Low-energy Line Research Articles

Scientific issues in future induction linac accelerators for heavy-ion fusion

The worldwide heavy-ion fusion effort has made a large amount of progress over almost three decades of research on the intense beam physics and concepts needed for an inertial fusion driver. In the 1970s, ‘80s, and ‘90s in the US, most experiments used driver parameters scaled to low energy and low line charge density in order to study, most often with a single beam, the physics of high-perveance beams. More recently, the US program has introduced higher line charge density experiments, where the beam space charge potential is significant. This allows investigation of the production, and effect on the beam, of stray electrons. Increasingly, the possibilities for neutralizing plasmas are also being investigated, both just upstream and in the target chamber. This talk will outline the scientific issues that remain to be examined. These include issues of longitudinal compression, dynamic aperture, electron/ion instabilities, effect of electrons and gas on the beam, longitudinal emittance evolution, halo production, inductive effects at high energy, multiple beams, innovative final focus approaches, and driver issues for solenoidal transport. An outline of future experiments to address these issues will be presented and discussed.

Transport and cooling of singly charged noble gas ion beams

The transport and cooling of noble gas singly charged ion beams by means of a Radio Frequency Quadrupole Cooler Buncher have been studied at the LIMBE low-energy beam line of the GANIL facility. Ions as light as 4 He + have been cooled and stored before their extraction in bunches using H 2 as buffer gas. Bunches characteristics have been studied as a function of the parameters of the device. Sizeable transmissions of up to 10% have been obtained. A detailed study of the lifetime of ions inside the buncher has been performed giving an estimate of the charge exchange cross-section. Results of a microscopic Monte Carlo transport code show reasonable agreement with experimental data.

Midinfrared intersubband electroluminescence of Si/SiGe quantum cascade structures

Unipolar intersubband lasers like quantum cascade laser structures might be realized not only in III–V semiconductors but also in Si/SiGe multiple layer structures since no optical transitions across the indirect band gap are involved. We report on well-defined intersubband electroluminescence emission of Si/SiGe quantum cascade structures with different active quantum wells parameters. The complex valence band structure and a nonradiative relaxation rate of about 400 fs were calculated by multiband k⋅p formalism including Si/Ge segregation effects. The observed spectral shift of the electroluminescence peak from 146 to 159 meV is described well by quantum confinement of the two lowest heavy hole subbands. The electroluminescence observed reveals transverse magnetic polarization, a spectral line shape that changes with the direction of the current, and low-energy line broadening with an increase in temperature and current. All these features are described well by the k⋅p model calculation.

Measurements of nonlinear harmonic generation at the Advanced Photon Source's SASE FEL

SASE saturation was recently achieved at the Advanced Photon Source's SASE FEL in the low-energy undulator test line at 530 nm and 385 nm. The electron beam microbunching becomes more and more prominent until saturation is achieved. This bunching causes nonlinear harmonic emission that extends the usefulness of a SASE system in achieving shorter FEL wavelengths for the same electron beam energy. We have investigated the intensity of the fundamental and second harmonic undulator radiation as a function of distance along the undulator line and present the experimental results and compare them to numerical simulations. In addition, we have measured the single-shot second harmonic spectra as well as the simultaneous fundamental and second harmonic spectra and present the experimental results.

Self-amplified spontaneous emission saturation at the Advanced Photon Source free-electron laser (abstract) (invited)

