Injector of the Turkish Light Source Facility TURKAY

SYNCHROTRON RADIATION TECHNOLOGY AND APPLICATIONS

Injector of the Turkish Light Source Facility TURKAY

Zafer Nergiz，

Avni Aksoy

Nuclear Science and Techniques

Vol.28, No.11

Article number 161

Published in print 01 Nov 2017

Available online 26 Oct 2017

DOI：10.1007/s41365-017-0306-8

35000

A synchrotron radiation source called TURKAY is proposed as a sub project of the Turkish Accelerator Center Project (TAC). The storage ring of TURKAY is a low emittance synchrotron and the radiation ranges between 0.01–60 keV and can be generated from the insertion devices and bending magnets placed on it. The injector system of the facility will mainly consist of a 150 MeV linac and full energy booster. In this study, we present design considerations and beam dynamics studies of the preinjector linac and booster ring for TURKAY.

Booster ringPreinjector linacBeam dynamics

1 Introduction

The Turkish Accelerator Center (TAC) is a project aimed to build some accelerator facilities in Turkey, and at first stage, an infra-red FEL facility named TARLA is under construction [1-3]. Another part of the TAC is to build a synchrotron radiation facility (TURKAY) and this facility is in a detailed design stage. The storage ring is designed and the results from the lattice optimization and the radiation properties have been presented in previous studies [4, 5]. The designed storage ring has a 0.51 nm rad emittance at 3 GeV electron energy, and the ring circumference is 477 m. This design is on par with other designs in the world community.

The major task of the injector is to generate and accelerate the electrons to an energy of 3 GeV. Then, the electron beam is extracted from the booster and transferred into the storage ring. A full energy injection system is the solution to reach design performance in an ultra-low emittance ring. A low energy linac with a full energy booster has been the choice for most light source injection systems because of its advantages in cost and reliability. Therefore, the design concept of TURKAY is composed of three parts: Injector, Full Energy Booster Ring and Storage Ring. The electron bunches generated by the electron gun are accelerated up to 150 MeV in a linear accelerator, and they are injected into a booster synchrotron which raises the beam energy to nominal operation value up to 3 GeV. Then, they are injected into the storage ring. The booster and storage ring RF systems will operate at approximately 500 MHz and the main reason for that is the availability of RF cavities that can be commercially supplied at this frequency.

2 Design Considerations

The storage ring shall has several fill-pattern modes according to user requirements such as uniform filling mode, half fill mode, and hybrit-fill mode. For instance, time resolved science requires operating modes with single bunch, full fill or hybrid fills, while diffraction applications requires uniform fill pattern. The maximum number of bunches in a SR is defined with N_b=C_SR/t_rep where C_SR is the circumference of the storage ring and t_rep is the minimum spacing between bunches t_rep=n/f_RF where f_RF is the operating frequency of the RF structure on storage ring and n is integer.

One of the important operating considerations is the fill-up method of the storage ring. There exist two type of filling method for a single bunch; refill mode and top-up fill mode. In refill mode, the pattern of bunches are filled to SR in time periods depending on lifetime. In top-up fill mode, stored beam current is maintained at a near constant level using frequent injection [6, 7]. Therefore the gun and pre-injector should provide a full bunch pattern equivalent to the booster ring circumference and should allow the flexible bunching system. Top-up injection is nowadays state of the art for storage ring based SR sources. For radiation safety reasons, the electron beam losses during injection with open photon beam shutters have to be minimized. Therefore, the booster emittance should be small to capture the full beam in the storage ring acceptance.

3 Preinjector Linac

The pre-injector linac of TURKAY consists of a 100 kV DC thermoionic gun, a sub-harmonic pre-buncher operating at 500 MHz, a pre-buncher operating at 3 GHz, a standing wave buncher operating at 3 GHz, and two traveling wave accelerating structures operating at 3 GHz, similar to ALBA project [8]. Schematic layout of injector is given with Fig. 1.

Figure 1:

(Color online) Layout of TURKAY pre-injector linac.

