Volume 72, Issue 1, January 2001
- review article
- proceedings of the 13th topical conference on high-temperature plasma diagnostics
- reflectometry, ece, rf, magnetics, probes, and runaway electrons
- engineering, data acquisition and analysis, diagnostics suites, ion beams, fast neutrals
- x-ray imaging and streak cameras
- fusion products and fast ions
- optical (ir, visible, uv)
- interferometry, polarimetry, and thomason scattering
- x-ray spectroscopy and diffraction
Index of content:
- REVIEW ARTICLE
72(2001); http://dx.doi.org/10.1063/1.1330575View Description Hide Description
In this article, we review the phenomenon of dispersion, paying particular attention to its impact in the optics of ultrashort pulses, as well as its measurement and management. At present, lasers generating coherent bandwidths of several hundred nanometers have been demonstrated and correspondingly short pulses of 10 fs or so are quite usual. The limits to the breadth of optical spectra and brevity of pulse durations that may be achieved are often set by the dispersive properties of the linear optical elements of which the source is constructed. Progress in ultrafast optics to date has therefore relied extensively on the development of ways to characterize and manipulate dispersion. The means by which this can be accomplished are significantly different for laser oscillators and laser amplifiers, as well as for nonlinear interactions that are used to extend the range of frequencies at which short optical pulses are available, but in all cases it is this phenomenon that determines the output of current optical sources.
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
72(2001); http://dx.doi.org/10.1063/1.1329904View Description Hide Description
A novel laser-based apparatus is presented utilizing high harmonic radiation for visible pump–EUV probe experiments on ultrafast processes. True femtosecond temporal resolution is achieved by a monochromator making use of dedicated narrowband multilayer mirrors rather than gratings for selection of single harmonic orders in the photon energy range between 66 and 73 eV. First applications of this new light source for electron spectroscopy on gas phase helium and xenon demonstrate the selection of a single high harmonic order with the intensity ratio between the selected and its adjacent harmonic not exceeding 10:1. A pump–probe study of hot electron production on a solid Pt(110) surface yields a cross-correlation corresponding to a temporal system resolution of 100 fs.
New picosecond laser system for easy tunability over the whole ultraviolet/visible/near infrared wavelength range based on flexible harmonic generation and optical parametric oscillation72(2001); http://dx.doi.org/10.1063/1.1326930View Description Hide Description
A new laser-based and time-correlated single photon counting(TCSPC) detection system which allows easy and fast tuning of excitation wavelengths over a broad range from 240 to 1300 nm, with small gaps from 335 to 360 nm and 660 to 720 nm, has been built. The unique combination of a mode-locked Ti:sapphire laser, an optical parametric oscillator, pulse selectors, and harmonic generators delivers ultrafast laser pulses (1–2 ps) with variable repetition rates and excitation wavelengths. Performance characteristics of the laser system at different excitation wavelengths are reported and the TCSPC setup, which is characterized by a total instrument response function of 25 ps full width at half maximum, is described. Typical TCSPC measurements demonstrate the capability of the system of deriving decay or species associated excitation spectra.
72(2001); http://dx.doi.org/10.1063/1.1327309View Description Hide Description
A compact, tabletop terawatt Ti:sapphire laser drive, ultrafast hard x-ray source for time-resolved x-ray diffraction studies is described. With a copper target the energy conversion efficiency from laser photons (800 nm) to copperKx-ray radiation (1.54 Å) is 0.008%. The optimal laser intensity for generating these x rays is lower than the highest laser intensity available from the laser system. These results are consistent with a theoretical model proposed on the basis that the x rays are produced as a result of laser driven electron ionization of core level electrons of Cu atoms near room temperature. This source also provides features such as ultrashort pulse duration, extremely small source size, variable wavelengths, high peak spectral brightness, and the potential for multiple beam line experiments. X-ray diffraction patterns from GaAs single crystals and amorphous Ni films recorded with this source are presented.
72(2001); http://dx.doi.org/10.1063/1.1331326View Description Hide Description
The required alignment tolerances and surface roughness for unit lens elements in a compound refractive lens (CRL) for x rays are discussed. Contrary to what one might expect and what has been stated in the patent literature, alignment tolerances are large and for typical parameter values the effect of misalignment is minor. For a parabolic lens the focusing properties of the CRL are unaltered by misalignment and there is a small increase in absorption. For a lens with spherical aberration, there is a slight change in focal length, a minor translation of the image, and a small increase in absorption. This article also shows that lens gain is not appreciably reduced if the phase shift that is introduced by the roughness is limited to ±π/4 or if the transverse period of the roughness exceeds a specified value. The CRL can benefit from a managed misalignment of the elements to reduce the phase error introduced by surface imperfections of the lens.
