Federico Roncarolo

Calibration of centre-of-mass energies at LEP 2 for a precise measurement of the W boson mass

Abstract.The determination of the centre-of-mass energies for all LEP 2 running is presented. Accurate knowledge of these energies is of primary importance to set the absolute energy scale for the measurement of the W boson mass. The beam energy between 80 and 104 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is defined in the NMR model, which is calibrated against precise measurements of the average beam energy between 41 and 61 GeV made using the resonant depolarisation technique. The validity of the NMR model is verified by three independent methods: the flux-loop, which is sensitive to the bending field of all the dipoles of LEP; the spectrometer, which determines the energy through measurements of the deflection of the beam in a magnet of known integrated field; and an analysis of the variation of the synchrotron tune with the total RF voltage. To obtain the centre-of-mass energies, corrections are then applied to account for sources of bending field external to the dipoles, and variations in the local beam energy at each interaction point. The relative error on the centre-of-mass energy determination for the majority of LEP 2 running is 1.2 x 10-4, which is sufficiently precise so as not to introduce a dominant uncertainty on the W mass measurement.The determination of the centre-of-mass energies for all LEP 2 running is presented. Accurate knowledge of these energies is of primary importance to set the absolute energy scale for the measurement of the W boson mass. The beam energy between 80 and 104 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is defined in the NMR model, which is calibrated against precise measurements of the average beam energy between 41 and 61 GeV made using the resonant depolarisation technique. The validity of the NMR model is verified by three independent methods: the flux-loop, which is sensitive to the bending field of all the dipoles of LEP; the spectrometer, which determines the energy through measurements of the deflection of the beam in a magnet of known integrated field; and an analysis of the variation of the synchrotron tune with the total RF voltage. To obtain the centre-of-mass energies, corrections are then applied to account for sources of bending field external to the dipoles, and variations in the local beam energy at each interaction point. The relative error on the centre-of-mass energy determination for the majority of LEP 2 running is 1.2 × 10 −4 , which is sufficiently precise so as not to introduce a dominant uncertainty on the W mass measurement.

Experimental results of the laserwire emittance scanner for LINAC4 at CERN

Abstract Within the framework of the LHC Injector Upgrade (LIU), the new LINAC4 is currently being commissioned to replace the existing LINAC2 proton source at CERN. After the expected completion at the end of 2016, the LINAC4 will accelerate H − ions to 160xa0MeV. To measure the transverse emittance of the H − beam, a method based on photo-detachment is proposed. This system will operate using a pulsed laser with light delivered via an optical fibre and subsequently focused onto the H − beam. The laser photons have sufficient energy to detach the outer electron and create H 0 /e − pairs. In a downstream dipole, the created H 0 particles are separated from the unstripped H − ions and their distribution is measured with a dedicated detector. By scanning the focused laser beam across the H − beam, the transverse emittance of the H − beam can be reconstructed. This paper will first discuss the concept, design and simulations of the laser emittance scanner and then present results from a prototype system used during the 12xa0MeV commissioning of the LINAC4.

Demonstration of a laserwire emittance scanner for hydrogen ion beams at CERN

A non-invasive, compact laserwire system has been developed to measure the transverse emittance of an H- beam and has been demonstrated at the new LINAC4 injector for the LHC at CERN. Light from a low power, pulsed laser source is conveyed via fibre to collide with the H- beam, a fraction of which is neutralized and then intercepted by a downstream diamond detector. Scanning the focused laser across the H- beam and measuring the distribution of the photo-neutralized particles enables the transverse emittance to be reconstructed. The vertical phase-space distribution of a 3 MeV beam during LINAC4 commissioning has been measured by the laserwire and verified with a conventional slit and grid method.

JACoW : Installation and Test of Pre-series Wire Scanners for the LHC Injector Upgrade Project at CERN

A new generation of fast wire scanners is being developed for the LHC Injectors Upgrade (LIU) project at CERN. These will be essential tools for transverse profile measurement with the higher brightness LIU beams and are planned for installation in all three LHC injector rings in 2019. An active period of development and test has resulted in prototype installations in the SPS and PSB rings. This paper will summarise the design and the results of tests to-date.

Beam Instrumentation Performance during Commissioning of CERN's Linac-4 to 50 MeV and 100 MeV

Linac-4, a 160 MeV H linear accelerator is designed to replace the aging 50 MeV proton Linac-2. It will consist of an H source and 45 keV LEBT, an RFQ and 3 MeV MEBT with a chopper, 3 drift tube linac (DTL) tanks accelerating the beam to 12, 30 and 50 Mev, cavity coupled structures (CCDTL) accelerating it to 100 MeV and a pi mode structure (PIMS) bringing it to its design energy of 160 MeV. This paper reports on the commissioning of the DTL and CCDTL with 2 dedicated temporary measurement lines, the first one adapted to the 12 MeV beam while the second one is dedicated to characterize the 50 MeV and the 100 MeV beams. The beam diagnostic devices used in these lines are described as well as results obtained. 3 AND 12 MEV MEASUREMENT LINE The Linac-4 accelerator is being assembled in stages. At each stage the beam is fully characterized, and the measurement results are compared to beam optics simulations. While many diagnostic devices are permanently installed in the machine and will be used for routine operation, dedicated temporary measurement lines were designed to determine beam characteristics after the RFQ at 3 MeV and after the first DTL tank at 12 MeV. Figure 1: 3 and 12 MeV measurement line. The following devices were installed on the 3 and 12 MeV measurement line: • Slit/grid emittance meter [1] (yellow) • 2 Beam Current Transformers (BCTs) (orange) • 2 Beam Position Monitors (BPMs) (light blue) • Spectrometer (green) • Bunch shape monitor (violet) • Laser emittance meter [2] (red) with a diamond detector (blue) The BCTs were the first instruments to see the beam as it passed through the RFQ and the first DTL tank. They have calibration windings into which a precise current is injected after each beam pulse, providing an absolute calibration. BCTs were extensively used to optimize transmission. Transverse Emittance The slit/grid device directly measured the transverse phasespace with the results compared to simulation. Similar measurements were made with the laser emittance meter, where the laser acts as the slit, neutralizing a small part of the H beam [3]. While the remaining H beam was deflected into the spectrometer line, the angular distribution of the neutralized Hs were detected with a diamond detector. Good agreement was found between both measurement techniques, providing important input for understanding the initial phase space distribution of the beam. 50 AND 100 MEV MEASUREMENT LINE Due to the energy-deposition in the slit, the slit/grid device could not be used at energies higher than 12 MeV. Equally the spectrometer could not handle higher energies because of the limited field in the spectrometer magnet. Figure 2: Phase space measured with slit/grid and laser emittance meter at 12 MeV. Proceedings of IPAC2016, Busan, Korea MOPMR026 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T03 Beam Diagnostics and Instrumentation ISBN 978-3-95450-147-2 293 C op yr ig ht