Published at PAC 93, Washington. A VXI/LabVIEW-based Beamline Tuner

Willem Blokland
Fermi National Accelerator Laboratory* P.O. Box 500, Batavia, IL60510USA

Abstract

Ageneral purpose beamline tuner is being developedto reduce betatron oscillations resulting from missteering during beam transfer. The tuner is based on VXI instruments con- trolled by a LabVIEW program running on a Macintosh com- puter. VXI digitizers take turn-by-turn data from beam posi- tion monitors followed by an analysis of the data in the time- and frequency- domains. The results, the phase and amplitude of the betatron oscillations, arecommunicated from LabVIEW to the control system over a tokenring network. An application program at a control console calculates the re- quired changes in the correction elements from the phase and amplitude to reduce the oscillations. The beamline tuner is self-contained and easy to adapt to other beamlines. Early re- sults indicate that the tuner outperforms the current system.

ton or forwardinjection. The current system is described in [2].

An advantage of using many, e.g. 1024, turn-by-turn measurements for injection tuning is that the current betatron tune can be determined and unwanted frequencies as noise or synchrotron oscillations due to dispersion, can be filtered out. Also, only a single detector per plane is needed, eliminating the error due to inaccurate betafunctions and phase advances. The turn-by-turn method has already been successfully ap- plied at Fermilab for the accumulator and is described in [3].

II. CONFIGURATION

The hardware centers around two Tektronix VX4240 dig- itizers, one for each plane. A 7.5 MHz clock for the digitizers is derived by a special CLOCK module from a beamsync sig- nal. A V177 carddecodes from the same beamsync signal an event that triggers the digitizers at a rate of 47 kHz, or once per turn. The digitizers sample on the first positive clock edge after a positive trigger edge. Since both the clock and trigger are derived from the beamsync signal, the beam is always sampled at the same point in the beam position waveform. Different bunches are selected by directing the V177 to change the delay of the trigger. To start the sampling on an in- jection, a V177 module decodes an injection event that arms the digitizers from the Tevatron Clock signal. To be able to look at Tevatron and at Main Ring injections, HP E1366A RF multiplexers switch the V177 cards betweenthe Main Ring and Tevatron bpm signals and betweenthe Main Ring beam- sync and Tevatron beamsync signals.

A Macintosh IIci controls the VXI crate through a MXI interface. The Macintosh connects to the accelerator control's Tokenring network, enabling it to exchange information with an application program on a control console. An independent control signal can cycle the power to the Macintosh and VXI crate to perform a remote reboot. The configuration is shown in figure 1.

I. INTRODUCTION

During a collider run it is important to achieve the highest possible luminosity. Betatron oscillations of the beam result in a growth of the beam emittance which decreases the luminos- ity during beam-beam collision. The betatron oscillationsoc- cur when the beam is not injected onto the closed orbit. The task of a tuner, therefore, is to adjust the correction elements so that the beam is injected onto the closed orbit. An overview of injection tuning methods is given in [1].

The current Tevatron injection tuning system uses a method that measures the beam position signal of 13 consecu- tive beam position monitors (bpm) at the first turn and then subtracts the beam positions ofthe closed orbit at those loca- tions. The resulting data represents the betatron oscillation to which a sinusoid is fitted to determine the phase and ampli- tude. To do a proper fit, the phase advances, the beta values and the dispersion values at each bpm location must be accu- rately known. Because the method assumes that all beta val- ues are the same and ignores the dispersion, it introduces er- rors in the calculations. The current system also suffers from the limitation that it can handle only one bunch per ring. The digitizers trigger on beam intensity and multiple bunches in the ring would lead to triggering on a wrong bunch. This means that the tuning of the injection must be separate from the actual loading of the ring for a collision shot. The time from tuning to loading the last bunch is often more than an hour. Meanwhile the drift of various elements can invalidate the calculated corrections resulting in betatron oscillations of up to one millimeter. The injection tuning of the anti-protons is also done in a separate step. To avoid wasting precious anti- protons,protons are reverse injected from the Tevatron into theMain Ring. The correction method is the same as forpro-

* Operated by the Universities Research Association under contract with the U.S. Department of Energy.

Figure 1. The configuration of the beamline tuner.