The electron gun is the standard triode gun and it is modulated by a 500 MHz amplifier. Therefore, the beam pulse structure at the gun is already matched to the booster and storage ring RF-bucket structure. After extraction of current from a cathode, the electron bunches can be created with a modulated voltage system at a length of σ_t ≈ 1 ns. The bunches will be compressed down to σ_t ≈ 10 ps in the drift sections by the energy modulation obtained in the sub-harmonic pre-buncher and the buncher. Final energy modulation and bunching will be performed in the buncher cavity in order to reduce the energy spread and to minimize the losses. The bunches will be accelerated to the final energy of 150 MeV by three traveling wave accelerating structures (Table 1).

Beam parameters of the TURKAY injector linac

Parameter	Single Bunch	Multi Bunch
Energy (MeV)	>150	>150
Pulse length (ns)	1	600
Pulse charge (nC)	1	6
RMS emitance (mm mrad)	<50	<50
Energy spread (%)	<1	<1

ASTRA [9] code has been used to analyze the beam behavior, such as the radial control, the bunching process, the energy spread, emittance, and transmission rate along the pre-injector. In the calculations, we have assumed that the maximum length of a single bunch of 1 nC is 1 ns at the exit of gun. The longitudinal distribution of bunches have inverted parabolic distribution and they have a 10 mm mrad emittance at maximum bunch charge.

The evolution of the transverse beam size and emittance from electron source to the end of the injector is given in Fig. 2. The energy increment, bunch length variation, and beam loss rate along the injector is given with Fig. 3. Final phase spaces and parameters of single bunch are shown in Fig. 4. As it is seen from the figure, the transverse emittance is lower than 50 mm.mrad. Figure 3 shows that more than 85 % of particles carried to the booster ring and bunch is compressed down to 2 mm. As it can be seen on the figure, almost 15 % of the particles are lost in the vacuum chamber between second pre-buncher and buncher cavity and on cavity walls of the initial two cell of the buncher cavity.

Figure 2:

(Color online) Transverse beam sizes and projected emittance variation of a single bunch along the injector linac.

Figure 3:

(Color online) Beam energy, bunch length, and transmission rate evolution of a single bunch along the injector linac.

Figure 4:

(Color online) Transverse beam distribution (top right is horizontal and top left is vertical), longitudinal phase spaces (bottom left), and the beam parameters of the bunch at the end of injector linac. The histograms associated with each macro particle distribution show the projection along the respective axis.

4 Booster

Almost all of the synchrotron radiation facilities in operation or in their design process have chosen a full energy booster synchrotron as an injector together with a low energy linac as the pre-injector. Some of the sources that have a booster share the same tunnel with the storage ring [10-13]. A long booster ring shares the same tunnel with the storage ring designed for TURKAY.

The circumference of the storage ring is 477.4 m. A reasonable tunnel space is needed to allow the installation of equipments, and it is found at least around 30-m difference in circumference is needed. Hence, the circumference of the booster is 445.6 m. With the 500 MHz RF frequency, the storage ring harmonic number is 795. The harmonic number of the concentric booster is 742 with the same RF frequency. The lattice of the booster ring is a combined function FODO lattice structure. There are eight superperiods and each superperiod consists of 6 cells of a combined function FODO lattice and two matching cells. Each bending magnets used in the FODO cells are 1.5 m long with a bending angle of 5.625 degree, and they have defocusing quadrupole and sextupole field components. So, the dipole field is 0.65 T at 3 GeV and 0.033 T at 150 MeV. The quadrupoles provide the focusing strength and they also include the focusing sextupole field components. The matching cell at the end of each superperiod consists of four matching quadrupoles and two combined function dipoles. The total bending angle of these two bending magnets is 5.625 degrees. Also, the matching cells include pairs of extra sextupoles. The optical functions are shown in Fig. 5, and the basic parameters of the booster are listed in Table 2. The circumference of the booster ring is 445.6 m and the lattice consists of eight (4.8 m) straight sections with zero dispersion suitable for the installation of RF cavity, injection, and extraction systems. The emittance value of the ring at 3 GeV is 2.92 nm rad. OPA, ELEGANT, BEAMOPTICS, and MADX codes are used to design a magnetic lattice of booster rings [14-17].

The main parameters of the booster ring.

