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arXiv2014

Possibilities for reduction of transverse projected emittances by partial removal of transverse to longitudinal beam correlations

We show that if in the particle beam there are linear correlations between energy of particles and their transverse positions and momenta (linear beam dispersions), then the transverse projected emittances always can be reduced by letting the beam to pass through magnetostatic system with specially chosen nonzero lattice dispersions. The maximum possible reduction of the transverse projected emittances occurs when all beam dispersions are zeroed, and the values of the lattice dispersions required for that are completely defined by the values of the beam dispersions and the beam rms energy spread and are independent from any other second-order central beam moments. Besides that, we prove that, alternatively, one can also use the lattice dispersions to remove linear correlations between longitudinal positions of particles and their transverse coordinates (linear beam tilts), but in this situation solution for the lattice dispersions is nonunique and the reduction of the transverse projected emittances is not guaranteed.

physics.acc-ph↗
arXiv2014

Emittance growth in linear induction accelerators

The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beams from two linear induction accelerators (LIAs) to radiograph large hydrodynamic experiments driven by high explosives. Radiographic resolution is determined by the size of the source spot, and beam emittance is the ultimate limitation to spot size. On the DARHT Axis-II LIA we measure an emittance higher than predicted by theoretical simulations, and even though this axis produces sub-millimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using particle-in-cell (PIC) codes, although most of these are discounted based on beam measurements. The most likely source of emittance growth is a mismatch of the beam to the magnetic transport, which can cause beam halo.

physics.acc-ph↗
arXiv2014

Time structure measurement of the storage ring with the time-resolved X-ray excited optical luminescence method at SSRF

Measuring the time structure of the storage ring on the sample spot inside the experimental hutch is a foundational step during the time-resolved experiments using the pulsed synchrotron X-rays with the time structure defined by the storage ring. In this work, the method of time-resolved X-ray excited optical luminescence was designed and implemented to do the measurement. This method is based on the principle of time-correlated single photon counting techniques. The measurement system consists of a spectrometer with a detector of photomultiplier tube, a timing system, a set of Nuclear Instrument Modules and a luminescent material of Zinc Oxide. The measurement was performed on the X-ray absorbed fine structure spectrum beamline at Shanghai Synchrotron Radiation Facility. The results show that this system can be used to measure the time structure of the storage ring with a precision of less than 1 ns.

physics.ins-det↗
arXiv2014

Semi-classical beam cooling in an intense laser pulse

We present a novel technique for studying the evolution of a particle distribution using single particle dynamics such that the distribution can be accurately reconstructed using fewer particles than existing approaches. To demonstrate this, the Landau-Lifshiftz description of radiation reaction is adapted into a semi-classical model, for which the Vlasov equation is intractable. Collision between an energetic electron bunch and high-intensity laser pulses are then compared using the two theories. Reduction in beam cooling is observed for the semi-classical case.

physics.plasm-ph↗
arXiv2014

Simulation of the Capture and Acceleration Processes in HIAF-CRing

In order to investigate the longitudinal beam dynamics during the adiabatic capture and acceleration processes of the CRing in HIAF project, a simulation of both processes above is carried out with $U^{34+}$ ions. The ions will be captured into a bucket adiabatically and accelerated from 800 MeV/u to 1130 MeV/u . Simulation of these processes by tracking appropriate distributions with the longitudinal beam dynamics code ESME has been used to find optimum parameters such as RF phase, RF voltage and RF frequency etc. An enhanced capture and acceleration efficiency can be gotten from the simulation results, with the optimized RF voltage and RF phase program.

physics.acc-ph↗
arXiv2014

Mode Error Analysis of Impedance Measurement using Twin Wires

Both longitudinal and transverse coupling impedance for some critical components need to be measured for accelerator design. The twin wires method is widely used to measure longitudinal and transverse impedance on the bench. A mode error is induced when the twin wires method is used with a two-port network analyzer. Here, the mode error is analyzed theoretically and an example analysis is given. Moreover, the mode error in the measurement is a few percent when a hybrid with no less than 25 dB isolation and a splitter with no less than 20 dB magnitude error are used.

physics.acc-ph↗
arXiv2014

Magnetic Shield Approach Toward Non-adiabatic Low to High Field Positron Beam Transition for Microtraps