72(2001); http://dx.doi.org/10.1063/1.1329901View Description Hide Description
In this article we describe the test of a zone platemonochromator for a laboratory soft x-raysource which is a laser produced plasma on a liquid jet target. The monochromator consists of a zone plate and a pinhole. Due to the special zone plate used (condensor zone plate KZP7) the monochromator is particularly suitable for laboratory sources since it collects a relatively large solid angle in the present setup. Depending upon the diameter of the pinhole a monochromaticity of up to λ/Δλ=600 can be achieved. The usefulness of the linear monochromator was proven on the basis of a filter transmission measurement. The monochromator can be used for several applications. In particular it is suitable for time-resolvedx-ray absorption spectroscopy and pump and probe experiments. The use for such investigations is discussed.
72(2001); http://dx.doi.org/10.1063/1.1327307View Description Hide Description
We describe a compact soft x-ray reflectometer for in-house characterization of water-window multilayer optics. The instrument is based on a line-emitting, liquid-jet, laser-plasma source in combination with angular scanning of the studied multilayer optics. With a proper choice of target liquid and thin-film filters, one or a few lines of well-defined wavelength dominate the spectrum and multilayer periods are measured with an accuracy of 0.003 nm using a multi-line calibration procedure. Absolute reflectivity may also be estimated with the instrument. The typical measurement time is currently 10 min. Although the principles of the reflectometer may be used in the entire soft x-ray and extreme ultraviolet range, the current instrument is primarily directed towards normal-incidence multilayer optics for water-window x-ray microscopy, and is thus demonstrated on multilayers for this wavelength range.
- PARTICLE SOURCES, OPTICS and ACCELERATION
72(2001); http://dx.doi.org/10.1063/1.1333042View Description Hide Description
Several measurements on a variety of semiconductorphotocathodes were performed in order to determine their photoelectric quantum efficiency. Two different excimer lasers (XeCl and KrCl) and a pulsed Xe lamp were used as light sources for electron photoextraction from doped and undoped samples of cadmiun telluride, indium antimonide, and indium phosphide. Large current densities were obtained up to the limit of the Child–Langmuir law. This suggests the use of these materials for the production of intense electron sources, which could also be used for purity measurements of noble liquids.
72(2001); http://dx.doi.org/10.1063/1.1328404View Description Hide Description
A Nd:YAGpulsed laser is employed to irradiate different metals in vacuum at the ECLISSE facility of the Laboratorio Nazionale del Sud, Catania, INFN. Laser pulse energy, 9 ns in width, ranges between 100 and 900 mJ. The ejection of atoms by means of laser irradiation is studied in terms of angular distribution, laser etching yield and film thickness deposited on a substrate. Light elements (Ni, Cu) show an angular distribution that is larger than heavy ones (W, Pb). A theoretical approach, applied to fit experimental data, indicates that the distribution depends on the high power of cos θ and that the flow velocity of ejected atom ranges between 27 000 and 88 000 m/s and the kinetic energy of ejected species ranges between 0.7 and 4.4 keV.
72(2001); http://dx.doi.org/10.1063/1.1331323View Description Hide Description
We describe a new concept for a microwave circuit functioning as a charged-particle accelerator at mm wavelengths, permitting an accelerating gradient higher than conventional passive circuits can withstand consistent with cyclic fatigue. The device provides acceleration for multiple bunches in parallel channels, and permits a short exposure time for the conducting surface of the accelerating cavities. Our analysis includes scalings based on a smooth transmission line model and a complementary treatment with a coupled-cavity simulation. We also provide an electromagnetic design for the accelerating structure, arriving at rough dimensions for a seven-cell accelerator matched to standard waveguide and suitable for bench tests at low power in air at 91.392 GHz. A critical element in the concept is a fast mm-wave switch suitable for operation at high power, and we present the considerations for implementation in an H-plane tee. We discuss the use of diamond as the photoconductor switch medium.
72(2001); http://dx.doi.org/10.1063/1.1287621View Description Hide Description
A microwaveprotonsource for sealed-tube neutron generator has been built in the radiation Technology Institute of Northeast Normal University. The plasma resonance chamber is made of ceramic material. The microwave absorption as a function of the magnetic field and the pressure is studied. The microwave absorption efficiencies, are more than 90% when the magnetic field at the microwave windows is 0.095 T and incident microwave power is 300–500 W, at the same time, the impedance between the microwave circuit and the plasma source is well matched. Two-grid multi-hole extraction electrodes are employed to extract ion. The maximum proton current of 30 mA is obtained when the extraction voltage is 5.4 KV and the incident microwave power is 300 W.
