Z. Huang
H.S. Sidhu
I.N. Towers
Z. Jovanoski
V.V. Gubernov
Highlights • Analysed a two-step adiabatic competitive exothermic reaction scheme. • Analysed the stability of the combustion waves using the Evan function method. • Determined the threshold values of parameters for the onset of pulsating behaviour. • Showed that the system exhibits pulsating and period-doubling behaviour. Abstract We consider travelling wave solutions of a reaction-diffusion system corresponding to an adiabatic two-step competitive exothermic reaction scheme. In such a scheme, a combustion process is assumed to be lumped into two different exothermic reactions. Although the rate constants of the reactions are distinct, both reactions occur simultaneously and feed on the same reactant. The travelling wave solutions are obtained via the shooting-relaxation method. The linear stability analysis is conducted using the Evans function technique and the compound matrix method. Further, threshold values of parameters corresponding to Hopf points are established. It is shown that the system exhibits pulsating behaviour when the parameter values are greater than the threshold values. The onset of instability is found for a broad range of parameter values. Two different numerical methods are then used to obtain solutions from the governing partial differential equations to validate the results.
Axel Brachmann
Christoph Bostedt
John D. Bozek
Ryan Coffee
F. J. Decker
Y. Ding
Darren Dowell
Paul Emma
Josef C. Frisch
Sasha Gilevich
George Haller
Greg Hays
Ph. Hering
Bernhard Hill
Z. Huang
Richard H. Iverson
Elliot P Kanter
Bertold Krässig
H. F. Machiel Van der Loos
Alan Miahnahri
H.-D. Nuhn
Antonio Perazzo
M. Petree
Daniel Ratner
Robin Santra
Todd L. Smith
Stephen H. Southworth
Jimmy L Turner
Juliet Welch
William E. White
J. Wu
John Byrd
Guang-Yao Huang
In addition to its normal operation at 250pC, the LCLS has operated with 20pC bunches delivering X-ray beams to users with energies between 800eV and 2 keV and with bunch lengths below 10 fs FWHM. A bunch arrival time monitor and timing transmission system provide users with sub 50 fs synchronization between a laser and the Xrays for pump / probe experiments. We describe the performance and operational experience of the LCLS for short bunch experiments. FEMTOSECOND OPERATION In normal operation the LCLS uses 250 pC electron bunches to produce X-ray pulses with lengths between 50 and 500 femtoseconds. The LCLS can also be operated at 20pC to produce very short pulses. The pulse length is below the resolution of the accelerator instrumentation but based on indirect measurements it is believed to be less than 10fs FWHM [1]. For short pulse mode the the photo-cathode laser is apertured to a 0.6mm diameter spot on the cathode with a 3.6 ps FWHM pulse. The source laser is operated 15 degrees from the Schottky edge produce an electron beam out of the gun with a 700fs RMS bunch length. Typical emittance is 0.14 microns RMS in each plane. The beam is compressed in two bunch compressors to near maximum peak current to produce a bunch length below the 20fs resolution of the transverse cavity. Simulations using measured beam paramters at 8.3 KeV photon energy give a pulse length around 5 femtoseconds FWHM. Figure 1: Simulated pulse length In Figure 2 the FEL pulse energy and (uncalibrated) peak electron beam current are plotted as a function of the phase of the L2 compression section. The FEL does not lase well at full compression, probably due to a combination of slice energy spread and CSR induced emittance growth. Maximum compression is at -32.25 degrees, and the FEL lases well at ;/1 degree from this peak. The bunch length when operating 1 degree from maximum compression can be calculated using a simple linear model and gives approximately 5 fs FWHM. Figure 2: FEL power (blue) and Ipk(red) vs. compression PUMP-PROBE EXPERIMENTS A variety of experiments can be preformed by pumping the sample with a visible laser and after a variable delay probing with a X-ray pulse. This requires synchronization of the FEL and a conventional laser and a system to measure their relative timing. The LCLS uses S-band RF cavities to measure the pulse to pulse beam time relative to a phase reference line. A slow feedback adjusts the timing of a signal that is transmitted to the experiment pump laser through an interferometrically stabilized fiber link [2]. A block diagram of the timing system is shown in figure 3. The electron beam timing jitter measured at the phase cavities relative to the 476 MHz phase reference line was 130 fs RMS for the 2009 run and 60 fs RMS for the 2010 run. The improvement was due to using a different source for the phase reference as shown in figure 3. The phase SLAC-PUB-14234 SLAC National Accelerator Laboratory, Menlo Park, CA 94025 cavity system reports the pulse to pulse relative timing of the electron beam and phase reference to allow offline improvement of the experimental data. The RF based beam arrival time measurement sytstm is described here. The fiber and laser locking systems are described in XXX Figure 3: Timing system BEAM ARRIVAL TIME MONITOR The Beam Arrival Time system uses