Today, many bright photon beams in the ultraviolet and x-ray wavelength range are produced by insertion devices installed in specially designed third-generation storage rings. There is the possibility of producing photon beams that are orders of magnitude brighter than presently achieved at synchrotron sources, by using self-amplified spontaneous emission (SASE). At the Advanced Photon Source (APS), the low-energy undulator test line (LEUTL) free-electron laser (FEL) project was built to explore the SASE process in the visible through vacuum ultraviolet wavelength range. While the understanding gained in these experiments will guide future work to extend SASE FELs to shorter wavelengths, the APS FEL itself will become a continuously tunable, bright light source. Measurements of the SASE process to saturation have been made at 530 and 385 nm. A number of quantities were measured to confirm our understanding of the SASE process and to verify that saturation was reached. The intensity of the FEL light was measured versus distance along the FEL, and was found to flatten out at saturation. The statistical variation of the light intensity was found to be wide in the exponential gain region where the intensity is expected to be noisy, and narrower once saturation was reached. Absolute power measurements compare well with GINGER simulations. The FEL light spectrum at different distances along the undulator line was measured with a high-resolution spectrometer, and the many sharp spectral spikes at the beginning of the SASE process coalesce into a single peak at saturation. The energy spread in the electron beam widens markedly after saturation due to the number of electrons that transfer a significant amount of energy to the photon beam. Coherent transition radiation measurements of the electron beam as it strikes a foil provide additional confirmation of the microbunching of the electron beam. The quantities measured confirm that saturation was indeed reached. Details are given in Milton et al., Science 292, 2037 (2001) (also online at www.sciencexpress.org as 10.1126/science. 1059955, 17 May 2001), and Lewellen et al., “Present Status and Recent Results from the APS SASE FEL,” to be published in the Proceedings of the 23rd International Free-Electron Laser Conference, Darmstadt, Germany, 20–24 August 2001.

Start-to-end simulation of self-amplified spontaneous emission free electron lasers from the gun through the undulator

It is widely appreciated that the performance of self-amplified spontaneous emission free-electron lasers (FELs) depends critically on the properties of the drive beam. In view of this, a multi-laboratory collaboration has explored methods and software tools for integrated simulation of the photoinjector, linear accelerator, bunch compressor, and FEL. Rather than create a single code to handle such a system, our goal has been a robust, generic solution wherein pre-existing simulation codes are used sequentially. We have standardized on the use of Argonne National Laboratory's Self-Describing Data Sets file protocol for transfer of data among codes. The simulation codes used are PARMELA, elegant, and GENESIS. We describe the software methodology and its advantages, then provide examples involving Argonne's Low-Energy Undulator Test Line and Stanford Linear Accelerator Center's Linac Coherent Light Source. We also indicate possible future direction of this work.

Condensation of bulk excitons on a magnetized two-dimensional electron gas in modulation-doped heterojunctions

The evolution of photoluminescence (PL) spectra with increasing magnetic field $(Bl~7 \mathrm{T})$ was studied in high-mobility, wide $\mathrm{GaAs}/{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ heterojunctions (HJ's) at lattice temperatures ${T}_{L}=1.9\char21{}25 \mathrm{K}.$ The two-dimensional electron-gas (2DEG) density in the studied samples is ${n}_{2D}^{0}=(0.9\char21{}3)\ifmmode\times\else\texttimes\fi{}{10}^{11} {\mathrm{cm}}^{\ensuremath{-}2},$ and it was varied with He-Ne laser illumination by optical depletion. For $B=0$ the PL is completely dominated by exciton recombination in the undoped GaAs layer. As B increases this band shows a typical exciton diamagnetic shift. For a filling factor $\ensuremath{\nu}l~2$ a strong PL transformation is observed: the exciton PL intensity decreases and a new, low-energy PL line abruptly appears and gains intensity at the expense of the exciton PL. We attribute this line to the 2DEG-free hole recombination, and propose that its appearance in the HJ's results from an increased free-exciton dissociation near the 2DEG at $\ensuremath{\nu}l~2.$ Thus, the evolution of the bulk free exciton to 2DEG-free hole PL with increasing B is due to ``condensation'' of the bulk excitons on the magnetized 2DEG layer at $\ensuremath{\nu}l~2.$

Tapered undulators for SASE FELs

We discuss the use of tapered undulators to enhance the performance of free-electron lasers (FELs) based upon self-amplified spontaneous emission, where the radiation tends to have a relatively broad bandwidth and limited temporal coherence. Using the polychromatic FEL simulation code GINGER, we numerically demonstrate the effectiveness of tapered undulators for parameters corresponding to the Argonne low-energy undulator test line FEL and the proposed linac coherent light source.

Present status and recent results from the APS SASE FEL

The Low-Energy Undulator Test Line (LEUTL) at the Advanced Photon Source, Argonne National Laboratory, is intended to demonstrate the basic operation of a SASE-based free-electron laser. Goals include comparison of experimental results with theoretical predictions and scaling laws, identification of problems relevant to fourth-generation light source construction and operation and the means of addressing them, the development of operational and diagnostic techniques to optimize SASE FEL performance and increase repeatability from run to run, and performance of initial pioneering experiments capable of exploiting the unique properties of the laser. The basic layout and operational philosophy of the LEUTL experiment is presented. A summary of past results, including saturation, is reviewed, and a description of recent results is presented. We conclude with future plans, which include pressing to shorter wavelengths and incorporating user experiments into the LEUTL experimental program.

