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 | JOURNAL OF SYNCHROTRON RADIATION |
Effect of the RF cavity temperature on low-energy injection at HLS
aNational Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui Province 230029, People's Republic of China
*Correspondence e-mail: daijp@hptc7.ihep.ac.cn
The resonant frequency shift caused by the temperature of the RF cavity at the Hefei Light Source has been measured and the results analysed. The effect of this frequency shift on low-energy injection with a low cavity voltage is discussed, and a new injection mode is proposed.
Keywords: RF cavities; frequency shifts; injection modes.
1. Introduction
Hefei Light Source (HLS) is a dedicated synchrotron radiation light source which adopts a low-energy injection mode. The main parameters relating to the RF system are listed in Table 1![[link]](../../../../../../logos/arrows/s_arr.gif)
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During the operation of HLS, we found that the temperature and the frequency of the cavity shifted noticeably with a change of the cavity voltage, and sometimes the frequency shift would lead to loss of the beam. This is discussed in the following.
2. Frequency shift of the cavity
The HLS cavity (Fig. 1) is a re-entrant cavity made of a copper/stainless-steel compound material. Using code URMELT (DESY M-85-11), we calculated the normalization distribution of the power along the skin of the cavity; the results are shown in Fig. 2.
![[Figure 1]](mi3340fig1thm.gif) | Figure 1 Shape of the RF cavity (dimensions in millimetres). |
![[Figure 2]](mi3340fig2thm.gif) | Figure 2 Distribution of the power of the HLS cavity. |
Since the cooling water of our cavity distributes poorly and its flow is not very large, the cavity temperature and its distribution will change noticeably when the cavity voltage, i.e. the power, changes. In addition, since our cavity is made of copper/stainless steel, it is relatively easily deformed as the temperature changes, leading to frequency shifts.
For our cavity the relation between the frequency shift and the movement of the tuner is
![[\Delta f_r = 0.0043\Delta L.\eqno{(1)}]](teximages/mi3340fd1.gif)
We define the tuner position, LR, as the position where the resonant frequency of the cavity is 204.035 MHz, i.e. the frequency of the RF source. By measuring the movement of LR caused by the change of the cavity voltage, we may obtain the frequency shift quantitatively.
From Fig. 3, we know that when the cavity voltage increases to 100 kV from 42 kV, the movement of LR is 7 mm, and this means that when the tuner is fixed the frequency of the cavity decreases by about 30 kHz.
![[Figure 3]](mi3340fig3thm.gif) | Figure 3 Movement of LR with the change of cavity voltage. |
3. Effect of frequency shift on injection
The RF system at HLS operates with no feedback system. In the injection procedure the cavity voltage changes greatly, and therefore the frequency also shifts greatly. Because the tuning angle, ψ, is defined in the following form,
/f_{\rm rf},\eqno{(2)}]](teximages/mi3340fd2.gif)
it has to be very large (near −90°) so that the ψ shift caused by the frequency shift is relatively small and the beam is far from the Robinson instability region. If the tuning angle was near the optimum tuning angle, ψm, then
![[\tan\psi_m=-I_0R_s\sin \varphi/[V_c(1+ \beta)],\eqno{(3)}]](teximages/mi3340fd3.gif)
and the frequency shift would easily lead to the Robinson instability and beam loss. This is injection mode I, shown in Fig. 4.
![[Figure 4]](mi3340fig4thm.gif) | Figure 4 Threshold current determined by the Robinson instability. |
Although injection mode I is simple and the RF system may operate without any feedback system, the reflected power is extreme. Usually, more than 50% of the power is reflected, which often damages the circulator or the RF source.
4. Improvement of the injection mode
In order to decrease the reflected power, we will adopt a new injection mode, mode II (Fig. 4).
In this mode, the cavity voltage is increased to 100 kV and fixed by an amplitude loop, and the tuning angle, ψ, is the optimum angle, ψm, and is controlled by a tuning loop. The reasons for choosing the cavity voltage to be 100 kV are as follows.
(i) The tuning angle, ψ, is the optimum angle, ψm, and the movement of the tuner is adjusted by the tuning loop as
![[L-L_R=6.1\times10^6 \,I_0/V_c.\eqno{(4)}]](teximages/mi3340fd4.gif)
If Vc = 100 kV, the tuner will move 18.3 mm altogether when the beam current accumulates to 300 mA from zero; however, if Vc = 40 kV or less, the tuner would move 45.6 mm or more, and it is impossible for our tuner to move such a long distance.
(ii) The threshold of the beam current, Imax, determined by the Robinson instability, is
![[I_{\rm max}=2V_c(1+\beta)\sin \varphi/R_s\sin|2\psi|. \eqno{(5)}]](teximages/mi3340fd5.gif)
If Vc is higher, the threshold of the beam current will be higher.
(iii) When the tuner is near the 25 mm position, the longitudinal multi-bunch instability driven by the TM012 mode of the cavity often leads to the difficulty of beam accumulation. If Vc is chosen as 100 kV, the tuner will be at 30–49 mm, and so this dangerous tuner position can be avoided.
Acknowledgements
The authors would like to express their thanks to the commissioning group for their cooperative work on this subject.
References
Dai, J. (1997). Thesis, University of Science and Technology of China, People's Republic of China.
Wilson, P. B. (1982). SLAC-PUB-2884. SLAC, Stanford, CA, USA.
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| JOURNAL OF SYNCHROTRON RADIATION |
