Beam Position Monitor with HOM couplers

Transcription

1 Beam Position Monitor with HOM couplers Masaru Sawamura and Ryoji Nagai Japan Atomic Energy Research Institute (JAERI) 2-4 Shirakata-Shirane, Tokai, Ibaraki , Japan Corresponding author: Masaru Sawamura JAERI 2-4 Shirakata-Shirane, Tokai, Ibaraki , Japan Phone: FAX: ABSTRACT A beam position monitor using HOM couplers is proposed. This has the possibility to detect beam positions through superconducting accelerators in long cryostats where normal beam position monitors cannot be installed. The primary measurement was done to show the correlation between the beam position and the HOM power. PACS: Qg, Et, Keywords: beam position monitor, HOM coupler

2 1. INTRODUCTION In a recirculating accelerator like an energy recovery linac (ERL) recirculating electron bunches pass through the same cavities and the off-axis beam excites higher order modes (HOMs). These HOMs can cause multi-pass, multi-bunch beam breakup (BBU) and limit the beam current especially for high Q-value superconducting accelerators (SCAs). HOM couplers are installed to decrease the Q-values of the HOMs and to rapidly dump the HOM power in the cavity. The HOM power can be dissipated in the cryostat for low current beam accelerators since the total dissipated power is not large. The HOM power for high current accelerators such as ERLs the HOM power should be brought out of the cryostat to reduce the load on refrigerator systems. The HOM power increases with the beam current but also with the beam position from the axis. The HOM power can be therefore used to characterize the beam position in the cavity. This indicates the possibility to detect the beam position along the long cryostat with many cavities between which the normal beam position monitors (BPMs) cannot be installed. Here we describe the basic idea of the BPM with HOM couplers as the primary measurement of the method. 2. Basic Idea of Beam Position Monitor When the beam passes through the center axis of cavities, the beam excites no transverse HOM fields. When the beam passes through the off axis, the beam excites the transverse HOM fields. Since these HOM fields are proportional to the distance from the axis, the power from the HOM couplers are related to the beam position. When the relation between the beam position and the HOM power is known, it becomes possible to detect the beam position in the every cavity where the BPMs are not usually installed. 1

3 This relation can be distorted when the HOM field kicks the beam strongly to affect the beam position. Under this condition the relation between the HOM power and the beam position is complicated. To avoid this situation the HOM field must be weak or measured in the pulse mode in which growth of the HOM fields can be practically retrained. 3. MEASUREMENT 3.1 Configuration The layout of the JAERI ERL-FEL is shown in Figure 1 [1]. Figure 2 shows the 5- cell cavity, which has five RF couplers such as a main power coupler, a pick-up coupler and three HOM couplers. Two HOM couplers are designed to dump transverse modes (TE modes) and the other to dump longitudinal modes (TM modes). All HOM couplers are terminated to the dummy loads outside of the cryostat 3.2 Setup The first 5-cell cavity was used to measure the HOM field. In this cavity many HOM fields can be excited. Table 1 shows the calculated transverse r/q and the measured Q-value for the groups of the TE111 and TM110 modes. The TE111 2π/5 mode has the highest Q- value among them. Figure 3 shows the HOM power spectrum extracted through the HOM couplers [2]. The TE111 2π/5 mode also has the largest power. For these reasons the TE111 2π/5 mode was selected to detect the HOM power. A stripline-type BPM is installed at each side of the cryostat of the 5-cell cavity. Though the BPM was not calibrated, the BPM has the response to increases the beam position signal with the beam offset from the axis. As the accelerator was operated in the pulse mode, the beam position was measured by averaging pulses of the electron bunches, which included both accelerated and decelerated 2

4 bunches. The HOM power was measured once during the macro beam pulse with a real-time spectrum analyzer which was triggered by the synchronized pulse with the macro beam pulse. 3.3 Results Figure 4 shows the HOM power and the beam position signals as a function of time. The beam positions were measured with the stripline-type BPM installed at the entrance of the cryostat. As the BPM was not calibrated, the unit of the beam position was arbitrary. The beam positions were measured when the accelerator was not stable so the beam position moved irregularly. Figure 5 shows the HOM power as a function of the beam positions in order to estimate the correlation between the beam position and the HOM power. The correlation coefficient was 0.25 for the horizontal direction, and 0.51 for the vertical. 4. CONCLUSION While there seems to be correlation between the beam position and the HOM power, there were many points departing from the relation. The correlation between the horizontal position and the HOM power is not as strong as the correlation observed in the vertical plane. This may come from the complicated beam position including the accelerating and decelerating beam. Hence the beam position should be shifted for intended position and measured separately for the accelerating and decelerating beam. It would be also more useful to compare the correlation to the other modes because the sensitivities for the beam position shift would be different among the modes. For more precious measurement BPM calibration is also required. ACKNOWLEDGEMENTS This work was supported in part by JSPS KAKENHI

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6 FIGURE CAPTIONS Figure 1. Layout of the JAERI ERL-FEL Figure 2. RF Couplers of the 5-cell cavity. Figure 3. HOM power spectrum from the HOM couplers. Figure 4. Beam positions and HOM power as a function of time. Figure 5. Correlation between HOM power and beam positions of horizontal (upper) and vertical (lower). 5

7 [1] R.Hajima et al., NIM A 507 (2003) REFERENCES [2] M.Sawamura et al., Proc. of PAC 2003 pp

8 1-cellSCA SHB Undu la to r 2nd A rc Ha l f Ch icane 1s t A rc BeamDump ElectronGun In jec t ionme rge r Ma in 5 -ce l l SCA Figure 1 7

9 Main Coupler TM mode Coupler TE mode Coupler #1 TE mode Coupler #2 Pickup Coupler Figure 2 8

10 20 0 TE mode coupler #1 TE mode coupler #2 TM mode coupler HOM Power (dbm) Frequency (MHz) Figure 3 9

11 -65 Power(dBm) Horizontal Beam Position (Arbitrary Unit) Vertical Beam Position (Arbitrary Unit) Time(min) Time(min) Time(min) Figure 4 10

12 -60 HOM Power(dBm) X position -60 HOM Power(dBm) Y Position Figure 5 11

13 Table 1 Calculated frequency and r/q and measured Q-value for TE111 and TM110 modes. freqency Mode r/q Qload (MHz) TE π/ E+03 2π/5 3π/5 4π/5 π E E E E+03 TM π E+03 4π/5 3π/5 2π/5 π/ E E E E+05 12