Observation and analysis of self-amplified spontaneous emission at the APS low-energy undulator test line

Exponential growth of self-amplified spontaneous emission at 530 nm was first experimentally observed at the Advanced Photon Source low-energy undulator test line in December 1999. Since then, further detailed measurements and analysis of the results have been made. Here, we present the measurements and compare these with calculations based on measured electron beam properties and theoretical expectations.

Design and performance simulations of the bunch compressor for the Advanced Photon Source Low-Energy Undulator Test Line free-electron laser

A magnetic bunch compressor was designed and commissioned to provide higher peak current for the Advanced Photon Source's Low-Energy Undulator Test Line free-electron laser [S. V. Milton et al., Phys. Rev. Lett. 85, 988 (2000)]. Of great concern is limiting emittance growth due to coherent synchrotron radiation. Tolerances must also be carefully evaluated to find stable operating conditions and ensure that the system can meet operational goals. Automated matching and tolerance simulations allowed consideration of numerous configurations, pinpointing those with reduced error sensitivity. Simulations indicate significant emittance growth up to 600 A peak current, for which the normalized emittance will increase from 5 to about $8.5\ensuremath{\mu}\mathrm{m}$. The simulations also provide predictions of emittance variation with chicane parameters and precompressor linac phase, which we hope to verify experimentally.

Simple method for particle tracking with coherent synchrotron radiation

Coherent synchrotron radiation (CSR) is of great interest to those designing accelerators as drivers for free-electron lasers (FELs). Although experimental evidence is incomplete, CSR is predicted to have potentially severe effects on the emittance of high-brightness electron beams. The performance of an FEL depends critically on the emittance, current, and energy spread of the beam. Attempts to increase the current through magnetic bunch compression can lead to increased emittance and energy spread due to CSR in the dipoles of such a compressor. The code elegant was used for design and simulation of the bunch compressor [M. Borland et al., in Proceedings of the 2000 Linear Accelerator Conference, Monterey, CA (SLAC, Menlo Park, CA, 2001), p. 863] for the low-energy undulator test line (LEUTL) FEL [S. V. Milton et al., Phys. Rev. Let. 85, 988 (1999)] at the Advanced Photon Source (APS). In order to facilitate this design, a fast algorithm was developed based on the 1D formalism of Saldin and co-workers [E. L. Saldin, E. A. Schneidmiller, and M. V. Yurkov, Nucl. Instrum. Methods Phys. Res., Sect. A 398, 373 (1997)]. In addition, a method of including CSR effects in drift spaces following the chicane magnets was developed and implemented. The algorithm is fast enough to permit running hundreds of tolerance simulations including CSR for 50 000 particles. This article describes the details of the implementation and shows results for the APS bunch compressor.

Ion-optics calculations of the LLNL AMS system for biochemical 14C measurements

A dedicated AMS system for biochemical 14C measurements is being built at the Center for Accelerator Mass Spectrometry. The system is centered around a National Electrostatics Corporation Model 3SDH-1 1-MV Pelletron accelerator and is designed to accept two ion sources. The LLNL 64-sample Cs-sputter ion source with its zoom lens beam line is attached to one port of the electrostatic switching element. To insure efficient coupling of the source to the acceptance of the accelerator, ion-optics calculations of the low-energy injection beam line have been conducted; the results of which were used to determine the layout of the ion-optical components of the beam line. Beam tests of the low-energy injection line show that beam behavior was accurately predicted by the calculations.

Observation of self-amplified spontaneous emission and exponential growth at 530 nm

Experimental evidence for self-amplified spontaneous emission (SASE) at 530 nm is reported. The measurements were made at the low-energy undulator test line facility at the Advanced Photon Source, Argonne National Laboratory. The experimental setup and details of the experimental results are presented, as well as preliminary analysis. This experiment extends to shorter wavelengths the operational knowledge of a linac-based SASE free-electron laser and explicitly shows the predicted exponential growth in intensity of the optical pulse as a function of length along the undulator.

