20 research outputs found
Sustainable early-stage lasing in a low-emittance electron storage ring
In this Letter, we report on the concept and analysis of a low-emittance
electron storage ring, in which the electron beams undergo an early-stage
self-amplified spontaneous emission lasing process on a turn-by-turn basis. The
lasing process for each pass through a long undulator in the ring is terminated
when the radiated power is still negligible compared to the total synchrotron
loss of each circulation, and the electron beams can be maintained in an
equilibrium state that supports sustainable lasing. A self-consistent model is
derived for evaluation of the properties of the electron beams, and a design
with numerical modeling is presented that demonstrates the feasibility of
generating short-wavelength radiation at the kW power level
Terahertz scale microbunching instability driven by nonevaporable getter coating resistive-wall impedance
Non-evaporable getter (NEG) coating is widely required in the next generation
of light sources and circular colliders for small vacuum pipes to
improve the vacuum level, which, however, also enhances the high-frequency
resistive-wall impedance and often generates a resonator-like peak in the
terahertz frequency region. In this paper, we will use the parameters of the
planned Hefei Advanced Light Facility (HALF) storage ring to study the impact
of NEG coating resistive-wall impedance on the longitudinal microwave
instability via particle tracking simulation. Using different NEG coating
parameters (resistivity and thickness) as examples, we find that the impedance
with a narrow and strong peak in the high frequency region can cause
micro-bunching instability, which has a low instability threshold current and
contributes to a large energy spread widening above the threshold. In order to
obtain a convergent simulation of the beam dynamics, one must properly resolve
such a peak. The coating with a lower resistivity has a much less sharp peak in
its impedance spectrum, which is helpful to suppress the micro-bunching
instability and in return contributes to a weaker microwave instability
Bunch lengthening affected by the short-range effect of resonant modes in radio-frequency cavities
Longitudinal bunch lengthening via higher harmonic cavities is essential for
the new state-of-the-art 4th generation of synchrotron light storage rings, as
it can effectively improve the Touschek lifetime and mitigate the transverse
emittance growth due to intrabeam scattering. In general, the optimum or
near-optimum bunch lengthening condition is widely adopted for the double
radio-frequency system. This paper reveals, under this optimum lengthening
condition, that the short-range effect of resonant modes of the main and
harmonic cavities has the potential to enhance or suppress the bunch
lengthening significantly. Using the planned Hefei Advanced Light Facility
storage ring as an example, it is particularly demonstrated that the
short-range effects of the main and harmonic fundamental modes can dramatically
degrade the bunch lengthening for the assumed case of high-charge bunches. This
degradation of bunch lengthening is again presented with a realistic example of
PETRA-IV that operated in timing mode with high bunch charge. It is found that
there exists a setting of harmonic voltage and phase quite different from the
conventional optimum lengthening setting, to get optimum bunch lengthening
Analytic formulas for the D-mode Robinson instability
The passive superconducting harmonic cavity (PSHC) scheme is adopted by
several existing and future synchrotron light source storage rings, as it has a
relatively smaller R/Q and a relatively larger quality factor (Q), which can
effectively reduce the beam-loading effect and suppress the mode-one
instability. Based on the mode-zero Robinson instability equation of uniformly
filled rigid bunches and a search algorithm for minimum, we have revealed that
the PSHC fundamental mode with a large loaded-Q possibly triggers the D-mode
Robinson instability [T. He, et al., Mode-zero Robinson instability in the
presence of passive superconducting harmonic cavities, PRAB 26, 064403 (2023)].
