Pohang Light Source (PLS) has 12 cells with one super-period. There are 70 vertical and 70 horizontal correctors for orbit correction. PLS has provided stable photon beam to beamline users since corrector magnet power supplies (CMPS) were upgraded to more stable ones. CMPS for synchrotron radiation source is required to have long-term stability, high precision current control, and control repeatability. The CMPS has been upgraded two times incorporating the available technology of magnet power supply at those times.

Until mid of 2000, power supplies for these correctors were based on 1990’s technology with 12 bit of current granularity and the hysteresis current controlled full-bridge converter type, so the global orbit feedback system was not possible with poor 12 bit resolution. With the 12-bit power supply, 1 bit current change of CMPS resulted in about 6 μm change of global orbit, which is not acceptable to beamline users. As the 12-bit power supply was too coarse to be used in global orbit feedback, we had to replace the power supply with the granularity of better than 16 bit.

In 2003, a task force team was assembled to develop new power supplies. At that time as a first upgrade model we decided to adopt DAC/ADC card developed for BESSY-II which is capable of 20-bit control granularity.

The first upgrade model was analog type. Only 22 among 70 power supplies for the vertical plane were replaced with the first upgrade model because of the budget limitation while 22 power supplies for horizontal plane were replaced with the modified module of the existing power supply whose DAC/ADC granularity was improved to 16 bit [1]-[3].

The selected corrector magnets which are located upstream and downstream of the straight sections can effectively counteract the orbit change due to gap change of insertion devices. The orbit stability after the first CMPS upgrade was less than 1μm forshort-term (<5min) and 5μm for long-term (>12hours).

However, this replacement was not enough to satisfy the beam stability requirement of 2 microns for PLS. In order to attain that requirement, more corrector power supplies should be replaced with better ones. Moreover some improvement was issued on the power supply itself; better stability and better control granularity. We have launched another power supply design based on all digital technology. This attempt was completed within a year, and 80 units were assembled in house. At that time we decided to adopt newly established-digital technology for magnet power supply which was developed in PSI [4]-[5].

We fabricated 70 power supplies with digital architecture for vertical plane and put them into operation successfully. The second upgrade of CMPS is fully digital technology and has a control resolution of ±1 ppm and short-term (< 5 min) stability of ±2 ppm and long term (< 24 hours) stability of 10 ppm. In this paper, the PLS new CMPS hardware and controller structure, experimental result and commissioning result are presented.

The control granularity of CMPS should be small enough to make the beam oscillation by orbit feedback invisible in beamline experiments. In order to get high precision control granularity, 19 to 24 bit DAC board was developed in BESSY-II. A power supply with digital control architecture was developed in PSI and successfully applied in DIAMOD for a few watts to 1 MW power supply. Digital-controlled power supply is capable of providing high resolution above 19-bit and good stability below 10 ppm, and fast control. The digital controller consists of DSP, FPGA, and high resolution ADC [6]-[8].

Figure 1 shows the circuit diagram of the old CMPS with the switching frequency of 1.6 kHz and the DAC bit of 12-bit. The hysteresis current control for stabilizing the load current can not compensate the voltage change well because the frequency changes as the load condition changes.

Figure 1: 
Circuit diagram of the old CMPS

Figure 2 shows the circuit diagram of the digital control power supply. The hardware is full-bridge switching type and is capable of four-quadrant action. IGBT is uni-polar type with 25 kHz switching and the 5-th order LC filter is used to get -80 dB attenuation of switching frequency at the output. ULTRASTAB 867-201 DCCT (Danfysic) is used to get higher resolution of load current. The requirements of the digital power supply are: (1) step resolution smaller than ±2 ppm, (2) 10 ppm long-term stability, (3) 4th or 5th output filter, (4) over 25 kHz switching frequency, (5) fast communication port for feedback operation. Table 1 lists the comparison of two type power supplies.

Figure 2: 
Circuit diagram of the digital control power supply

If the high frequency switching noise and low frequency noise from the power supply is not perfectively damped in the vacuum chamber of corrector magnet, the stored beam can be influenced by that noise.

Figure 3 and Figure 4 shows the output noise spectrum of the old CMPS and the digital CMPS. As the old model CMPS has no output filter, the switching frequency (1.6 kHz) and its higher harmonics are clearly shown, while the switching noise is perfectly damped in the digital CMPS as shown Figure 4.

Figure 3: 
Old output current noise spectrum

Figure 4: 
Digital output current noise spectrum

Figure 5 and Figure 6 shows the short-term stability of the old CMPS and the digital CMPS measured at the load current of 55 A. The old CMPS shows a stability of 20 ppm while the digital CMPS shows a stability of 0.5 ppm. The short-term stability is important in the dynamic correction power supply because the high resolution control can be assured after the short term stability is guaranteed.

Figure 5: 
Short-term stability old CMPS (20ppm/Div)

Figure 6: 
Short-term stability digital CMPS (1ppm/Div)

The short-term stability is determined by DDCT, ADC, Input/Output filter, and digital controller for current regulation. The long-term stability is influenced by external sources such as ambient temperature and the temperature coefficient of semi-conductor components.

Figure 7 shows the current variations of the setting and reading of two CMPS (P08CV1 and P08CV2) used in the orbit feedback for 48 hours. It is seen that the load current follows well the setting value during the orbit feedback run.

Figure 7: 
Current variations of the setting and reading of two CMPS (P08CV1 and P08CV2)

PLS CMPS has been upgraded continuously based on digital technology, and played an important role in orbit stabilization. The high performance digital power supplies installed have been working well up to now and the orbit stability has improved remarkably. The power supply itself shows a good reliability. Currently, the global orbit feedback system is running successfully with the digital power supplies.