ALIGNMENT AND VIBRATION ISSUES IN TeV LINEAR COLLIDER DESIGN G. E. FISCHER Stanford Linear Accelerator Center Stanford University, Stanford, CA Abstract The next generation of linear colliders will require alignment accuracies and stabilities of component placement at least one, perhaps two, orders of magni- tude better than can be achieved by the conventional methods and procedures in practice today. The magnitudes of these component-placement tolerances for cur- rent designs of various linear collider subsystems are tabulated. In the micron range, long-term ground motion is sufficiently rapid that on-line reference and mechanical correction systems are called for. Some recent experiences with the upgraded SLAC laser alignment systems and examples of some conceivable solutions for the future are described. The so called “girder” problem is discussed in the light of ambient and vibratory disturbances. The importance of the quality of the underlying geol- ogy is stressed. The necessity and limitations of particle-beam-derived placement information are mentioned. INTRODUCTION The surface of Mother Earth, on which we wish to build well aligned accelerators, is not Terra Firma but is constantly in motion. The purpose of this talk is to review some facets of this motion, to examine whether they present fundamental limitations and to estimate whether the problems encountered can be solved by sound engineering in the foreseeable future.I submit that, in the case of the next linear collider, the answer to this question is“Yes,” but it will take a good deal of effort and money. The earth is a glob of material held together by its own gravitation and subject to a variety of internal and external forces.Portions of its interior are thought to be liquid (will not support shear waves), and its relatively thin, elastic, wrinkled and, in places, broken crust swims around on an outer mantle that can hardly be called an engineering material. Peoples who live along the Pacific rim have become quite used to this concept, but I nevertheless thought you might be interested to know that Japan and California, separated by almost one billion centimeters, are approaching each other at the rate of between 2 and 5 cm/year depending on where and how this measurement is made’. I might add that California is drifting away from Europe at a similar rate. * Work supported by Department of Energy contract DE-AC03-76SF00515 -275- About ten years ago, when we first began to think about the bizarre world of ground motion in relation to the construction of linear colliders, we became alarmed about motion in the frequency band ~0.1 to 50 Hz. The reasons for concern in specifically this domain were (a) that high frequency jitter appeared to present difficult engineering problems and (b) that disturbances at slower rates. that is with wavelengths greater than site dimensions, would move the machine monolithically causing no relative motions between focussing elements and hence produce no deleberious orbit distortions. In the intervening years we have learned to live with microseismic disturbances. For example: The magnitudes and signatures of natural and man-made sources were identified2. From such considerations project site tolerance recommendations could bemade3.Common sense solutions to local groundnoise abatement followed. Great strides were made in the understanding of how circular machines react to random plane wave excitation through the extension of the pioneering work of Aniel and Laclare4 and by Rossbach5. The importance of resonances in magnet supports was quantified by Chou6 and more recently by Rossbach7. Vibration isolation of final focus elements by means similar to those used by workers in Gravitational Wave detection was discussed by Ash8, and recently we have heard of the Active Alignment System at KEK9 and of the commercial vibration isolation system “TACMI”10. For such reasons and the fact that the general level of consciousness regarding vibration problems has been raised, I wish to turn to another region of the motion spectrum: — namely slow drift. We will see that in the world of micron-level alignment, these problems can become pernicious. COMPONENT PLACEMENT TOLERANCES In this section we will attempt to define the general level of alignment tolerances likely to be found in various linear collider concepts, designs and subsystems. I say it this way because tolerance setting is an unpopular and odious task — very often an ongoing debate between the desired and the achievable that may still be carried on even after the collider is in operation when effects, not dreamed of at conception, rear their ugly heads. All values listed in the table below were kindly provided with this caveat in mind and are meant, at this time, to be only guidelines to workers developing alignment techniques. I should now try to explain what I mean