COD Correction by Novel Back-leg at the KEK-PS Booster

The correction of closed orbit distortion is performed by using new driving system of back-leg windings. Two back-leg coils of separate magnets are connected to make a closed circuit in which the induced voltages of the two magnets have opposite phases to each other. When the current source is inserted into the closed loop, the current drives the two magnets with opposite polarities. If the pair of magnets is properly selected, the current effectively corrects the orbit distortion. The selection rule of the pair is as follows; one is the magnet at the maximum distortion and the other magnet is that separated with −90deg in betatron phase. The correction system at the KEK-PS Booster reduced the closed orbit distortion to less than 1/6 of that without correction, and increased the capture efficiency. The average beam intensity of our booster is increased from 2.0×10ppp to 2.8×10ppp. Advantages of the drive system are as follows; (1) this scheme has no interaction between the power supply of main magnet and that of correction system, therefore (2) this scheme is very stable and reliable, (3) this reduces the cost for the orbit correction, and (4) this scheme is applicable to compact synchrotrons. 1. PRINCIPLE AND POWER SUPPLY The KEK-PS Booster consists of eight magnets (M1 ~M8), and the magnets are separated with straight sections (S1 ~ S8) having the length of only 1.9m. Nearly all straight sections are occupied with the instruments, such as, RF systems, injection and extraction systems, and other correction magnets. That is, we have no space to install the magnets to correct the closed orbit distortion (COD). Therefore operation of the back-leg orbitcorrection system is indispensable for our booster. We use the back-leg windings of a pair of magnets, and connected them with current source as shown in Fig. 1; the two induced voltages at the back-leg windings due to the change of main magnet power supply are cancelled at the current source. Let the separation of the magnets be 90°-betatron phase angle and orbit bump at the center of magnet-2, then the current of back-leg winding at magnet-1 cancels the orbit bump at magnet-2, and the extra bending angle of the magnet-1 is cancelled by that of the magnet-2. Thus the pair of kicks minimizes the effect onto the orbit at the other position of the machine. The second scheme is shown in Fig. 2; where windings of the three magnets are used to compensate COD. We installed terminal boards at the yoke of magnets. On a board eight terminals of the ends of four back-leg windings, and four terminals of four ends of cables connecting from magnet to magnet are attached. Therefore we can easily change the connection at this terminal board. For example, we selected the connection of 4-turns at each magnet for scheme-1, and connection of 2turns for scheme-2. The bend angle of Booster magnet is 45° (= 785.4 mrad) at 67.9kAT at the top energy, and 16.48kAT at injection. Therefore the kick angle due to the current at the back-leg winding is 11.6[μrad/(AT)] at the top, and 47.6[μrad/(AT)] at the injection. In order to compensate 0.5% of bend angle, the current of 340AT is necessary. Because we only have a COD-data at injection, we designed the power supply having output current of ±100A to make some overhead. Table 1 summarizes the constants and specifications of the power supply.