single cell resonant cavities operating at 2805MHz which are excited by the bunch passage. This produces a decaying exponential tone which is mixed with the 2856 MHz 6 th harmonics of the 476 MHz reference line to produce a 51MHz IF. That signal is digitized with the 119MHz 4 th subharmonic of the 476 MHz. Figure 4. The input attenuator is used to control the signal level sent to the mixer. The desired level is a trade-off between noise and nonlinearity that would convert bunch charge fluctuations into timing noise. A signal level of -10dBm into the mixer (Marki Microwave T3-03) gives good performance. The cavity produces signals larger than this for bunch charges above approximately 50 pC. The mixer output is followed by a 20dB gain low noise, high linearity amplifier (MiniCircuits ZHL-2010;) to match the signal to the digitizer input level. The digitizer is a VME module constructed at SLAC based on a Linear Technology LTC2028 16 bit chip. The external trigger to the digitizer is synchronous with the beam so the waveform is reproducible shot to shot. If the divide-by-4 circuit jumps buckets it is reset by software. Figure 4: Bunch Arrival Time system block diagram Phase Measurement Algorithm The phase cavities are constructed from copper and will exhibit a temperature Dependant frequency shift that will result in a change in the average phase of the signal on the order of several picoseconds per degree C, much larger than the tolerance for the system. Fortunately the signal provides a direct measurement of the cavity frequency on each pulse which can be used to correct for the temperature coefficient. The digitized signal is down-converted to provide a measurement of phase vs. time. The projection of this phase back to the beam arrival time provides a signal that is first order independent of of cavity frequency. Beam Arrival Time Monitor Performance Comparison of the 2 phase cavity systems provides a measure of their phase drift and noise. Note that while the systems use independent hardware after the reference line, they are exposed to similar environments so some drifts may not be seen in this comparison. In addition drifts of the X6 multipliers will show up in the cavity to cavity comparison, but will not effect the operation in the real system: the fiber system is locked to the ouput of the X6 multiplier. Figure 5 shows the difference between the times measured by the two systems and the 100 pulse shot-toshot RMS difference plotted over 4 days of operation at the normal 250pC operating charge. Figure 5: Phase cavity system performance over 4 days showing 13fs RMS noise (blue) and 40fs drift (magenta). When the accelerator is operating at 20pC for short bunches, the phase cavity noise increases due to the small signal level. Figure 6 shows the performance over 3 hours. This is the longest run at 20pC with the present version of the phase cavity system. Figure 6: Operation at 20pC showing 27fs RMS noise and 25fs drift over 3 hours. OVERALL SYSTEM PERFORMANCE The noise and drift performance of the fiber system cannot be measured as installed – there is no alternate system for comparison. Based on measurements at LBNL the RMS noise and drift are believed to be below 10fs. The experiment laser phase noise is 25fs RMS in a 1KHz bandwidth, but increases to 120fs RMS at broadband (up to 100 KHz), believed to be due to acoustic noise. There is no direct way to measure the laser timing drift. Preliminary results from laser pump / X-ray probe experiments performed in 2009 indicate a beam timing jitter of approximately 150 fs between the pump laser and the X-rays. This is consistent with the electron beam measurements preformed at the same time. The reference line has been upgraded in 2010 (see figure 3), but new experiment results are not yet available. The 2009 experiments indicated sub 50-femtosecond pump laser to X-ray timing noise and drift(after off-line correction). This was likely dominated by the noise of the pump laser relative to the fiber system reference. [3] PERFORMANCE IMPROVEMENTS The low frequency noise and drift performance of RF components is not generally specified by the manufacturer. Work is under way to test components to find improved performance substitutes. The algorithm used is non-ideal as it does not make use of the fact that the cavity frequency varies slowly with time. It also discards some of the data due to phase ringing from components in the RF system. Initial tests indicate that a factor of 2 improvement in noise may be possible from algorithm improvements. The existing fiber system includes some RF components that are outside of the stabilization loop and could probably be optimized. The laser locking system is believed to be the largest contributor to timing noise. Work is underway to isolate the laser from acoustic noise and to use low drift and temperature stabilized cables and components in the locking system. The low (68MHz) repetition rate of the laser mode locked oscillator limits the noise performance if its phase detection system. Experiments are underway to use an etalon system to multiply the beam rate on the photodiode to improve the measurement phase noise.
