Resolution of a high performance cavity beam position monitor system

Abstract

International Linear Collider (ILC) interaction region beam sizes and component position stability requirements will be as small as a few nanometers. It is important to the ILC design effort to demonstrate that these tolerances can be achieved -ideally using beam-based stability measurements. It has been estimated that RF cavity beam position monitors (BPMs) could provide position measurement resolutions of less than one nanometer and could form the basis of the desired beam-based stability measurement. We have developed a high resolution RF cavity BPM system. A triplet of these BPMs has been installed in the extraction line of the KEK Accelerator Test Facility (ATF) for testing with its ultra-low emittance beam. A metrology system for the three BPMs was recently installed. This system employed optical encoders to measure each BPM's position and orientation relative to a zero-coefficient of thermal expansion carbon fiber frame and has demonstrated that the three BPMs behave as a rigid-body to less than 5 nm. To date, we have demonstrated a BPM resolution of less than 20 nm over a dynamic range of +/-20 microns. THEORYWhen a bunch transits a cavity BPM, the field of the bunch excites the eigenmodes of the electromagnetic fields within the cavity. The amplitude of the TM 110 mode has a linear dependence on the transverse offset of the beam rela- * This work was performed under the auspices of the U. † walston2@llnl.gov tive to the electrical center of the cavity; the phase depends on the direction of the offset. The TM 110 mode also has a linear dependence on both the angle of attack and angle of obliquity (collectively referred to as "tilt") of a finite length bunch relative to the z-axis of the cavity. This is discussed in more detail elsewhere [1,2,3]. The intrinsic resolution of a BPM is limited by the signal to noise ratio of the system: The signal voltage of the BPM is determined by the beam's energy loss to the TM 110 mode and by the external coupling of the waveguide; the overall noise of the system comes from thermal noise as well as contamination from the symmetric TM 010 mode. It has been estimated that an RF cavity BPM could have a resolution below one nanometer [4]. EXPERIMENTThis experiment employed three identical cavity BPMs [5]. Single bunch extractions from the ATF ring of typically between 6 and 7 × 10 9 e − at an energy of 1.28 GeV were used for our tests. The machine repetition rate was ∼ 1 Hz.Because the beam passed through the apparatus in a straight line, the beam's position in BPM 2 was related in a linear way to the beam's positions in BPMs 1 and 3. BPM resolution was determined by measuring the residual -that is the difference between the position of the beam as measured by BPM 2 and the predicted position as calculated from the beam's parameters measured by BPMs 1 and 3. The coefficients used to calculate the beam's position at BPM 2 were determined by regressing the beam's y position measured by BPM 2, y over many events. The resolution was then proportional to the standard dev...

Design and cold test of a rectangular cavity beam position monitor

“…The most commonly used CBPMs now are cylindrical ones, which are easy to design and make. Many labs have designed cylindrical CBPMs whose resolution have reached micron and even sub-micron levels [1][2][3][4][5][6][7][8]. However, because of processing limits, for a cylindrical CBPM, the transverse section will always be an elliptical one.…”

Design and test of SX-FEL cavity BPM

“…The output signal will have a large amplitude and a short time at the same time. There are many CBPM's experiment reports with nanometer level resolution [1][2][3]. Considering the bunch-by-bunch beam position diagnostic, the high Q cavity will cause signal mixing and can't give an adequate position resolution.…”