Plasmons and the quantum limit in semiconductor wires

We report on the observation by Raman scattering of one-dimensional plasmons in deep-etched wires with lateral sizes w down to 30 nm. We determine the dispersion of these collective excitations down to 45 nm, well beyond the transition to a single one-dimensional subband occupancy $(w\ensuremath{\sim}60\mathrm{nm}),$ and demonstrate that they can be well described both within the random-phase approximation and the L\"uttinger liquid theory. In thinner wires, the Raman spectra reduce to a single undispersive low-energy line which we attribute to a plasmon localized by longitudinal disorder in the external potential.

Towards the definition of the optimum height of voluminous cylindrical sources in gamma-ray spectroscopy

We show that for cylindrical voluminous sources in gamma-ray spectroscopy the relative error of the spectral intensity of a low-energy spectral line, as the height of the source increases, saturates faster than the spectral intensity itself. This significantly reduces the quantity of the required sample material as compared to that based on the usual considerations of the spectral intensity only. We also demonstrate that there exists a weakly pronounced height of the source (the source size) which minimizes the relative error of the intensity of a given low-energy spectral line. Due to the smallness of the effect the benefit of introducing the concept of the truly optimum size of the source is more on a psychological side, one should feel easier using an optimum quantity of the source material than merely a sufficient one, as is otherwise the case.

Status of the Advanced Photon Source low-energy undulator test line

A Low-Energy Undulator Test Line (LEUTL) is under construction at the Advanced Photon Source (APS). Its purpose is to fully characterize innovative, future-generation undulators, and to act as a test line in pursuit of generating coherent radiation at wavelengths down to a few tens of nanometers. The line will use a high-brightness RF electron gun as its source and the APS linac to accelerate these electrons up to >650 MeV. This beam will then be directed through undulators located in a new 50 m long enclosure. In addition, there is an end station external to the radiation environment which will house the equipment necessary to directly study the light generated by the beam and undulator system. At present, the new enclosures are complete, the electron rf gun is installed and operational, and a part of the necessary transport line is installed. A prototypical undulator cell, suitable for testing the requirements of single-pass free-electron laser operation, is near completion and will be installed during the winter shutdown of 1998. This paper will describe the LEUTL project and, in particular, its capabilities for exploring the self-amplified spontaneous emission process.

Low-energy beam transport of intense and partially space-charge-neutralized ion beams

High-perveance ion beams are required for heavy-ion inertial-fusion drivers and cannot be transported without a reduction of space charge forces by space charge neutralization. The rearrangement of beam ions due to the self-field of partly neutralized beams is one source of r.m.s. emittance growth in low-energy sections. Calculations including the influence of compensating electrons have been carried out. A low-energy beam line is being set up to investigate the influence of partial space charge neutralization on the transverse beam emittance of high-perveance beams (typically 9keV, 3.5 mA He+). Theoretical assumptions and results of numerical simulations are presented and compared with first experimental results.

The atomic physics facility at the LBL ECR source

A low-energy beam line facility for atomic and surface physics research has been added to the injection line of the ECR source at the LBL 88 in. cyclotron. Beams throughout the periodic table can be delivered to three beam line stations at energies up to 15 QkeV. Experiments using this facility include forbidden line emission from the ion beam, ion-atom collisions, merged electron-ion beams and sputtering by highly charged ions.

The polarised adsorption and cathodoluminescence associated with the 1.40 eV centre in synthetic diamond

Synthetic diamonds that are grown using nickel or a nickel alloy as the solvent-catalyst contain 1.40 eV centres which are strongly segregated in the (111) growth sectors. Absorption and cathodoluminescence measurements made using polarising optics show that the dipole moment of the centre is preferentially aligned with the (111) growth planes. The absorption and luminescence systems have a zero-phonon doublet at 1.4008 and 1.4035 eV, and, surprisingly, the high-energy component exhibits a much higher degree of polarisation than the low-energy line. Polarisation-dependent structure is also observed in the vibronic absorption and luminescence bands. The zero-phonon splitting is due to a splitting of the electronic ground state, and a speculative model is proposed that may form a basis for understanding the different polarisation behaviours of the two lines.