson spheres. Moreover, the detection of the pattern is a complicated assignment due to overlapping diffraction patterns.
The Airy disk is also a diffraction pattern used in accelerator alignment. It emerges by passing the light beam through a circular aperture. The pattern can be propagated for long distances and was used at SPring [28] and KEK [29]. The system is very useful to reduce beam divergence up to 0.25 mrad, but it appears without diffraction only 20 m after the generator, which is relatively far away. In addition, it demands a precise generator alignment and a perfect iris geometry to produce a symmetrical pattern.

Besides diffraction patterns, there were attempts to use a simple gaussian beam at CERN. A gaussian beam was used by Quesnel [30], in collaboration JINR [31] and the context of the CLIC study [32]. The biggest disadvantage of using a gaussian beam is high divergence. However, it can be partially mitigated by a beam expander [32]. The divergence does not allow the use of a Gaussian beam for long-distance applications. The answer to this drawback might be found while looking at the properties of non-diffractive laser beams.

The non-diffractive laser beams are an important tool for a short-distance alignment of optical systems [33]–[35]. The Bessel beam is one of the most known non-diffractive beams that is used for alignment, but the main limitation is the maximal length of propagation not exceeding 20 m [36]. For longer distances, the Structured Laser Beam (SLB) was proposed as a new paradigm for creating a non-diffractive laser beam [37]. The optical intensity profile of SLB is similar to that of a Bessel beam. The SLB has a narrow central spot of high intensity surrounded by concentric circles. Thanks to the small divergence of a central spot, experimentally confirmed 10 μrad, it can be used for the alignment of accelerators.

To take advantage of the SLB, common cons of laser alignment systems should be addressed. The main obstacles are atmospheric refraction, multipoint measurement adaptation and techniques integration.

Atmospheric refraction is the prior obstacle to a long-distance optical alignment because it influences an alignment reference. The alignment assumes the offset measurement with respect to the straight reference line. For optical systems, that reference line is a line defined by electromagnetic radiation rays. In the atmosphere, the rays get bent due to differences in the local refractive index of air. To mitigate the effect of refraction the vacuum pipe was introduced [38], with the pressure of around 1 Pa. Other institutes followed pioneering works at SLAC installing vacuum systems at KEK [39], DESY [40] and NIKEF [41]. Although the low pressure is very effective to reduce the refraction influence, it induces several issues connected to the pumping procedure and the mechanical link between a detector in the vacuum and an aligned component. The answer could be refraction modelling known from land surveying, but the majority of models are adapted to surface work. Furthermore, the atmosphere in the tunnel is a dynamic system and should be monitored in real-time to gain accuracy. In general, the vertical alignment component is more sensitive to changes in refractive index than the horizontal one because of air convection [42].

The multipoint measurement adaptation allows the acquisition of a position of more than a single object between two points defining the beginning and the end of a reference line. There are two approaches to multipoint measurement. The first approach is a physical shutter that consists of a detector/target. In the past, the detector was inserted end extracted manually [20]. Later, there were introduced automatic systems for quick and firm replacement of detectors/targets [43]. The solution based on shutters is the most intuitive and simplest, but it prevents observing all points at the same time thus it reduces the acquisition frequency of a system. The mechanical reproducibility of a shutter position remains unaddressed however, that can be mitigated using encoders or photogrammetry [32]. The opposite approach is to use an optical system to detect the laser spot without stopping its further propagation. These systems are usually based on beam splitters [44]. Even though the modern pellicle beam splitters reduce changes in a ray path, they still play a role for a long distance [45].

To assure the metrological monitoring of an accelerator, a variety of techniques might be integrated into the machine setup. The gravitational and reference line techniques have to be designed and connected in a way allowing determining all necessary degrees of freedom with a given accuracy. An example of such a layout is an alignment system for CLIC [7]. The important part of designing the complex system is coordinate transfer and tolerances connected to it [46]. An optical reference line gives huge advantages thanks to the long-range propagation, but other techniques will be complementary to the weaknesses of a laser system. The techniques mentioned in this text the HLS, the WPS, inclinometers but also Frequency Scanning Interferometry (FSI) [47] or BCAMs [48] might be integrated to assure an optimal solution.


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