when one says “alignment tolerance”. There has been some confusion about this term. I hope you will bear with me for the following, somewhat pedantic, definitions, Quite often simple calculations or computer simulations are performed in which the effects of offsetting any single element from its correct “on-axis” position are measured in terms of, say, a 10% loss of luminosity. Since making luminosity is the purpose of a collider, this seems like a not unreasonable way to proceed and immediately gives the relative sensitivities of element placement. For reasons that will become clearer later, let me call such displacements incoherent jitter tolerances. There are, however, at least two serious conceptual flaws with this method. No ac- -276- count is taken of the facts that in a real machine (a) all elements (not just one at a time) are out of place simultaneously, and (b)elaborate orbit and tune correction systems are applied to recover the lost luminosity. Permissible alignment errors (ei- ther systematic or random) under these more realistic assumptions are much harder to estimate because they require an understanding of all conceivable interactive ef- fects that go into a simulation and a detailed scenario of tuning and correcting. Let us define survey and alignment tolerances to be those values of placement error which, if exceeded, lead to a machine that is uncorrectable - with its unacceptable loss of luminosity.In recent years, experience with higher order optical systems has shown that alignment tolerances derived in this manner tend to be about an order of magnitude looser than those derived for one component at a time. The reason for choosing the name jitter is now more apparent. Jitter tolerances are the magnitudes of errors that are NOT amenable to compensation by either static or dynamic (including BNS)methods. To recapitulate: It is the loosest survey errors that we are now looking for, not those tolerances associated with keeping an already operating machine running at peak luminosity. TABLE I. Estimated Transverse Component Survey and Alignment Tolerance (pm) * System\ Project CLIC*(CERN) VLEPP+ (USSR) JLCt(KEK) TLC?‘(SLAC) Injector Accelerator: 200 standard Damping Rings: Quads (Horiz.) 100 100 Quads (Vert.) 50 30 Compressor #1: 50 ? Intermediate Linac: R.F.Structure 50 Magnetic Foc.(H) 50 Magnetic Foc.(V) 50 Compressor #2: 50 ? Main Linac: R.F.Structure 10 .3* 50 10 - 50 Magnetic Foe.(H) 1.7 .03* 10 100 Magnetic Foe.(V) 1.0 .03* 10 20 Final Focus Elements (V) 5 10 Final Focus Elements (H) 5 30 Final Lenses (V) 5 2nm* Final Lenses (H) 5 200nm* Beam Analysis before dump 5 ? -277- * 1 (I value of the population unless otherwise indicated, jitter values are denoted with an * . * Provided by H.Henke. t Provided by V.Balakin, for a bunch population of 2 x 1011. 1 Provided by K.Takata, Values are tentative and are meant to be maximum deviation from central orbit. w Provided by R.Ruth. In the above table, distinction is drawn between the linac’s RF structure and its Magnetic lattice elements; both of which may focus and steer. Further, for those groups who engage in asymmetric beams, vertical and horizontal tolerances may be substantially different. This is of import for those wishing to align, since techniques in these planes may be quite different. Finally, there is clearly something very special about the final lenses. REFERENCE SYSTEMS An examination of periodic realignment records of several large accelerators [Linac, PEP (SLAC), PS, SPS (CERN) and others] shows that, on the average, tunnel floor motion is at least about 50~ per 6 month interval. Much higher rates are seen in certain well known locations which are associated with the trauma inflicted on the ground by construction activity resulting in rebound, recompaction etc.. Ground water levels, varying with rainfall, have been shown to cause both vertical and horizontal tunnel displacements11,12. Wesee therefore that in the micron world, the rate of drift, albeit slow compared to conventional vibratory motion, is faster than can be corrected by conventional survey and realignment. We need servo- realignment, and for this we need reference systems more or less independent of the earth! Four such systems are listed below. The Large Rectangular Fresnel Lens System l3 The principle of this system is shown in Figure 1.Divergent laser light is made to impinge on lenses which are inserted into an optical path one by one, and the relative displacements of their images is measured at the far end. The resolution of such a system depends on three terms: (1) the size of the diffraction limited images, w x Xs/D , in which D is the effective width of a lens, (2) the ability of a scanner to locate the centroid of an image, and (3) the optical lever arm (r + s)/r. A 3 km long system of this kind has been used to keep the SLAC linac aligned for 20 years.