This D-mode Robinson instability is unique because it is anti-damped by the
radiation-damping effect. In this paper, analytical formulas for the frequency
and growth rate of the D-mode Robinson instability are derived with several
appropriate approximations. These analytical formulas will facilitate analyzing
and understanding the D-mode Robinson instability. Most importantly, useful
formulas for the D-mode threshold detuning calculation have finally been found
Complex unit lattice cell for low-emittance storage ring light source
To achieve the true diffraction-limited emittance of a storage ring light
source, such as ~10 pm.rad for medium-energy electron beams, within a limited
circumference, it is generally necessary to increase the number of bending
magnets in a multi-bend achromat (MBA) lattice, as in the future upgrade plan
of MAX IV with a 19BA replacing the current 7BA. However, this comes with
extremely strong quadrupole and sextupole magnets and very limited space. The
former can result in very small vacuum chambers, increasing the coupling
impedance and thus enhancing the beam instabilities, and the latter can pose
significant challenges in accommodating the necessary diagnostics and vacuum
components. Inspired by the hybrid MBA lattice concept, in this paper we
propose a new unit lattice concept called the complex unit lattice cell, which
can reduce the magnet strengths and also save space. The complex unit cell is
numerically studied using a simplified model. Then as an example, a 17BA
lattice based on the complex unit cell concept is designed for a 3 GeV storage
ring light source with a circumference of 537.6 m, which has a natural
emittance of 19.3 pm.rad. This 17BA lattice is also compared with the 17BA
lattice designed with conventional unit cells to showcase the benefits of the
complex unit cell concept. This 17BA lattice also suggests a new type of MBA
lattice, which we call the MBA lattice with semi-distributed chromatic
correction
Minimizing the fluctuation of resonance driving terms in dynamic aperture optimization
Dynamic aperture (DA) is an important nonlinear property of a storage ring
lattice, which has a dominant effect on beam injection efficiency and beam
lifetime. Generally, minimizing both resonance driving terms (RDTs) and
amplitude dependent tune shifts is an essential condition for enlarging the DA.
In this paper, we study the correlation between the fluctuation of RDTs along
the ring and the DA area with double- and multi-bend achromat lattices. It is
found that minimizing the RDT fluctuations is more effective than minimizing
RDTs themselves in enlarging the DA, and thus can serve as a very powerful
indicator in the DA optimization. Besides, it is found that minimizing
lower-order RDT fluctuations can also reduce higher-order RDTs, which are not
only more computationally complicated but also more numerous. The effectiveness
of controlling the RDT fluctuations in enlarging the DA confirms that the local
cancellation of nonlinear effects used in some diffraction-limited storage ring
lattices is more effective than the global cancellation
EMITTANCE OPTIMIZATION USING PARTICLE SWARM ALGORITHM*
Abstract In this paper we use a swarm intelligence algorithm, Particle Swarm Optimization (PSO), to optimize the emittance directly. Some constraint conditions such as beta functions, fractional tunes and dispersion function, are considered in the emittance optimization. We optimize the strengths of quadrupoles to search for low emittances. Here an FBA lattice studied in the design of the Hefei Advanced Light Source storage ring is used as the test lattice. The PSO is shown to be beneficial in the optimization
Mode-zero Robinson instability in the presence of passive superconducting harmonic cavities
A higher harmonic cavity (HHC) is popularly employed in synchrotron light storage rings to enhance the machine performance, which requires its fundamental mode resonant frequency to be tuned above the radio-frequency harmonic. However, this detuning is likely to cause Robinson instability. In this paper, we focus on a mode-zero Robinson instability driven by the fundamental mode of a passive superconducting harmonic cavity (PSHC). This instability oscillates slightly below the detuning frequency of PSHC and was recently observed in tracking simulations or experiments for several synchrotron light sources, but the underlying mechanisms have not been well understood. To investigate this instability, we modify the conventional Robinson instability equation with the inclusion of the damping effect. By solving directly this modified equation combined with performing macroparticle tracking simulation, it is found that this instability is largely dependent on the momentum compaction factor, the Q value and detuning of PSHC, and even the radiation damping time. Most importantly, this instability can be significantly enhanced by a higher Q of PSHC and a lower radiation damping time, which is completely contrary to the conventional Robinson instability