AAI/ANL, Lemont, Illinois, USA
ANL, Lemont, Illinois, USA
The initial observations of optical synchrotron radiation (OSR) emitted over millions of passes from a few electrons circulating in the Particle Accumulator Ring (PAR) at the Advanced Photon Source have been done with a digital CMOS camera and a synchroscan streak camera operating at 117.3 MHz. The discrete changes of integrated counts in the CMOS image region of interest are ascribed to single electron steps at ~3500 cts per electron. Circulations of a single electron at 375 MeV and at 425 MeV were demonstrated in the 12-bit digital FLIR USB3 camera images. The Hamamatsu C5680 streak camera operating at the 12th harmonic of the fundamental revolution frequency at 9.77 MHz was used to measure the zero-current bunch length from 0.5 nC circulating charge down to 10s of electrons or <10 aC. The latter cases were performed with 6-ps temporal resolution for the first time anywhere, to our knowledge. We report a preliminary effective bunch length of 276 ± 36 ps for 57 electrons (9.1 aC) stored based on a fit to a single Gaussian peak. The results will be compared to the standard zero-current model for the ring.
ANL, Lemont, Illinois, USA
The Advanced Photon Source (APS) is building a fourth-generation storage ring (4GSR), replacing the present double-bend achromat lattice with a multibend achromat system thereby allowing the production of ultra-bright x-ray beams. The new lattice enables a two-order-of-magnitude reduction in horizontal beam emittance and a factor of two increase in beam current. The result is an electron beam of very high energy- and power-densities. Initial predictions suggest virtually any material struck by the undiluted electron beam will be damaged. Two experimental beam abort studies have been conducted on collimator test pieces in the present APS SR to inform the design of a fully-functional machine protection system for APS 4GSR operations at 200 mA. A comprehensive suite of diagnostics were employed during the studies The diagnostics used in these experiments are not new, but employed in different ways to obtain unique data sets. With these data sets now in hand, we are developing new numerical tools to guide collimator design.
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ANL, Lemont, Illinois, USA
Relativistic electron beams in storage rings radiate a significant fraction of beam energy per turn. As demonstrated in previous experiments, with the radiofrequency accelerating structures off, the turn-by-turn time of arrival of the electron bunch can be observed from the synchrotron radiation that it produces using a streak camera. In the present work, we present measurements of the energy loss per turn of an initially short electron bunch (~1 ps RMS) from a photocathode electron gun in the Advanced Photon Source Particle Accumulator Ring (375 MeV, 102 ns revolution period). With the streak camera synchroscan locked to the twelfth harmonic of the revolution frequency (117.3 MHz), we observe an injection transient in the horizontal direction.
Fermilab, Batavia, Illinois, USA
SLAC, Menlo Park, California, USA
The preservation of the low emittance of electron beams during transport in the accelerating structures of large facilities is an ongoing challenge. In the cases of the TESLA-type superconducting rf cavities currently used in the European X-ray Free-electron Laser (FEL) and the under construction Linac Coherent Light Source upgrade (LCLS-II), off-axis beam transport may result in emittance dilution due to transverse long-range and short-range wakefields (SRW)*. To investigate such effects, experiments were performed at the Fermilab Accelerator Science and Technology (FAST) facility with its unique two-cavity configuration after the photocathode rf gun. We used optical transition radiation (OTR) imaging with a UV-visible synchroscan streak camera to display sub-micropulse y-t effects in the 41-MeV beam. The head-tail transverse kicks within the 11-ps-long micropulses were observed at the 100-micron level for steering off-axis in one cavity and several 100 microns for two cavities. Since the SRW kick angles go inversely with energy, these results may inform the commissioning plans of the LCLS-II injector where beam will be injected at ~1 MeV into a cryomodule.
* W.K.H. Panofsky and M. Bander, Rev. Sci. Instr. 39 , 206 (1968).
Fermilab, Batavia, Illinois, USA
We have recently extended our ability to explore submicropulse effects in relativistic electron beams to energy-time (E-t) correlations. The Fermilab Accelerator Science and Technology (FAST) facility consists of a photoinjector, two superconducting TESLA-type capture cavities, one superconducting ILC-style cryomodule, and a small ring for studying non-linear, integrable beam optics called IOTA. The linac contains, as part of its instrumentation, an optical transport system that directs optical transition radiation (OTR) from an Al-coated Si surface to an externally located streak camera for bunch length measurements. For the first time, an OTR screen after the spectrometer magnet was used for measurements of submicropulse E-t correlations. The projected, micropulse time profile was fit to a single Gaussian peak with σ = 11.5 ± 0.5 ps for 500 pC/micropulse and with a 200-micropulse synchronous sum, in agreement with the upstream bunch-length measurement at a non-energy-dispersive location. The submicropulse E-t images were explored for four rf phases of CC1, and the E vs. t effects will be presented.
Fermilab, Batavia, Illinois, USA
The University of Texas at Austin, Austin, Texas, USA
HZDR, Dresden, Germany
Private Address, Silver Spring, USA
A significant advance in laser-driven plasma accelerator (LPA) electron-beam diagnostics has recently been demonstrated based on coherent optical transition radiation (COTR) imaging*. We find COTR signal strengths from a microbunched subset of beam exiting the LPA to be several orders of magnitude higher than that of incoherent optical transition radiation (OTR). The transverse sizes are only a few microns as deduced from the point-spread-function-related lobe structure. In addition, the far-field COTR interferometric images obtained on the same shot provide beam-size limits plus divergence and pointing information at the sub-mrad level when compared to an analytical model** with a recent revision. The integrated image intensities can be used to estimate the microbunching fraction and relatable to the LPA process. Initial results in a collaborative experiment at the Helmholtz-Zentrum Dresden-Rossendorf LPA will be reported for electron beam energies of about 215 MeV.
* A.H. Lumpkin, M. LaBerge, D.W. Rule et al., "Interferometric Optical Signature.", subm. to Phys. Rev.Lett.(2019).
** D.W. Rule, A.H. Lumpkin, Proc. of PAC2001, Vol. 2, pp. 1288-1290 (IEEE 2001).
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