Parameters	Value
Energy (GeV)	0.15-3.0
Circumference (m)	445.6
H. Emittance (nm rad)	2.92
Energy Loss/part./turn (3 GeV) (keV)	461.6
Max.βx (m)	11.0
Max. βy (m)	16.5
βx in the mid. of straight sect. (m)	3.35
βy in the mid. of straight sect. (m)	12
Dx in the mid. of straight sect. (m)	0.00
Betatron Tunes Qx/Qy	30.85/13.92
Natural Chromaticity ξx/ξy	-29.63/-20.18
Number of superperiod	8
Damping Time H/V/Long. (ms)	10.680/19.323 /16.228
Harmonic Number	742

Figure 5:

(Color online) Optical functions in one superperiod of the booster ring.

Natural chromaticity should be corrected by the sextupole magnets. However, the nonlinearities of strong sextupole magnets affect the dynamic aperture, which is the transverse area of the particle in which it can execute its oscillations safely without getting defocused or lost. This is due to the quadratic field distribution in the sextupole magnets. The nonlinear forces on the particle, which lead to anharmonic betatron oscillations arise and particle loses above a certain amplitude. Particles with nominal momentum but wrong oscillations will be deflected by the sextupole fields and it will no longer be possible to find a stable trajectory [18]. This affect can be compensated by the use of other sextupoles. The dynamic aperture, including synchrotron radiation effect for different momentum offsets at the injection point, are shown in Fig. 6. The beam sizes at injection are σx=0.82 mm and σx=1.3 mm. The dynamic aperture is large enough for the requirements of injection and extraction processes.

Figure 6:

(Color online) Dynamic aperture of the booster ring for different momentum offsets (magnet errors is zero, 10000 turns).

The condition for optical resonance in both planes is expressed as

m Q_{x} + n Q_{y} = p

(1)

where m, n, and p are integers and Qx and Qy are horizontal and vertical tunes. m+n is the resonance order and is generally concerned about resonances up to the 5th order. For stable operation, the horizontal and vertical tune values, named working point, should be chosen carefully and they should be away from resonance lines. Frequency Map Analysis (FMA) [19] is a numerical method for understanding the effect of resonance on the dynamic aperture. For this purpose the diffusion rate,

D = L o g_{10} \sqrt{Δ ν_{x}^{2} + Δ ν_{y}^{2}},

(2)

can be used as a stability index [20]. Δνx and Δνy are the transverse tune shifts of surviving particles in tracking. Fig. 7 shows the frequency map of a single particle tracked by the code ELEGANT. As is seen from the figure, especially in the central region of the aperture, the effect of the resonant is so limited.

Figure 7:

(Color online) Frequency map of the booster obtained from tracking of two thousands turns

The misalignment and field errors of the magnets can be caused by the closed orbit distortion (COD) which is one of the error sources in the high intensity synchrotron ring. The distortion can be corrected by the use of beam position monitors and horizontal and vertical correctors. Simulation results for distorted and corrected closed orbits are shown in Figs. 8 and 9, respectively. The errors applied to the magnets have a gaussian distribution with a cut-off in three sigma and accepted misalignment deviations (displacement and roll of magnetic elements) as shown in the Table 3. The maximum corrector strength is limited to 1 mrad.

Deviations of the errors applied to the magnets of the booster ring.

Error Type	Dipoles	Quadrupoles	Sextupoles
Δx (mm)	0.2	0.2	0.2
Δy (mm)	0.2	0.2	0.2
Δϕ (mrad)	0.5	0.5	0.5
Dipole Field Err. (%)	0.1	-	-

Figure 8:

(Color online) Closed orbit distortion along the booster ring for 100 seeds of random errors.

Figure 9:

(Color online) Corrected closed orbit along the booster ring for 100 seeds of random errors.

In a booster ring, the emittance and the energy spread change during acceleration. Time evolutions of the emittance and the energy spread during ramping is given by [21]

\frac{d ε}{d t} = - \frac{1}{γ (t)} \frac{d γ (t)}{d t} ε - \frac{2}{τ_{x}} ε + \frac{1}{J_{x}} \frac{2}{τ_{x}} C_{q} γ^{2} (t) \frac{I_{5}}{I_{2}},

(3)

\frac{d σ^{2}}{d t} = C_{q} γ^{2} (t) \frac{2}{J_{z} τ_{z}} \frac{I_{3}}{I 2} - (\frac{2}{τ_{z}} + \frac{1}{γ (t)} \frac{d γ (t)}{d t}) σ^{2},

(4)

where Cq is 3.832 × 10^-3 m/(GeV)², Jx and Jz are the partition numbers, τx and τz are the damping numbers, and I₂, I₃, I₅ are the synchrotron integrals. Evolution of the emittance and energy spread of the beam during the ramping from Eq. (3), Eq. (4) and particle tracking simulation with ELEGANT are shown in Fig. 10. The Fig. 11 shows the beam energy evolution and number of particles during the ramping from a tracking simulation with 10,000 particles. All particles survived the acceleration process.