This simulation study sheds light on effectiveness of a magnetic shield on non-adiabatic transport of a positron beam from 3700G to 100G where we were able to maximize the low to high field transmission efficiency for 6.5KeV positrons. An initially brighter beam can be transported even more efficiently. Cross-sectional uniformity of the beam is also studied for different beam energies. This is important for filling microtraps consist of thousands tubes with diameters of tens of microns each. Transverse momentum of the particles is also studied since it causes mirroring in the high field and compromises efficiency. Beam remoderation is required as ~eV energy positrons will be trapped in the Microtrap. The observed magnetic shield assisted low to high filed transition opens new avenue for high efficiency manipulating of non-neutral beams.

physics.plasm-ph↗
arXiv2014

Beamline Instrumentation for Future Parity-Violation Experiments

The parity-violating electron scattering community has made tremendous progress over the last twenty five years in their ability to measure tiny asymmetries of order 100 parts per billion (ppb) with beam-related corrections and systematic errors of a few ppb. Future experiments are planned for about an order of magnitude smaller asymmetries and with higher rates in the detectors. These new experiments pose new challenges for the beam instrumentation and for the strategy for setting up the beam. In this contribution to PAVI14 I discuss several of these challenges and demands, with a focus on developments at Jefferson Lab.

nucl-ex↗
arXiv2014

Planar Ultrananocrystalline Diamond Field Emitter in Accelerator RF Electron Injector: Performance Metrics

A case performance study of a planar field emission cathode (FEC) based on nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, was carried out in an RF 1.3 GHz electron gun. The FEC was a 100 nm (N)UNCD film grown on a 20 mm diameter stainless steel disk with a Mo buffer layer. At surface gradients 45-65 MV/m, peak currents of 1-80 mA (equivalent to 0.3-25 mA/cm$^2$) were achieved. Imaging with two YAG screens confirmed emission from the (N)UNCD surface with (1) the beam emittance of 1.5 mm$\times$mrad/mm-rms, and (2) longitudinal FWHM and rms energy spread of 0.7% and 11% at an electron energy of 2 MeV. Current stability was tested over the course of 36$\times$10$^3$ RF pulses (equivalent to 288$\times$10$^6$ GHz oscillations).

physics.acc-ph↗
arXiv2014

Baseline LHC machine parameters and configuration of the 2015 proton run

This paper shows the baseline LHC machine parameters for the 2015 start-up. Many systems have been upgraded during LS1 and in 2015 the LHC will operate at a higher energy than before and with a tighter filling scheme. Therefore, the 2015 commissioning phase risks to be less smooth than in 2012. The proposed starting configuration puts the focus on feasibility rather than peak performance and includes margins for operational uncertainties. Instead, once beam experience and a better machine knowledge has been obtained, a push in $β^*$ and performance can be envisaged. In this paper, the focus is on collimation settings and reach in $β^*$---other parameters are covered in greater depth by other papers in these proceedings.

physics.acc-ph↗
arXiv2014

Optics options for the 2015 LHC run

A review of the possible optics configurations for the 2015 LHC run will be made. The rationale behind the various scenarios will also be presented together with the latest results of the validation studies. Special runs, such as Van der Meer and high-beta, will be discussed too. Finally, the next steps and the related milestones will be discussed with the goal of achieving a consensual decision on the optics configuration to be used for the LHC in the coming weeks.

physics.acc-ph↗
arXiv2014

Cylindrical Circular and Elliptical, Toroidal Circular and Elliptical Multipoles Fields, Potentials and their Measurement for Accelerator Magnets

Recent progress in particle accelerator tracking has shown that the field representation is one of the major limits of the prediction accuracy especially for machines, whose aperture is fully filled by the beam and thus higher the artefacts created by higher order modes have to be thoroughly understood. The standard tool for field presentation today are cylindrical circular multipoles due to their straight forward correspondence to the Cartesian coordinates. In this paper we extend the standard approach to other coordinate systems, show how these can be measured next to their realisation in measuring the SIS100 Magnets for the FAIR project.