Stefan Möller
Jessica Arthur
Axel Brachmann
Yun Feng
Alan S. Fisher
Josef C. Frisch
John N. Galayda
Sasha Gilevich
Jerome B. Hastings
Greg Hays
Ph. Hering
Z. Huang
Richard H. Iverson
Jacek Krzywiński
Steffan Lewis
H. F. Machiel Van der Loos
J. Robinson
Dmitri D. Ryutov
Shu-zhong Shen
The LCLS hard x-ray Free Electron Laser at SLAC reported first lasing in April of 2009. Since then two successful user runs have been completed at the two soft x-ray stations. The first hard x-ray station has started commissioning in July of 2010. Beam diagnostics play an essential role for tuning the machine and delivering the requested beam properties to the users. An overview of the LCLS photon diagnostics will be presented including some selected commissioning results. Plans for future improvements and upgrades will be briefly discussed.
Andrew L. Aquila
Anton Barty
Christoph Bostedt
Sébastien Boutet
Gary Carini
Daniel P Deponte
Persis S . Drell
Sebastian Doniach
Kenneth H. Downing
Thomas N Earnest
Hans Elmlund
Veit Elser
Markus Guehr
János Hajdu
Jerome B. Hastings
Stefan P. Hau-Riege
Z. Huang
Eaton E. Lattman
F. R. N. C. Maia
Stefano Marchesini
Abbas Ourmazd
Claudio de Pellegrini
Robin Santra
Ilme Schlichting
Christy Schroer
J. C. H. Spence
I A Vartanyants
Soichi Wakatsuki
William I Weis
Garth J. Williams
Intense femtosecond x-ray pulses from free-electron laser sources allow the imaging of individual particles in a single shot. Early experiments at the Linac Coherent Light Source (LCLS) have led to rapid progress in the field and; so far; coherent diffractive images have been recorded from biological specimens; aerosols; and quantum systems with a few-tens-of-nanometers resolution. In March 2014; LCLS held a workshop to discuss the scientific and technical challenges for reaching the ultimate goal of atomic resolution with single-shot coherent diffractive imaging. This paper summarizes the workshop findings and presents the roadmap toward reaching atomic resolution; 3D imaging at free-electron laser sources.
Katherine C Harkay
Michael Borland
Y. C. Chae
Louis Emery
Z. Huang
E. S. Lessner
Alex H. Lumpkin
Stephen Val Milton
N. S. Sereno
B. Yang
TUNE SLOPE
The single-bunch current limit and tune shift with current have been documented over time in the 7-GeV Advanced Photon Source (APS) storage ring as a function of lattice; chromaticity; and number of small-gap insertion device (ID) chambers. The contribution to the machine coupling impedance of the 8-mm-gap ID chambers was reported earlier [1]. One 5-mm-gap ID chamber was installed in December 1997. This required changing the lattice to preserve the vertical acceptance. The new lattice reduced the average vertical beta function at the 5-mm chamber as well as at all the other ID chambers and so has also lowered the effect of the vertical coupling impedance. As additional 8-mm and 5-mm chambers are planned; a more detailed characterization of the impedance is essential. This includes separating the effects of the transitions between the small-gap chambers and the standard chambers from the resistive wall impedance of the small-gap chambers themselves. In this paper; we report on the transverse instabilities and thresholds observed in the vertical and horizontal planes. From these observations; various contributions to the coupling impedance are derived.
H. F. Machiel Van der Loos
Ron Akre
Axel Brachmann
Ryan Coffee
F. J. Decker
Y. Ding
Darren Dowell
Steve Edstrom
Paul Emma
Alan S. Fisher
Josef C. Frisch
Sasha Gilevich
Greg Hays
Ph. Hering
Z. Huang
Richard H. Iverson
Manuela Messerschmidt
Alan Miahnahri
Stefan Möller
H.-D. Nuhn
Daniel Ratner
J. Rzepiela
Todd L. Smith
Pollak Stefan
H. Tompkins
Jimmy L Turner
Juliet Welch
William E. White
J. Wu
Gerald Yocky
The Linac Coherent Light Source (LCLS) X-ray FEL utilizing the last km of the SLAC linac has been operational since April 2009 and finished its first successful user run last December. The various diagnostics for electron beam properties including beam position monitors; wire scanners; beam profile monitors; and bunch length diagnostics are presented as well as diagnostics for the x-ray beam. The low emittance and ultra-short electron beam required for X-ray FEL operation has implications on the transverse and longitudinal diagnostics. The coherence effects of the beam profile monitors and the challenges of measuring fs long bunches are discussed.
