Design of a 250 MeV linac injector system for the Southern Advanced Photon Source

The Southern Advanced Photon Source (SAPS) is a 4th generation storage ring based light source under design started several years ago, which is planned to be constructed at Guangdong province at China. The equilibrium emittance of the storage ring(SR) will be below 100 pm.rad and the beam energy is determined to be 3.5 GeV. Over the past two years, the nominal current of the SR was increased from 200 to 500 mA, requiring the injector system to provide more bunch charge. Additionally, the injection beam energy for the booster has been increased from 150 to 250 MeV, which means two more accelerating cavities have to been added. This paper presents an update of the linac injector. And also a beam transfer line from linac to booster is presented.


Introduction
The Southern Advanced Photon Source (SAPS) is a mid-energy fourth generation synchrotron radiation light source [1] which is proposed to be built in Guangdong, the south of China.The project is still on the design stage and several key technologies are continuing moving ahead.The beam energy of the storage ring is determined to 3.5 GeV and the natural emittance of the SAPS is designed to be below 100 pm.rad in order to provide high performance synchrotron light.
For the storage ring (SR) of synchrotron light source, there are basically two options of injector.One option is a booster ring with a lower energy linac which has been used in many light sources [2] [3] [4] [5] [6], and another option is a full energy linac which has been used by MAX IV [7] and Spring8 [8].For our project, two options are being designed in parallel.In this paper, we focus on the design of the low energy linac for the first option.
Two years ago, we presented a design of a low energy linac [9].The injector was designed to providing electron bunch with charge 1.33 nC to the SR, as the average current was temporally determined as 200 mA.However, the average current of the SR has recently been increased to 500 mA, while the circumference has remained almost the same, So the injector has to provide bunch with charge 3.33 nC.Additionaly, to ensure reliable operation of the booster ring, the beam energy at the linac end has been increased from 150 to 250 MeV.In this paper, the new linac design fulfilling these changes is presented.A transfer line between the linac to the booster ring is also shown.

Parameters and layout of the linac system
The low energy linac injector serves as the pre-injector of the synchrotron light source.After several rounds of discussion with the designers of the booster and storage ring, The main parameters of system are determined as shown in Table 1.With these requirements, the low energy linac system is designed as shown in Fig. 1.This system consists of three sections: one DC electron gun section, one bunching section and one accelerating section contains four accelerating cavities.

The DC electron gun
The electron gun of the SAPS will be a thermionic high voltage DC gun, which can provide the required bunch charge to SR without accumulation.The required bunch charge at the end of linac is 5.2 nC.We assume a 80% capture efficiency from the DC gun to end of linac, then it should be enough that the DC gun provide bunch charge of 6.5 nC.
The DC voltage of the gun is the same as the HEPS gun [10], i.e., V = 150 kV.Given the radius of cathode r = 8 mm and distance between cathode to anode d = 0.64 m, the Child-Langmuir law will give the current in the electron gun as 6.6 A.
With these initial parameters, we designed an electron gun with help of the program EGUN [11].The final parameters of the gun are shown in

The bunching section
As shown in Fig. 1, the main components of the bunching section are the same as the previous design.The differences come from the larger bunch charge, which means the bunching and focusing has to be re-optimized.
To improve the transmission efficiency, the layout of the solenoids has been changed.Due to the large bunch charge, the transverse emittance of electron bunch will grow rapidly when the electron beam gets out the gun.Therefore, it is important to put the first solenoid as close as possible to the exit of the gun.Other solenoids before the accelerating cavities are equalspacing arranged if there no interfere with other components.In total, 23 solenoids are used for the transverse focusing, with a maximum magnetic field of the solenoid of 0.1 T.
The optimization is based on beam tracking with the algorithm simplex using the program ASTRA [12].After several rounds of optimization, the optimized emittance and beam size along the bunching system are shown in Fig 3 .The rms emittance at the end of the bunching system is 41 mm.mrad.The position of the maximum beam emittance and beam size is about 10 cm from the cathode.
The optimized parameters of the cavities used in the buching system are listed in Tab. 3. The particles grouped in the range ±5 ps is 92% and the relative energy spread of those particles is 1.1%

