MINDS@UW Madison
http://digital.library.wisc.edu/1793/8334
2019-04-26T02:36:40ZPartial design of a 980-MeV Energy Recovery Linac (ERL)
http://digital.library.wisc.edu/1793/79066
Partial design of a 980-MeV Energy Recovery Linac (ERL)
Bosch, Robert A.; Bisognano, Joseph J.; Medley, M.D.
We describe the partial design of a 980-MeV energy recovery linac (ERL) with radiofrequency (RF)
of 1.5 GHz. We model the linac, recirculation arcs, beam spreader/combiner, and beam compression.
The electron gun, gunline, and beam dump are not modeled.
We consider a one-up/one-down design in which 20-MeV bunches are accelerated and decelerated by
a 960-MeV superconducting linac. We also consider a two-up/two-down design in which 20-MeV
bunches are accelerated by two passages through a 480-MeV superconducting linac and decelerated by
two subsequent passages. For both designs, which incorporate graded-gradient linac focusing with cavity
gradients of 15 MeV/m, the beam breakup (BBU) thresholds exceed 100 mA.
In the two-up/two-down design, we achieved a large degree of longitudinal compression in both
recirculation arcs by accelerating and decelerating off-crest in the linac. For low current operation with
normalized emittances in both transverse directions of 0.1 mm-mrad, this high-compression design was
studied by tracking without consideration of synchrotron radiation using two codes. In tracking with the
MAD-with-acceleration code, bunches with initial rms bunchlength σ = t 1.85 ps are compressed to 22 fs
in the 500-MeV arc, to 15 fs in the 980-MeV arc, and to 32 fs in the second passage through the 500-MeV
arc. The compressed bunch transverse dimensions are slightly larger than those given by conservation of
the normalized emittance. In tracking with the elegant code, bunches with initial rms bunchlength σ = t
1.85 ps are compressed to 8.5 fs in the 500-MeV arc, to 9.2 fs in the 980-MeV arc, and to 48 fs in the
second passage through the 500-MeV arc.
Incoherent synchrotron radiation (ISR) was then included in tracking with elegant, indicating a slight
increase in the compressed bunch lengths and horizontal emittance. The effect of coherent synchrotron
radiation (CSR) was also studied by tracking with elegant. For bunch charge of 1 pC (corresponding to
average ERL current of 1.5 mA), the CSR has a small effect, while for bunch charges of 2 and 4 pC, the
longitudinal bunch compression and horizontal focusing are both significantly worsened by CSR. For
isochronous transport at 500 MeV and high compression (factor of ~180) at 980 MeV, bunch charges ≤ 2
pC suffered little degradation from CSR. With isochronous transport in both recirculation arcs, bunch
charges ≤ 10 pC suffered little degradation from CSR. The tracking results suggest that average ring
currents in the tens of milliamperes will be strongly affected by CSR, degrading high-current operation.
For ring currents of several mA, high performance operation with a large degree of longitudinal
compression is expected from our design.
2006-12-06T00:00:00ZLongitudinal broadband impedance of the Aladdin vacuum chamber and new dipole chamber design
http://digital.library.wisc.edu/1793/79065
Longitudinal broadband impedance of the Aladdin vacuum chamber and new dipole chamber design
Bosch, Robert A.; Fisher, Mike V.
An estimate of the longitudinal broadband impedance is obtained by summing estimated impedance contributions from the radiofrequency (RF) cavities, bellows, beam position monitors, Q electrode, vacuum chamber transitions, dipole chambers, pumping holes and slots, kickers, flanges, sector valves, resistive walls and space charge. The estimated total reduced impedance is |Zn/n| = 11.5 Ω. The RF cavities are estimated to contribute 5.8 Ω to the reduced impedance |Zn/n|, while the vacuum chamber is estimated to contribute 5.7 Ω. The vacuum chamber impedance of 5.7 Ω is expected to excite the microwave instability.
If the dipole vacuum chambers are replaced by a design with an antechamber connected by a slot of height 24 mm, and the attached large formed bellows are effectively shielded (or shortened), the estimated ring impedance is decreased by 0.5 Ω. This impedance reduction may improve performance of the standard low-emittance lattice at high ring currents, where increased beam dimensions attributed to the microwave instability are observed.
2004-06-30T00:00:00ZInstabilities driven by higher-order modes in an RF system with a passive higher harmonic cavity
http://digital.library.wisc.edu/1793/79064
Instabilities driven by higher-order modes in an RF system with a passive higher harmonic cavity
Bosch, Robert A.
A passive higher-harmonic cavity may be used to suppress parasitic longitudinal coupled-bunch
instabilities. Our analytic modeling already predicts whether a parasitic higher-order mode (HOM) will
cause longitudinal coupled bunch instability for a worst-case scenario where a synchrotron sideband has
the same frequency as the HOM. An analytic prediction of whether a broadband HOM will cause
microwave instability has also been added to the modeling.
The capability to include a higher-order mode (HOM) has also been added to a simulation code that
models Robinson instabilities with a higher harmonic cavity. To speed the computations, the fundamental
modes in the RF cavities and the HOM wake fields are represented by in-phase and quadrature
components.
The analytic modeling and simulations are compared for passive harmonic-cavity operation of
Aladdin with the base lattice and low-emittance lattice, for the UVSOR ring, and for MAXlab rings. For
the parasitic coupled bunch instability, the analytic predictions are in approximate agreement with
simulations. For the microwave instability, the analytic predictions are in rough agreement with
simulations.
For low-emittance operation of the Aladdin ring, the bunchlength in simulations of the microwave
instability is consistent with experimental measurements. This suggests that our estimate of the reduced
longitudinal broadband impedance (5.7 Ω) is reasonable.
In modeling of MAX-II, a typical parasitic coupled-bunch instability is suppressed by the harmonic
cavity in the new 100-MHz/500-MHz RF system and the previously installed 500-MHz/1500-MHz RF
system. Both RF systems are expected to show little or no evidence of microwave instability at full ring
energy, provided that the reduced longitudinal broadband impedance is less than 4 Ω.
The simulations confirm that a passive harmonic cavity may be used to suppress parasitic coupled
bunch instabilities, in approximate agreement with the analytic model.
2004-10-14T00:00:00ZInstability modeling for passive harmonic-cavity operation at MAXlab
http://digital.library.wisc.edu/1793/79063
Instability modeling for passive harmonic-cavity operation at MAXlab
Bosch, Robert A.
Instability modeling is performed for passive harmonic-cavity operation at MAXlab. For the MAX-II 1.5 GeV ring, we consider the 100-MHz/500-MHz RF system now being constructed with three fundamental cavities and one harmonic cavity. We also model the existing 500-MHz/1500-MHz system on MAX-II. For the MAX-III 700 MeV ring, we model a 100-MHz/500-MHz RF system with one harmonic cavity, and a system with two identical harmonic cavities. For the proposed MAX-IV 1.5-GeV ring, we model a 100-MHz/500-MHz RF system with four fundamental cavities and one harmonic cavity. For the proposed MAX-IV 3-GeV ring, we model a 100-MHz/500-MHz RF system with ten fundamental cavities and two harmonic cavities.
Instability plots are shown which predict the current range where optimal bunchlengthening by the harmonic cavity is stable. We note the calculated coupled-dipole Robinson frequency, at which a large amount of phase noise on the beam may be expected. In addition, the bunchlengths are calculated for the case without a harmonic cavity and for optimal bunchlengthening.
2004-03-26T00:00:00Z