A NOVEL DUAL-CHANNEL KICKER FOR THE HEFEI ADVANCED LIGHT FACILITY

Hefei Advanced Light Facility (HALF) is designed as a fourth-generation light source based on the diffraction-limited storage ring (DLSR). The pre-research for this design is complete. Due to the smaller beam dynamic aperture, about 6 mm, a novel dual-channel kicker has been purposed and designed, with other two traditional kicker, combined the new injection system bump system. This paper presented the principle and layout and the detail of the novel dual-channel kicker.


INTRODUCTION
The Hefei Advanced Light Facility (HALF) will be built as a world-advanced VUV and soft X-ray diffraction limited storage ring light source [1].The traditional beam-filling method for the storage ring is to use a closed-bump kicker magnet system to inject fresh bunches of electrons off-axis from the already stored beam.This method required the dynamic aperture large than 10 mm or increasing  in the injection straight section [2].
The SLS has proposed a new closed-bump injection method, which utilizes two traditional kicker and a "antiseptum" [3].With this method, the injected beam can be made very close to the stored beam.So this new closedbump can reduced the requirements on the beam dynamics aperture.This paper presented a new magnet to realize the function of "anti-septum", is called dual-channel kicker.

INJECTION SYSTEM DESIGN FOR HALF
The relevant parameters of the HALF injection system are listed in Table 1.As extremely small dynamic aperture and limited straight section, the traditional closed-bump injection system cannot be used smoothly.
We presented a dual-channel kicker for the new closedbump injection system, as shown in Fig. 1. the dual-channel kicker is placed downstream of the septum.The stored beam passes through K1, K2, and K3 to form a closedbump, and the maximum height of about 6 mm is obtained at K2.The bend angle provided by K2 is the sum of K1 and K3.The injected beam is deflected by the septum with a height of 11.8 mm and an angle of -3.2 mrad.When it reaches K3, the injection beam is deflected together with the stored beam by the magnetic field of K3, with an angle of 3.2 mrad.At the exit of K3, the height of the injection beam is 3.8 mm and the angle is 0, which can be successfully captured by the acceptance of the storage ring.

Dual-channel Kicker Design
An oxygen-free copper block is placed in the dual-channel kicker vacuum chamber and a hole was drilled inside it.The right side of the block is curved with the same curvature as the inner diameter of the vacuum chamber, leaving a gap about 1 mm with the inner wall.The detail is shown in Fig. 2. It can be seen that if the copper block is thick enough, the eddy current will shield most of the magnetic field, which means the magnetic field in the hole would be zero.
In addition, the core consists of two C-shaped blocks that use different materials in an asymmetric layout The left Cshaped is made of Ni-Zn ferrite with high magnetic permeability.Meanwhile the right C-shaped block is made of copper.The majority of magnetic lines will pass through the ferrite magnetic core, and the impact of the copper block will be reduced.

Coating Ceramic Vacuum Chamber
The ceramic vacuum chamber is made of 95% alumina (Al2O3), and the interior is coated with a layer of titanium metal to ensure electrical connection with the storage ring vacuum chamber and to reduce the impedance.The pulsed magnetic shielding effect of the coating also needs to be considered.
The square resistance of the coating layer is  , whose value is determined by the following equation: Where d is the thickness of the coating layer and σ is the resistivity of the coating material, it can be seen that when the coating material is chosen, the square resistance is inversely related to the thickness of the coating.
The skin depth can be calculated by the following equation: where σ is the conductor resistivity,  2,  is the current frequency, and the conductor's permeability    .The dual-channel kicker adopts a 5.6us half sine 5 kA excitation current, so the equivalent frequency f is about 0.18 MHz, and the titanium's resistivity  4.2 10 Ω m ⁄ , and the skin depth  of titanium can be calculated as 427.9 µm.
The thickness of the coating is determined by Eq. (1).For example, when the square resistance is 1 Ω, the coating thickness is 0.42 µm, when d＞ , due to the eddy currents, the magnetic field cannot penetrate the coated ceramic tube.When the coating thicknesses is 0.42 µm, the calculated peak magnetic field intensity are shown in Fig. 3.It can be seen that the 0.42 µm Ti coating has no significant shielding effect on the magnetic field, and can be considered to have no effect on the magnetic field.The influence of coating thickness on the drop of peak magnetic field intensity is shown in Fig. 4. It can be seen from the figure that as the coating thickness increases, the peak magnetic field strength continuously decreases.When a certain thickness is reached, the peak magnetic field drops sharply.

Simulation Results
The main field of the dual-channel kicker is simulated by Opera-2D, as shown in Fig. 5.The majority of magnetic field lines pass through the ferrite magnetic core, and the magnetic field of the shielding channel almost zero.The distribution of the magnetic field is shown in Fig. 5 The result of the main field in time domain shown as Fig. 6.There is almost no delay between the pulsed magnetic field and the excitation current.Therefore, the consistency and repetitive stability of the pulse power supply will have a major influence on the injection system.

CONCLUSION
The simulation results of a dual-channel kicker indicate that its magnetic shielding channel can effectively shield the pulsed magnetic field.The optimized dimensions have little effect on the magnetic field distribution outside of the magnetic shielding channel, suggesting that this approach is feasible.The next step will be to proceed with the magnetic field measurement of the prototype magnet.

Figure 1 :
Figure 1: Schematic of HALF injection system with dualchannel kicker.

CFigure 2 :
Figure 2: a) Cross section of the dual-channel kicker and b) details of the kicker.

Figure 3 :
Figure 3: Comparison of magnetic field distribution inside the ceramic vacuum chamber with and without coating.The influence of coating thickness on the drop of peak magnetic field intensity is shown in Fig.4.It can be seen from the figure that as the coating thickness increases, the peak magnetic field strength continuously decreases.When a certain thickness is reached, the peak magnetic field drops sharply.

Figure 4 :
Figure 4: Effect of coating thickness on the peak field.The main parameters of the dual-channel are listed in Table 2. Table 2: Dual-Channel Kicker Main Parameters Parameter Dual-channel kicker a 3 mm b 13.7 mm Curvature of the copper block' right side 0.0588 mm-1 Hole diameter 8 mm Left C-shaped material Ni-Zn ferrite Right C-shaped material oxygen-free copper Coating thickness 0.42 µm Excitation current 5 kA/5.6 us/half sine Opera-2D simulation results of the main field: a) magnetic field distributions; b) at the peak of the current pulse.

Figure 6 :
Figure 6: Simulation of the main filed at different time.The results of the simulation by Opera 3D are shown in Fig. 7.The calculated magnetic along the Z-direction is consistent with the 2D results.

Figure 7 :
Figure 7: Opera-3D simulation results.Based on the results of the simulation, we carried out the design of the prototype, as shown in Fig.8.The copper block is longer than the ceramic vacuum chamber and extends to the bellows.

Table 1 :
Relevant Parameters