Applications for the beam stabilization in the linac

The linac BPM data acquisition system has been applied to an actual linac operation since December 2003.

It has continuously acquired all the BPM data for every shot and has accumulated all the acquired data into database throughout the linac operation including injection both to the booster and the NewSUBARU storage ring.

The accumulated BPM data is available for data analysis by retrieving arbitrary BPM data from the database. Fig. 4.14(a) exhibits horizontal beam trajectories along the length of the linac at 10pps operations, and Fig. 4.14(b) shows their variations. The trajectory variation was caused by a kicking perturbation at the injector section where beam energy was about 60MeV. The analysis is available to retrieve a time-specified beam shot data using a web browser.

In addition to off-line analysis, the BPM data can be applied to on-line automatic feedback control for beam trajectory and beam energy stabilization.

4.6.1. Automatic beam position feedback control

A beam trajectory drift in the linac makes injection efficiencies to the booster and the NewSUBARU storage ring worse. A room temperature variation causes the trajectory drift. Since the SPring-8 top-up operation highly requires constant injection current, the linac beam trajectories have to be as stable as possible. In order to achieve such stable injections, a beam position feedback control program was introduced in September 2004 [64]. Before introducing the feedback control program, operators manually corrected the beam trajectory in every two or three days. This program stabilized beam positions and angles at an injector section located upstream of the linac, a matching section located downstream of the ECS, and a beam transport (BT) section to the NewSUBARU. These sections have two sets of steering magnets for x and y-plane in the drift space, which is located upstream of two non-dispersive BPMs. Fig. 4.15 represents the matching section and the BT section to the NewSUBARU. Beam positions and angles to the booster are finally stabilized at the matching section, and those to the NewSUBARU are finally stabilized at the BT section.

The position feedback program cyclically retrieved the BPM data from the database, calculated correction values for the steering magnets to maintain the beam positions within a tolerance, and applied the correction values to the magnets. The beam position tolerance is currently set to ±30µm and the feedback control cycle is set to 5 times of the injection interval. In the case of an every minute injection for several bunch operations at the SPring-8 storage ring, the feedback control cycle was set to 5 minutes.

Fig. 4.16(a) shows time variation of horizontal beam position at a LSBT-1 BPM in the matching section (see Fig. 4.15) before introducing the automatic beam position feedback control. Fig. 4.16(b) shows a result of horizontal beam position drift at the same BPM after applying the feedback control. The comparison of the Fig. 4.16 indicates that the position feedback control program succeeds in stabilization of the beam position in the matching section.

4.6.2. Automatic beam energy feedback control

As describing in 2.1.2, the SPring-8 linac has the ECS which is located upstream of the matching section (see Fig. 4.15) in order to compensate accidental energy variation and to compress energy spread of injection beam to the booster and the NewSUBARU. However, unexpected small and slow energy drift at the downstream of the ECS was observed. It was caused by a room temperature variation. The energy drift affected the beam injection efficiency both of the booster and the NewSUBARU. In order to stabilize the beam energy, a beam energy feedback control program was introduced as a supplemental system of the ECS in September 2004.

The energy feedback program retrieves the accumulated BPM data from the database and adjusts the ECS phase shifter as the horizontal beam position within a tolerance at the LSBT-3 BPM, which is installed in a

dispersive section located downstream of the ECS (see Fig. 4.15). The adjustment was executed only when the beam positions in the matching section exceeded the double tolerance defined by the position feedback control. The tolerances of the position feedback control and the energy feedback control are currently set to

±30µm and ±300µm (~ ±0.03% of beam energy), respectively.

Fig. 4.17(a) shows time variation of horizontal beam positions at a LSBT-3 BPM before introducing the automatic beam position feedback control. Fig. 4.17(b) shows a result of horizontal beam position drift at the same BPM after introducing the feedback control. Fig. 4.16 indicates the energy feedback control program succeeded in stabilizing the beam energy.

4.6.3. Investigation of unexpected burst current emissions

On 5 October 2005, a phenomenon that the linac electron gun very occasionally emitted unexpected burst current was found. Fig. 4.18 shows the burst current observed by using the beam current monitors installed just behind the buncher section and the electron gun. The burst current was emitted 40nsec after the normal 1nsec beam and it had the length of 2msec. The total charge of the burst current was nearly 10nC. Beam current monitors for radiation safety control to limit the transported charge to the NewSUBARU storage ring couldn’t record the burst current because these monitors measured the normal 1nsec beam with a narrow gate signal. Therefore, investigation of the burst current had to be made how many times the phenomena occurred in the past and whether the burst currents were injected to the NewSUBARU storage ring.

The BPM data collected by the shot-by-shot data acquisition system were utilized for the investigation because the signal processor of the linac BPM system didn’t use the gate signal to the 1nsec beam and the shot-by-shot data acquisition system accumulated all the acquired BPM data into the database.

Table 4.5 shows the result of the investigation since November 2004. It was found that there occurred 35 times of the burst currents. The charge of the burst current at the downstream of the ECS was 5.8nC ~ 8.5nC before a cathode exchange in the summer of 2005 and 8.9nC ~ 9.7nC after the September 2005.

Table 4.5 represents that the burst current toward the NewSUBARU storage ring occurred only once on 13^th of November 2004. And the charge of the beam shot was evaluated by using the BPM data, as shown in table 4.6. A beam slit installed behind a bending magnet in the upstream of the L3BT reduced the burst current to about a fifth. And a beam slit installed in the L4BT finally reduced the burst current to 0.5nC or less. This investigation result proved that the injection of the burst current into NewSUBARU didn’t become an issue of radiation safety control. Thus the shot-by-shot data acquisition system for the linac BPM played an essential role to trace the entire burst current in the past because the system accumulated all the BPM data synchronizing to all the beam shot.

At present, the burst current emission is not observed as the result of adjustment of the grid voltage of the electron gun. The cause of the burst current doesn’t become clear but fault of the grid of the electron gun is suspected.