physics.acc-ph↗
arXiv2014

The Case for Muon-based Neutrino Beams

For the foreseeable future, high energy physics accelerator capabilities in the US will be deployed to study the physics of the neutrino sector. In this context, it is useful to explore the sensitivities and limiting systematic effects of the planned neutrino oscillation program, so that we can evaluate the issues that must be addressed in order to ensure the success of these efforts. It is only in this way that we will ultimately be able to elucidate the fundamental physics processes involved. We conclude that success can only be guaranteed by, at some point in the future, being able to deploy muon accelerator capabilities. Such capabilities provide the only route to precision neutrino beams with which to study and mitigate, at the sub-percent level, the limiting systematic issues of future oscillation measurements. Thus this analysis argues strongly for maintaining a viable accelerator research program towards future muon accelerator capabilities.

hep-ex↗
arXiv2014

Untrapped HOM Radiation Absorption in the LCLS-II Cryomodules

The superconducting cavities in the continuous wave (CW) linacs of LCLS-II are designed to operate at 2 K, where cooling costs are very expensive. One source of heat is presented by the higher order mode (HOM) power deposited by the beam. Due to the very short bunch length--especially in L3 the final linac--the LCLS-II beam spectrum extends into the terahertz range. Ceramic absorbers, at 70 K and located between cryomodules, are meant to absorb much of this power. In this report we perform two kinds of calculations to estimate the effectiveness of the absorbers and the amount of beam power that needs to be removed at 2 K.

physics.acc-ph↗
arXiv2014

Wakefield effects of the bypass line in LCLS-II

In LCLS-II, after acceleration and compression and just before entering the undulator, the beam passes through 2.5 km of 24.5 mm (radius) stainless steel pipe. The bunch that passes through the pipe is extremely short---with an rms of 8 um for the nominal 100 pC case. Thus, even though the pipe has a large aperture, the wake that applies is the {\it short-range} resistive wall wakefield. The bunch distribution is approximately uniform, and therefore the wake induced voltage is characterized by a rather linear voltage chirp. It turns out that the wake supplies needed dechirping to the LCLS-II beam before it enters the undulator. In this note we calculate the wake, discuss the confidence in the calculation, and investigate how to improve the induced chirp linearity and/or strength. Finally, we also study the strength and effects of the transverse (dipole) resistive wall wakefield.

physics.acc-ph↗
arXiv2014

Longitudinal instabilities affecting the moving critical layer laser-plasma ion accelerators

In this work we analyze the longitudinal instabilities of propagating acceleration structures that are driven by a relativistically intense laser at the moving plasma critical layer [1]. These instabilities affect the energy-spectra of the accelerated ion-beams in propagating critical layer acceleration schemes [2][3]. Specifically, using analytical theory and PIC simulations we look into three fundamental physical processes and their interplay that are crucial to the understanding of energy spectral control by making the laser-plasma ion accelerators stable. The interacting processes are (i) Doppler-shifted ponderomotive bunching [1][4] (ii) potential quenching by beam-loading [2] and (iii) two-stream instabilities. These phenomenon have been observed in simulations analyzing these acceleration processes [5][6][7]. From the preliminary models and results we present in this work, we can infer measures by which these instabilities can be controlled [8] for improving the energy-spread of the beams.

physics.plasm-ph↗
arXiv2014

Optimization of neutrino fluxes for future long baseline neutrino oscillation experiments

One of the main goals of the Long Baseline Neutrino Oscillation experiment (LBNO) experiment is to study the L/E behaviour of the electron neutrino appearance probability in order to determine the unknown phase $δ_{CP}$. In the standard neutrino 3-flavour mixing paradigm, this parameter encapsulates a possibility of a CP violation in the lepton sector that in turn could help explain the matter-antimatter asymmetry in the universe. In LBNO, the measurement of $δ_{CP}$ would rely on the observation of the electron appearance probability in a broad energy range covering the 1$^{st}$ and 2$^{nd}$ maxima of the oscillation probability. An optimization of the energy spectrum of the neutrino beam is necessary to find the best coverage of the neutrino energies of interest. This in general is a complex task that requires exploring a large parameter space describing hadron target and beamline focusing elements. In this paper we will present a numerical approach of finding a solution to this difficult optimization problem often encountered in design of modern neutrino beamlines and we will show the improved LBNO sensitivity to the presence of the leptonic CP violation attained after the neutrino beam optimization.

physics.ins-det↗