Resonant BPM for continuous tune measurement in RHIC

Abstract

A movable Beam Position Monitor (BPM) using shorted stripline Pick-Up Electrode (PUE) elements has been resonated using matching stub techniques to achieve a relatively high Q resonance at about 230MHz. This PUE has been used in a feasibility study of phase-lockedloop tune measurement [1], using a lock-in amplifier and variable frequency generator to continuously track betatron tune in RHIC, as well as to observe Schottky signals of the Gold beam. The approach to providing a high Q PUE for difference mode signals, simulation studies, and the results of initial tests will be presented. BACKGROUNDMonitoring tune during ramp -up of RHIC using a high Q resonator (Q=4000) to observe Schottky signals at 2GHz requires tracking and averaging FFT analysis of signals that are sweeping over a large range of frequencies. An alternate approach was conceived that would utilize a BPM with a resonant difference mode, and a damped sum mode. By resonating the difference mode, the BPM can be made more sensitive to transverse beam motion, and the tune can be extracted from the betatron oscillations by analyzing the beam spectrum.To accomplish this a Q near 100 is required to develop sufficient signal sensitivity. RESONATING THE BPMA resonant frequency near 230MHz was chosen to provide sensitivity to the Schottky spectral lines near a low spectral power frequency for the beam. It is necessary to resonate the BPM in the difference mode. High Qs are required to obtain reasonable signal levels. Utilizing a discrete component tuned circuit results in Qs that are lower than required. An alternate technique was investigated utilizing the reactive characteristics of a transmission line. To obtain high sensitivity a very high impedance at the pick-off nodes of the shorted strip line PUE is required. Assuming these nodes "open", a half wave cable would provide an "open" at the other end. Therefore, placing a half wave cable from one PUE to the opposite PUE, could resonate the difference mode of a BPM. It would then become a matching problem to extract power from the PUE nodes. SimulationTo aid in the design of a resonant BPM, a simulation was developed using ORCAD and PSpice. The simulation schematic is shown in Figure 1. Lossy transmission line models were used to simulate the coaxial cables and the BPM stripline. The characteristics of the coax were taken from an Andrew® catalog for Heliax® cables [2]. The resistance was modified for frequency effects by estimating the losses per 100 meters at about 250 MHz. An equivalent cable resistance was then calculated and used as a resistance parameter per meter (0.98 Ohms). To simulate the response to the difference mode, two current sources were used to inject signals into the stripline termination points. These were set to provide differential excitation in an AC analysis. To investigate matching impedance, two current sources were used at the Hybrid input ports and set to provide differential excitation. The AC sources were set to +/-1 Amp thereby yielding output voltages that read direc...

Schottky measurements during RHIC 2000

“…Solutions to the problems arising out of the non-stationary spectrum were also hampered by limitations in the interface in the FFT analyzer, which permitted transfer of spectra at a maximum rate of about 1Hz. All of these problems have been effectively dealt with in a 238MHz Schottky/PLL/Tune Feedback system, which is described briefly at the end of this paper and in more detail elsewhere in these proceedings [2,3].…”

“…For a variety of reasons, including the limitations discussed earlier in this paper, it was decided to resonate a BPM [3] at about 230MHz. Figure 5 shows a horizontal spectrum obtained from that BPM.…”