I. Hsu

Bio: I. Hsu is an academic researcher from Stanford University. The author has contributed to research in topics: Particle accelerator & Betatron. The author has an hindex of 5, co-authored 8 publications receiving 68 citations.

The introduction of trajectory oscillations to reduce emittance growth in the SLC linac

Abstract: Emittance growth of accelerated beams in the 50 GeV linear accelerator of the Stanford Linear Collider (SLC) arises from the effects of transverse wakefields and momentum dispersion. These effects are caused by small misalignments of the beam position monitors, lattice quadrupoles, and accelerating structure and by the energy spectrum of the beam which changes along the accelerator. The introduction of strategically placed trajectory oscillations over finite lengths of the linac has been used to generate beam errors which cancel the emittance accumulation from these small unknown, random alignment errors. Induced oscillations early in the linac cancel effects which filament along the accelerator affecting mostly the beam core. Induced oscillations located at the center of the accelerator or beyond cancel wakefield and dispersion errors which do not completely filament but cause the beams to have, in addition, an apparent betatron mismatch and transverse tails. The required induced oscillations of a few hundred microns are reasonably stable over a period of several weeks. Of course, the optimum induced oscillations depend upon the beam charge. Emittance reductions of 30 to 50% have been obtained.

Dispersion and betatron matching into the linac

Abstract: In high-energy linear colliders, the low-emittance beam from a damping ring has to be preserved all the way to the linear accelerator (LINAC), in the LINAC and to the interaction point. In particular, the ring-to-LINAC (RTL) section of the SLAC Linear Collider (SLC) should provide an exact betatron and dispersion match from the damping ring to the LINAC. A beam with a nonzero dispersion shows up immediately as an increased emittance, while with a betatron mismatch the beam forms filaments in the LINAC. Experimental tests and tuning procedures have shown that the linearized beta matching algorithms are insufficient if the actual transport line has some unknown errors not included in the model. Also, adjusting quadrupole strengths steers the beam if it is offset in the quadrupole magnets. These and other effects have led to a lengthy tuning process, which in the end improves the matching, but is not optimal. Different ideas are discussed to improve this matching procedure and make it a more reliable, faster, and simpler process. >

Summary of emittance control in the SLC linac

Abstract: The Stanford Linear Collider (SLC) electron-positron collider requires micron-size beams at the collision point in order to make maximum luminosity, which requires small beam emittances. These small emittances must be produced in the damping rings and accelerated down the linac without significant enlargement. The authors describe measurements of the beam emittance at various locations along the beam's trajectory and the techniques used to diagnose and correct errors. Measurements of the emittances of the beams exiting the damping rings as a function of the storage time show that the injected emittances are about three times larger than the design. Injection oscillations and betatron mismatches into the damping rings are the suspected causes of this enlargement. >

Characterization and monitoring of transverse beam tails

Abstract: Characterizations of the beam shapes are difficult because the shapes are often asymmetric and change with betatron phase. Several methods to describe beam distributions are discussed including an accelerator physics model of these tails. The uses of these characterizations in monitoring the beam emittances in the SLC (SLAC Linear Collider) are described. First, two dimensional distributions from profile monitor screens are reviewed showing correlated tails. Second, a fitting technique for non-Gaussian one dimensional distributions is used to extract the core from the tail areas. Finally, a model for tail propagation in the linac is given. >

Multibunch energy and spectrum control in the SLC high energy linac

Abstract: Three intense bunches (two electron and one positron) are accelerated on each RF pulse in the SLC (SLAC Linear Collider) linac. Careful control of the energy and energy spectrum of each bunch is needed to provide acceptable beams at the collision point and the positron production target. The required RF amplitude, timing, and phase adjustment can be calculated and adjusted in real time to correct for changing conditions. BNS damping and energy feedback systems reduce the available reserve energy, which is limited. Observations and stability of actual beams are reviewed. Implications for a future collider are discussed. >

The History of X-ray Free-Electron Lasers

Abstract: The successful lasing at the SLAC National Accelerator Laboratory of the Linear Coherent Light Source (LCLS), the first X-ray free-electron laser (X-ray FEL), in the wavelength range 1.5 to 15 A, pulse duration of 60 to few femtoseconds, number of coherent photons per pulse from 1013 to 1011, is a landmark event in the development of coherent electromagnetic radiation sources. Until now electrons traversing an undulator magnet in a synchrotron radiation storage ring provided the best X-ray sources. The LCLS has set a new standard, with a peak X-ray brightness higher by ten orders of magnitudes and pulse duration shorter by three orders of magnitudes. LCLS opens a new window in the exploration of matter at the atomic and molecular scales of length and time. Taking a motion picture of chemical processes in a few femtoseconds or less, unraveling the structure and dynamics of complex molecular systems, like proteins, are some of the exciting experiments made possible by LCLS and the other X-ray FELs now being built in Europe and Asia. In this paper, we describe the history of the many theoretical, experimental and technological discoveries and innovations, starting from the 1960s and 1970s, leading to the development of LCLS.

AKATSUKI returns to Venus

Abstract: AKATSUKI is the Japanese Venus Climate Orbiter that was designed to investigate the climate system of Venus. The orbiter was launched on May 21, 2010, and it reached Venus on December 7, 2010. Thrust was applied by the orbital maneuver engine in an attempt to put AKATSUKI into a westward equatorial orbit around Venus with a 30-h orbital period. However, this operation failed because of a malfunction in the propulsion system. After this failure, the spacecraft orbited the Sun for 5 years. On December 7, 2015, AKATSUKI once again approached Venus and the Venus orbit insertion was successful, whereby a westward equatorial orbit with apoapsis of ~440,000 km and orbital period of 14 days was initiated. Now that AKATSUKI’s long journey to Venus has ended, it will provide scientific data on the Venusian climate system for two or more years. For the purpose of both decreasing the apoapsis altitude and avoiding a long eclipse during the orbit, a trim maneuver was performed at the first periapsis. The apoapsis altitude is now ~360,000 km with a periapsis altitude of 1000–8000 km, and the period is 10 days and 12 h. In this paper, we describe the details of the Venus orbit insertion-revenge 1 (VOI-R1) and the new orbit, the expected scientific information to be obtained at this orbit, and the Venus images captured by the onboard 1-µm infrared camera, ultraviolet imager, and long-wave infrared camera 2 h after the successful initiation of the VOI-R1.

Features and futures of X-ray free-electron lasers.

Abstract: Linear accelerator-based free-electron lasers (FELs) are the leading source of fully coherent X-rays with ultra-high peak powers and ultra-short pulse lengths. Current X-ray FEL facilities have proved their worth as useful tools for diverse scientific applications. In this paper, we present an overview of the features and future prospects of X-ray FELs, including the working principles and properties of X-ray FELs, the operational status of different FEL facilities worldwide, the applications supported by such facilities, and the current developments and outlook for X-ray FEL-based research.

A 2 to 4 nm high power FEL on the SLAC linac

Abstract: We report the results of preliminary studies of a 2 to 4 nm SASE FEL, using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy up to 10 GeV. Longitudinal bunch compression is used to increase ten fold the peak current to 2.5 kA, while reducing the bunch length to the subpicosecond range. The saturated output power is in the multi-gigawatt range, producing about 1014 coherent photons within a bandwidth of about 0.2% rms, in a pulse of several millijoules. At 120 Hz repetition rate the average power is about 1 W. The system is optimized for X-ray microscopy in the water window around 2 to 4 nm, and will permit imaging a biological sample in a single subpicosecond pulse.