The accelerating section
The main components of the accelerating section are four S-band accelerating cavities, each 3 meters long.Two adjacent cavities will be driven by one RF power source and the accelerating gradient can be greater than 20 MV/m.Three quadrupole triplets are used to match and focus the beam along this section.Beam instrumentation elements, like BPM, ICT and profile  monitors are also put along the sections.The program elegant [13] is used to match and track the beam along the accelerating section.The beta function along the accelerating section is shown in Fig. 4. For the tracking, the particles within the range of ±5 ps from the reference particles are used as input for the accelerating section.The beam energy after this section is about 250 MeV and the energy spread is 0.24%.The normalized emittances are 39.1 and 39.5 mm • mrad in the horizontal and vertical plane, respectively.The beam phase space at the end of the linac are shown in Fig. 5 and 6.It should be noticed that there are some particles located outside the time range ±5 ps and their energy will be much lower than 250 MeV.Those particles will be collimated in the transfer line.
The simulation shows that there is no particle loss during this stage.

The Transfer line to booster
The transfer line has to transport the beam from linac to the booster ring.There are several functions that the transfer line should provide: the beam matching between the linac and booster, the dispersion-free design, the momentum collimation and maybe the bunch length compression.Here, we only have to deal with the matching and collimation.The designed transfer line is composed by two DBA structures which guarantee the dispersion free condition.The first DBA consists two bending magnets and a matching quadrupole.The second DBA includes a bending manget and the injection lambertson magnet of the booster.Three quadrupoles are put between the bending magnet and the lambertson.The angles of all dipole magnets are 200 mrad.
Three quadrupoles are put before the first DBA structure and another six quadrupoles are put between the two DBAs for matching.The input Twiss parameters are from the linac injector system and the output Twiss parameters are from the booster ring.After some optimization.the beam optics of the transfer line is shown in Fig. 7.The maximum dispersions of the two DBAs are -0.2 and 0.6 m, respectively.Because there are some particles whose energy is far from the 250 MeV, it is important to put collmiation system in the first DBA in order to control the beam loss.This work will be done in the next step.

The error study
For the entire low energy linac, together with the transfer line, a start-to-end error analysis has been performed.All kinds of static errors are applied: the rms of the all offsets and angle are set to 0.1 mm and 0.1mrad, the relative field error 0.1%.and the rms phase error of all cavities 1.0 deg.
For the bunching section, the gradient, phase and offset of cavities, the offset and field error of solenoids are manually setup during the astra simulation.Here, there is no correction performed.
For the accelerating section and the transfer line, the offset and field errors are applied to all cavities and magnets.Then the beam-based correction is performed in the elegant simulation.The machine layout with 300 random errors settings are simulated.
At the of the transfer line, the normalized horizontal and vertical emittances are smaller than 63 and 55 mm • mrad for 90% of random machines, as shown in Fig. 8.The rms energy jitter is less than 0.1%.The energy spreads for 90% of random machines are less than 5.5‰, which can be largely reduced after momentum collimation.And the overall effective transmission rate is higher than 90%.

Summary
In this paper, a update of the low energy linac system for the project SAPS is presented.The delivered bunch charge has been increased to 5.2 nC and the beam energy become 250 MeV.A design of the transfer line between the linac and booster is also presented.With a start-to-end error analysis, it is shown that the injector system can fulfill the requirement of the booster ring.

Figure 1 .
Figure 1.The layout of the 250 MeV injector linac system.

Figure 2 .
Figure 2. The geometry and optics of the electron gun.

Figure 3 .
Figure 3.The rms transverse beam emittacne and beam size along the bunching system.

Figure 4 .
Figure 4.The beta function along the accelerating system.

14thFigure 5 .
Figure 5.The transverse phase space at the end of the accelerating section.The left hand side plot is for the horizontal plane and right hand side plot is for the vertical plane.

Figure 6 .
Figure 6.The longitudinal phase space at the end of the accelerating section.

Figure 7 .
Figure 7.The beta function for the transfer line.

Table 1 .
The main parameters of the injector system

Table 2 .
The geometry and optics of gun are shown in Fig 2.

Table 2 .
The parameters of the high voltage DC gun

Table 3 .
Parameters of the RF cavities in bunching system