Multiturn Injection Design and Optimization for XiPAF-Upgrading Synchrotron

XiPAF (Xi’an 200MeV proton application Facility) synchrotron is using H- stripping injection and phase space painting scheme. With the demand of more particle species for single event effect study, XiPAF synchrotron has been upgraded to multiturn injection from stripping injection, the injection system must be redesigned. This paper report XiPAF-Upgrading synchrotron multiturn injection scheme, simulation results by PyORBIT[1] show that the injection efficiency is ∼85% for proton and ∼80% for heavy ions. The influence of space charge and magnet errors on accumulated particle number has been studied by simulation.

With the increasing demand for space radiation experiments, multiple ions are needed.XiPAF synchrotron is planning to upgrade to a proton and heavy ion synchrotron, i.e.XiPAF-Upgrading synchrotron.In order to inject all kind of particles, XiPAF's original stripping injection system could not meet the requirements and multiturn injection scheme must be used.This article introduced multiturn injection design and simulation results of XiPAF-Upgrading synchrotron.

Injection System 2.1. Layout
In order to achieve full particle injection, the original negative hydrogen stripping injection scheme needs to be changed to a multiturn injection scheme.
As shown in Fig. 1, input beam is deflected by a injection magnetic septum (MSeptum), a electrostatic septum (ESeptum), then injected into the synchrotron.Phase space painting parameters are the key for injection efficiency and gain in multiturn injection scheme, which is affected by synchrotron injection bump orbit at injection point, injected beam twiss and so on.In order to control the position and angle of the bump orbit more conveniently to improve injection efficiency, the number of injection bump magnets is increased to four (BP1∼BP4) from original two, allowing for independent adjustment of the bump orbit position and angle at the injection point.The injection system consists 6 components, include 1 injection magnetic septum, 1 injection electrostatic septum and 4 injection bump magnets.

Equipment Parameters
Injection electrostatic septum is set at 40 mm from synchrotron reference orbit in horizontal (x) dimension, outside the ring, guaranteed horizontal acceptance of 200 mm • mrad.Injected beam center coordinates (  ,  ′  ) must be close to septum for maximum injection efficiency, we choose   = 45 mm and  ′  = 0 mrad, to avoid stored beam envelope becomes too large at focusing quadrupole, which is limited by quadrupole magnet pole aperture.The injection bump orbit distribution in synchrotron is shown as Fig. 2. The maximum injection bump strength is 0.0243 T • m for Bi 32+ , the effective length of BP1, BP2, BP3 is 0.2 m, and 0.25 m for BP4.Main parameters of ESeptum and MSeptum are shown as Table 1.

Input Beam Parameters
For higher injection efficiency, a injection mismatch condition [5], as Eq.( 1) and Eq.( 2), must be satisfied, where   ,   ,   for injected beam and   ,   ,   for synchrotron matched twiss parameters and acceptance.The first condition (Eq.( 1)) promised that the injected beam is upright ellipse in normalized phase space so the bump reduce step is smallest in phase space painting, the second condition (Eq.( 2)) make sure that beam always stay in acceptance circle in normalized phase space.2.

Parameters
Proton Heavy Ions

Multiturn Injection Simulation
In multiturn injection, injection orbit bump is reduced with time so that the early beam occupies the central region of the horizontal acceptance and the later beam the periphery of Multiturn injection simulations based on PyORBIT are carried out.Initinal beam distributions come from LRBT beam dynamics simulation.After painting curve optimization, injection efficiencies have been improved to ∼80%, with injection turn limited to 6∼12 turn, as shown in Fig. 3.More stored particles can be obtained by increasing injection turn, but comes a lower injection efficiency, which may affect vacuum status and beam lifetime in synchrotron.Typical beam distribution after injection painting process is shown in Fig. 3, with longitudinal coasting beam.Stored protons are less than original XiPAF synchrotron situation, which is 2 × 10 11 .We will explain it in discuss section.

Space Charge
The stored proton number of the XiPAF-Upgrading synchrotron is less than the original proton synchrotron, which is due to the limitation of Liouville's theorem, as the beam cannot overlap turn by turn in phase space.The injection gain of the multiturn injection scheme is smaller compared to stripping injection.
With an increasing proton beam intensity of 8 mA, the stored particles reach 2.5×10 11 , which is similar to the original XiPAF proton synchrotron level.With this beam intensity, beam loss is critical in RF capture and acceleration, resulting in a total acceleration efficiency of 5%, which is unacceptable.
After single particle motion and beam tune analysis, the main beam loss is found to be due to the transverse couple resonance   + 2  = 6.After injection,   /  ≈ 10, and the small vertical emittance generates a huge tune shift (∼0.4) in the y dimension.The tune footprint overlaps the couple resonance and space charge induced resonance crossing occurs when the bare tune is (1.73, 2.26).The vertical emittance then blows up and particles are lost at the dipole vertical vacuum chamber.
Space charge tune optimization is based on   scan.Injection and acceleration efficiencies are shown in Fig. 4

Errors and Transverse Coupling
After injection,   /  ≈ 10, while the vertical acceptance is limited by the dipole vertical vacuum chamber, transverse coupling is the main reason of beam loss in XiPAF-Upgrading synchrotron, it comes from magnet field errors, rotate alignment errors, and space charge.
For heavy ions, only magnet field errors and rotate alignment errors exists.XiPAF proton synchrotron measured field errors and alignment errors are used to evaluate the transverse coupling effect.Simulation results show that rotate errors and field errors are mutually suppressed under this particular error setting.However, due to the random nature of alignment errors, maximum 30% beam loss caused by transverse coupling can be expected in the future real machine.
For proton, field errors and alignment errors have no significant influence on the stored particle number, space charge is the main limit.

CONCLUSION
Multiturn injection is employed in XiPAF-Upgrading synchrotron for all ions accumulation, the main injection system parameters and injection simulation results have been introduced, injection efficiencies of ∼85% for proton and ∼80% for heavy ions have been achieved.
Transverse coupling is the main limit of stored beam intensity.For proton, space charge is the main reason causing transverse coupling, space charge optimization has been carried out for more stored particles during proton acceleartion, and tune has changed to (1.73, 2.11) for proton injection to avoid resonance   + 2  = 6.For heavy ions, magnet field errors and rotate alignment errors can induce a maximum of 30% beam loss.

Figure 2 .
Figure 2. Injection bump orbit distribution along the ring.

14th
International Particle Accelerator Conference Journal of Physics: Conference Series 2687 (2024) 052034 IOP Publishing doi:10.1088/1742-6596/2687/5/0520344 the acceptance ellipse, which is known as phase space painting.The painting is finished when beam emittance reach the designed value.Extraction emittance is set to 100 mm • mrad for all ions to avoid extraction spill, different extraction energy means different beam emittance after injection.Emittance after injection are classified to 3 types: 200 mm • mrad, 170 mm • mrad and 120 mm • mrad.Beam emittance after injection has a non-uniform distribution on particle momentum because of dispersion mismatch.So a slightly bigger painting emittance is accepted for more usable particles.

Figure 4 .
Figure 4. Space charge tune optimization for proton acceleration.Blue for injection efficiency, red for acceleration efficiency, solid lines represent proton number after injection is 6.7 × 10 10 , dashed lines represent proton number after injection is 13.5 × 10 10 .

Table 1 .
Main parameters of injection ESeptum and MSeptum 5 × 10 10 .particles, but acceleration efficiencies are quite different, best tune is (1.73, 2.11) to obtain the higest acceleration efficiency, which is below the couple resonance   + 2  = 6.