Y. Ding
Z. Huang
Daniel Ratner
Philip H. Bucksbaum
Hamed Merdji
Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. Several schemes have been proposed based on manipulations of electron bunches with extremely short laser pulses. In this paper; we extend the attosecond twocolor ESASE scheme proposed by Zholents et al. to the long optical cycle regime using a detuned second laser and a tapered undulator. Both lasers can be about ten-opticalcycles long; with the second laser frequency detuned from the first one to optimize the contrast between the central and side current spikes. A tapered undulator mitigates the degradation effect of the longitudinal space charge (LSC) force in the undulator and suppresses the FEL gain of all side current spikes. Simulations using the LCLS parameters show a single attosecond x-ray spike of ∼ 110 attosecond can be produced with a good contrast ratio.
Stefano Coda
J. Ahn
Raffaele Albanese
Stefano Alberti
E. Alessi
Sabrina Allan
Himyanshu Anand
Gerasimos Anastassiou
Y. Andrèbe
Carlo Angioni
Marco Ariola
M. Bernert
M. N. A. Beurskens
W. Bin
T . Bolzonella
F. Bouquey
Falk Braunmueller
Hugo Bufferand
P. Buratti
G. Calabro
O. Chellai
Dalsu Choi
C . Cianfarani
Guido Ciraolo
Jonathan Citrin
Stefan Costea
F. Crisanti
Nicolás Cruz
Agata Czarnecka
Joan Decker
Giulia De Masi
Gianmaria De Tommasi
D. Douai
Matt Dunne
Andrea Fasoli
Nicolas FEDORCZAK
Federico Felici
O. Février
O. Ficker
Susanne Fietz
Michele Fontana
L. Frassinetti
Ivo Furno
Sergio Galeani
Alejandro Gallo
C. Galperti
S. Garavaglia
Izaskun Garrido
Bettina Geiger
Edmondo Giovannozzi
Marco GOBBIN
T. Goodman
G. Gorini
Mateusz Gospodarczyk
Gustavo Granucci
J. P. Graves
R. Guirlet
Amy R. Hakola
Chad Ham
J Harrison
Jeffrey Hawke
P. Hennequin
B. Hnat
Z. Huang
V . G . Igochine
Paola Innocente
C. Ionita Schrittwieser
Heinz Isliker
R. Jacquier
Alain Jardin
P. Maget
E. Maljaars
AlexeyA. Malygin
Marc Maraschek
Camille Marini
Philippe Martin
Michalis Mavridis
Didier Mazon
Roy McAdams
R M McDermott
Alexander Merle
Hellmuth-A. Meyer
F . Militello
C. Rapson
Juul Rasmussen
Michael T. Reich
H . Reimerdes
Cedric Reux
P. Ricci
F. Riva
Terence Robinson
Samuli Saarelma
Francine Saint-Laurent
O. Sauter
Richard Scannell
MORELLI SILVA
Jitendra Sinha
C. Sozzi
M Spolaore
Torsten Stange
T. Stoltzfus
S L Vartanian
Geert Verdoolaege
K. Verhaegh
Paul lez Durance
The TCV tokamak is augmenting its unique historical capabilities (strong shaping, strong electron heating) with ion heating, additional electron heating compatible with high densities, and variable divertor geometry, in a multifaceted upgrade program designed to broaden its operational range without sacrificing its fundamental flexibility. The TCV program is rooted in a three-pronged approach aimed at ITER support, explorations towards DEMO, and fundamental research. A 1-MW, tangential neutral beam injector (NBI) was recently installed and promptly extended the TCV parameter range, with record ion temperatures and toroidal rotation velocities and measurable neutral-beam current drive. ITER-relevant scenario development has received particular attention, with strategies aimed at maximizing performance through optimized discharge trajectories to avoid MHD instabilities, such as peeling-ballooning and neoclassical tearing modes. Experiments on exhaust physics have focused particularly on detachment, a necessary step to a DEMO reactor, in a comprehensive set of conventional and advanced divertor concepts. The specific theoretical prediction of an enhanced radiation region between the two X-points in the low-field-side snowflake-minus configuration was experimentally confirmed. Fundamental investigations of the power decay length in the scrape-off layer (SOL) are progressing rapidly, again in widely varying configurations and in both D and He plasmas; in particular, the double decay length in L-mode limited plasmas was found to be replaced by a single length at high SOL resistivity. Experiments on disruption mitigation by massive gas injection and electron-cyclotron resonance heating (ECRH) have begun in earnest, in parallel with studies of runaway electron generation and control, in both stable and disruptive