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# Transverse synchrotron light monitors - BSRT

The LHC is equipped with two synchrotron radiation systems, one per beam, used to measure the transverse bunch distributions. The light emitted by a superconducting undulator or by a the edge of dipole magnet, depending on the beam energy, is intercepted by an in vacuum extraction mirror and sent through a viewport to the synchrotron radiation telescope (BSRT). The first version of the telescope, used from 2009 to mid 2012, was based on spherical focusing mirrors in order to minimize chromatic aberrations. However, this required a very complicated delay line in order to switch the focus between the two different light sources as a function of beam energy. A new system based on optical lenses was designed and installed in mid 2012 in order to simplify the optical line and thus reduce misalignment and focusing errors.

The BSRT monitor images the synchrotron light generated by beam particles traversing two supeconducting magnets (an undulator and a dipole) located one after the other. From the LHC injection energy (450 GeV) to about 1.5 TeV, the radiation generated by the undulator is in the visible range, and shifts to the X-rays for the top energy. Particles traversing the dipole emit light in the visible range from 1.2 TeV onwards. The BSRT extraction mirror located 27m downstream of the magnets collects the light coming from the undulator and the dipole, as sketched in Fig. 1 . Therefore, the imaging system must focus objects at different distances depending on the beam energy. A non proper focusing results in a blurring of the image.

Figure 1 - Light sources of the LHC BSRT

The BSRT light extraction system is based on a retractable mirror in vacuum, that re-directs the intercepted light through a view port to an optical table located below the beam pipe. The first mirror on this table is motorized and is used to adjust the light steering for the following elements, in order to cope with beam position drifts and any fluctuation/vibration of the table. The table is also equipped with the optics for reconstructing the transverse beam profiles. Those are acquired by an intensified camera which allows the measurement of a single pilot bunch of 5 109 charges at LHC injection energy in a single turn. Given the large distance needed between the light sources and the extraction mirror to separate the photons from the beam, a two-focusing element system is necessary to achieve the imaging on a reasonably short table and with the desired magnification (camera acceptance). At the LHC top energy, the resolution of the system is limited by optical diffraction, and as this is proportional to the wavelength, a 400 nm bandpass filter is placed in front of the camera. At two different stage, the light is splitted in order to send part of the radiation to two other detectors for longitudinal diagnostics (the abort gap and the longitudinal density monitors). A laser beam following the same path as the synchrotron light in the beam pipe allows the optical elements, including the extraction mirror, to be precisely aligned. In addition, a light source and a calibration pattern can be used on the laser line to verify the focusing and optical magnification of the system. Figure 2 shows the layout of the BSRT optics.

Figure 2 - Optical layout of the LHC BSRT

# Calibration procedure

• Inject bunches of decreasing emittances. This is achieved by inserting BTV screens in TT10. The order of screens decreasing emittances is: TT10.BTV.1003 & TT10.BTV.1001, TT10.BTV.1003, TT10.BTV.1001, No screen.
• Perform wire scans all along during the calibration fill.
• Scan the position of the L2 lens (combined scans of L2 and camera not used anymore as the camera is kept in the far position)
• Bunch sizes from wire scanners are obtained by a linear fit vs. time: sigma_i(t)= a_i*t+b_i
• Plot sigma_bsrt_i^2 vs. sigma_ws_i^2, and calculate a linear fit
• The BSRT scale is obtained from the slope of the fit, while the line spread function from the intercept

$$\sigma_{BSRT} = \sqrt{\varepsilon \beta_{BSRT}}$$

$$\sigma_{WS} = \sqrt{\varepsilon \beta_{WS}}$$

$$\sigma_{BSRT}^2 = s^2 \sigma_{BSRT_{pixels}}^2 - LSF^2$$

$$\sigma_{BSRT_{pixels}}^2= \frac{1}{s^2} \frac{\beta_{BSRT}}{\beta_{WS}} \sigma_{WS}^2 - \frac{LSF^2}{s^2}$$

# Hardware documentation

Telescope layout STEP file

The BSRT contains a large number of optical elements: mirrors, lenses, filters, light splitters associated with precise mechanical mounts. Many of these mounts contain motorised adjustments, based on stepping motors, that can be controlled remotely.

Typical remote controlled devices are linear translation stages, lab-jacks and Gimbal mirror mounts. While most of these opto-mechanics components are commercial product we have also developed a pneumatic actuator system that we use to control optical filters and lens mounts. In the LHC the stepping motor controller is hundreds of meters away from the devices it controls. For this reason even the commercial products have to be adapted as they are usually not suitable for such long control cables. The most common problem is the control of the limit switches due to the interference from the motor power.

Until 2017 a MIDI Engineering stepping motor controller-driver was used to control 64 motors. In 2017 this obsolete system has been replaced by in house solution based on a VME board and commercial power drivers (Phytron ZMX+). BStepMotorVME FESA class documentation

Beta at BSRT (updated 23.04.2016 E.B.)

B1 450 GeV H= 205.5, V= 320.0

B1 6.5 TeV H= 214.0, V= 328.0

B2 450 GeV H= 191.5, V= 387.8

B2 6.5 TeV H= 205.3, V= 344.0

Beta at Wire scanners (updated 21.06.2015 E.B. source Optics Display)

B1 H= 194.02, V= 368.07

B2 H= 188.67, V= 411.68