Precision Measurement of Transport Components*

Were~ontbemethodsandmsultsofmagneticmessufemats of the centers and moments of magnetic focusing elements for the Final Focus Test Beam at SLAC. The magneticcentaislocatedbyobserktganekuomotiveface (EMF) generated on a vibrating wire within the magnetic apenme.Itisfoundthatthecentercanbelocatbdwitha precision of a few microns. The multipole coefficients can alsobemeasumdbyusingagridofstretched-wiresweeps,and mapping the timGintegraud voltage throughout the apemue. By fitting directly to this map, the dipole, quadrupole, and sextupole terms of the magnetic field ate extracted The design fields of quadrupoles and sextupoles can be measumd with a precision better than 0.1%. and the resolution of sextupole aberrations of quadrupole magnets is well below design tolerances. This method has been used to pmxxs twenty-five quadrupoles and four sextupoles. Results of these measuremaltsareplesealted .-I; INTRODUCI’ION The Final Focus Test Beam (FFTB) is a transport line designed to test both concepts and advanced technologies for application to future linear colliders. It is currently near completion at SLAC in the straight-ahead tunnel at the end of the linsc. The primary optical elements of the FFTB are a family of quadrupole magnets and a family of sextupole magnets used fur chmmaticity cxrrection. In order to achieve the desired spot sizes at the focal point (ox = 1 pm, cry = 60 nm), tolerances -on alignment, &sign field strength, and aberration field strength must be met [ 11. We describe below our use of a stretched-wire t chnique for measuring each magnet’s compliance with its tolemnces, and discuss our experiences with the appfmmstodate. II. CONCEPT OF THE METHOD Figure 1 below depicts a schematic of our apparatus. A computer-controlled Coordinate Measuring Machine (CMM) is used to establish a coordinate system parallel to the mecbanicalaxisofthemagnet,withx=y=Oontheaxisand z=Oatthelongitudinalmidpointofthemagnet.Athinwire which has been stretched lengthwise down the magnet apertute is then made parallel to the longitudinal axis to _ withinafewtensofmiawadkns The technique for measuring the magnetic center of the magnetfehtivetothemechanicalcenterhasbeendesuibedin detail elsewhere [21. In brief, the wire is set to vibrate at its resonant freqmy to. and the rewhing EMF is monitored on aspectrumanalyzu.BynullingtheEMFatthedrivingfrequency in horixontal and vertical planes, the magnetic enter can be located and measured relative to the mechanical . cfoadua system, via theCMM. *Work Supported by the Department of Energy Contract DEACQ3-76SFXKlS15 Inor&rtomeasurethemagneticmomentsoftbemagnef thewireisswq%byaknownhorixontalaverticaldistance (S, or %, respectively) about a point (x,y). The expansion to second (sextupole) ordex of an arbitrary beambne component’s magnetkf&liX Bx = Box + Gny + GSX + 2Snxy + Ss(xZ-y2) , By = Boy + Gnx Gsy + S&‘-y’) ~SSXY , (1) where Box and Boy are the horixontal and vertical field components at the migin, respectively; Gn is the normal quadrupole (preserving midplane symmetry); Gs is the skew qutulrupole (violating midplane symmetry); and Sn and Ss are the normal and skew sextupole components, respectively 131. Whentkwireissweptthn>ughthemagneticfieldthetimeintegrated EMF across tbe wire is a function of the center point of the motion and of the total distance traveled in each dift!CtiOll: Ivd@L { S, [Boy+Gnx-Gsy+Sn(x2-y2)-2Ssxy+Sndx2/12] dy [Box+Gny+Gsx+2Snxy+Ss(x~y2~S~y2/121 ) . 0’ where L is the effective length of the magnet. The wire is moved to locations throughout the aperture of the magnet, and swept by known amounts, vertically and horizontally. The potential difference across the wire is monitored by an integrating voltmeter, which allows direct measurement of the timeintegrated voltage as a function of position and sweep vector. By fitting the mapped measurements directly to the equation above, it is possible to extract the coefftcients of the magnetic expansion directly. Note that Eq. (2) is only correct up to an overall sign, which is influenced by the connection between the voltmeter and the wire, among other variables. t KY -tm I TUmonblled I . . . . . . ..w-.a..-......----11-w ..I cwnRoanT-2oio.5~c u -1 Figure 1. Schematic diagram of apparatus, including Stage Controller (SC), Integrating Digital Voltmeter (IDVM), Spectnun Analyzer (SA), frequency generator (FG) and switch box (SB). Not shown: mounting table and CMM. -Presented at the Particle Accelerator Conference (PAC 93), Washington, DC, May 17-20, 1993 III. DETAILS OF THE APPARATUS As shown in Figure 1, the principal elements of the measurement apparatus are the wire, the oscillation drivers, the stages which move the wire, the integrating voltmeter, the mounting table upon which the apparatus rests, and the magnets themselves. Many of these components have been described in detail in Reference [2], so for these only supple mentary infotmation is given.