169 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
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
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
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
Intrinsic Electrical Transport Properties of Monolayer Silicene and MoS2 from First Principles
The electron-phonon interaction and related transport properties are
investigated in monolayer silicene and MoS2 by using a density functional
theory calculation combined with a full-band Monte Carlo analysis. In the case
of silicene, the results illustrate that the out-of-plane acoustic phonon mode
may play the dominant role unlike its close relative - graphene. The small
energy of this phonon mode, originating from the weak sp2 bonding between Si
atoms, contributes to the high scattering rate and significant degradation in
electron transport. In MoS2, the longitudinal acoustic phonons show the
strongest interaction with electrons. The key factor in this material appears
to be the Q valleys located between the {\Gamma} and K points in the first
Brillouin zone as they introduce additional intervalley scattering. The
analysis also reveals the potential impact of extrinsic screening by other
carriers and/or adjacent materials. Subsequent decrease in the actual
scattering rate can be drastic, warranting careful consideration. Finally, the
effective deformation potential constants are extracted for all relevant
intrinsic electron-phonon scattering processes in both materials
Low-density nanoporous iron foams synthesized by sol-gel autocombustion
Nanoporous iron metal foams were synthesized by an improved sol-gel autocombustion method in this report. It has been confirmed to be pure phase iron by X-ray diffraction measurements. The nanoporous characteristics were illustrated through scanning electron microscope and transmission electron microscope images. Very low density and quite large saturation magnetization has been performed in the synthesized samples
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