Figure 10:

(Color online) Time evolution of the beam emittance and energy spread during ramping in the booster.

Figure 11:

(Color online) Energy evolution and number of particles during ramping in the booster.

The variation of the magnetic field in the magnets, especially bending magnets, will induce an eddy current, and as a result, a sextupol field occurs in this region. Due to this sextupolar gradient, the chromaticity of the ring will change during the ramping. This effect is investigated for an elliptic stainless steel vacuum chamber 1 mm thick. The evolution of the sextupole field and chromaticity during the ramping are shown in Figs. 12 and 13. The effect of the eddy current is maximum at 0.04th second. The chromticities can easily be corrected with two lattice sextupole families.

Figure 12:

(Color online) Eddy current induced sextupole field (2 Hz).

Figure 13:

(Color online) Chromaticities induced by eddy currents on the dipoles vacuum chambers due to the ramping process.

5 Linac to Booster Transfer Lines

The linac to booster (LTB) transfer line consists of 2 bending magnet, 1 septum, 1 kicker magnet, and several quadrupoles for optical matching. The bending angle of the septum is 10 degrees, and the kick angle of the kicker magnet is 1 degree. Figure 14 shows the optical functions along the LTB transfer line. The horizontal and vertical betatron values are matched to the parameters of the injection point of the booster. The maximum betatron functions are controlled within 35 m under the constraints of well dispersion matching. Maximum beam sizes along the line are σx=5 mm and σy=2.3 mm. The beam stays clear which determines the vacuum chamber size is defined as 3σ of the beam size and 5 mm tolerance for the trajectory distortion. Therefore, the aperture requirements are ±20 mm horizontal and ±15 mm vertical. Figure 15 shows the beam phase spaces from the particle tracking simulation in injection. According to the simulation results, 100% transmission is achived.

Figure 14:

(Color online) Optical parameters of the linac to booster transfer line.

Figure 15:

Phase spaces of the injected beam to the booster, after and before the kicker magnet and after first turn at the booster (Reference point is the kicker in the coordinate of the booster).

There are two methods for injection from the booster into the storage ring. First, off axis injection with an orbit bump generated by several kicker magnets. Second, on axis injection with a single non-linear kicker, as developed recently at BESSY II [22]. For top-up injection, the second process can be very helpful to minimize residual beam excitation by the injection process. The design study for the booster to the storage ring transfer line continue and the injection process is under discussion.

6 Summary

In this paper, we have described the design of the beam dynamics of the TURKAY injector. Each component of the injector linac, such as solenoids, RF structures are preliminary designed in order to make proper simulation with the ASTRA code. Simulations show that the injector linac can deliver more than 85 % of particles to the booster within the requested parameters of the booster ring. However, no static or dynamics imperfections has been taken into account during the simulation of pre-injector linac at present. In the presence of the imperfections, such as RF and magnet errors, the efficiency is expected to be more than 75%. It will be studied in detail in the next stage of the work. An eight fold symmetry booster ring is in design, and this ring will share the same tunnel with storage ring. Top up injection is nowadays state of the art for storage ring based SR sources. For radiation safety reasons, the electron beam losses during injection with open photon beam shutters have to be minimized. Therefore, the booster emittance should be small to capture the full beam in the storage ring acceptance. 2.92 nm rad horizontal emittance is sufficiently low for this purpose. The tracking simulations show that the particles can be ramped to 3 GeV nominal energy without particle looses.

References

[1]

S. Sultansoy, M. Yilmaz, O. Cakir, et al.

The Status of Turkic Accelerator Complex Proposal

, Proceedings of PAC05, Knoxwille, Tennessee, USA, p. 449 (2005). doi: 10.1109/PAC.2005.1590465