- NUCLEAR PHYSICS, FUSION and PLASMAS
72(2001); http://dx.doi.org/10.1063/1.1333040View Description Hide Description
Laser two-color heterodyne interferometry is a proven method to measure electron density in fusion plasmas. Though only used in tokamaks with high electron densities, the idea of also using two-color laser interferometers for stellarators and small machines to replace far-infrared laser interferometers is being proposed. This will lead to low-cost, reliable, and easy to operate diagnostics for electron-density measurements. In this article, we present the interferometric experiments we have used for calibration of a two-color laser heterodyne system for electron-density measurements in the TJ-II stellarator. These experiments have been based on the use of a novel interferometric scheme: the heterodyne/homodyne interferometer. Finally, we describe the interferometer we have installed in the TJ-II stellarator and present the first results on mechanical-vibration subtraction and electron-density measurements with a two-color laser interferometer in a stellarator (TJ-II, Madrid, Spain).
72(2001); http://dx.doi.org/10.1063/1.1329897View Description Hide Description
The Alcator C-Mod divertor bypass has for the first time allowed in situ variations to the mechanical baffle design in a tokamak. The design utilizes small coils which interact with the ambient magnetic field inside the vessel to provide the torque required to control small flaps of a Venetian blind geometry. Plasma physics experiments with the bypass have revealed the importance of the divertor baffling to maintain high divertor gas pressures. These experiments have also indicated that the divertor baffling has only a limited effect on the main chamber pressure in C-Mod.
The determination of absolutely calibrated spectra from laser produced plasmas using a transmission grating spectrometer at the Nike laser facility72(2001); http://dx.doi.org/10.1063/1.1332113View Description Hide Description
A transmission grating spectrometer has been developed that is capable of recording time-resolved spectra from laser-irradiated target foils in the wavelength range from 10 to 150 Å. Since the primary goal is to quantify the radiation flux from the resulting plasma, a detailed investigation has been conducted to evaluate the estimation of the absolute radiation intensity. Time-integrated spectra were observed on photographic film using a 5000 l/mm grating and clearly show the superposition of four diffraction orders. The deconvolution of the observed spectrum is based on a quantitative description of the spectrometer’s performance using the calibrated transmission efficiencies of the grating’s diffraction orders. This deconvolution procedure can provide an estimate of the absolute spectral intensity with a residual error of ∼20%. Results from a high spectral resolution grazing-incidence spectrometer have been used in the development of this method. A streak camera (not absolutely calibrated) was also used with the transmission grating spectrometer to record the time-resolved spectra.
72(2001); http://dx.doi.org/10.1063/1.1332115View Description Hide Description
Recent development of the JT-60U charge exchange recombination spectroscopy(CXRS) diagnostic system is reported. The measurements of the radial profiles of the ion temperatures, the toroidal and poloidal rotation velocities, and the impurity density are based on CXR spectra of fully stripped carbon ions induced by neutral beam injection. Measurements are made simultaneously at 59 spatial points (23 toroidal and 36 poloidal) with a time resolution of 1/60 s throughout the full discharge pulse of 15 s. Considerable effort has been expended to ensure the automatic provision of reliable profile data, where the mechanical components of the system must cope with both a baked tokamak vessel (∼300 °C) and an expected disruption-induced acceleration of up to 60 g, where g denotes the gravitational acceleration, i.e., 9.8 m2/s.
- MICROSCOPY and IMAGING
A very low temperature scanning tunneling microscope for the local spectroscopy of mesoscopic structures72(2001); http://dx.doi.org/10.1063/1.1331328View Description Hide Description
We present the design and operation of a very low temperature scanning tunneling microscope(STM) working at 60 mK in a dilution refrigerator. The STM features both atomic resolution and micron-sized scanning range at low temperature. We achieved an efficient thermalization of the sample while maintaining a clean surface for STMimaging. Our spectroscopic data show unprecedented energy resolution. We present current–voltage characteristics and the deduced local density of states of hybrid superconductor–normal metalsystems. This work is the first experimental realization of a local spectroscopy of mesoscopic structures at very low temperature.
72(2001); http://dx.doi.org/10.1063/1.1333045View Description Hide Description
We have designed and built a low-temperature (1.3–4.2 K) scanning-tunneling microscope which is capable of collecting light that is generated in the tunneling region. Light collection is done by means of two fibers whose cleaved front is in close proximity mm) to the tunneling region. The whole system can be operated in high magnetic fields (11 T) without loss of optical signal strength. As a demonstration, we measured the electroluminescencespectra of an InGaAs quantum well at various temperatures. At 4.2 K, we found an electron-to-photon conversion factor that is three orders of magnitude higher than at room temperature.
72(2001); http://dx.doi.org/10.1063/1.1326053View Description Hide Description
We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.
72(2001); http://dx.doi.org/10.1063/1.1333044View Description Hide Description
We introduce the concept of a multiple sensor stabilization system (MSS) applicable to all local probe microscopes. With this it is possible to separate distance control and sensor-sample-interaction signals with unprecedented stability. The MSS uses two or more sensors of an array of local probe sensors, which are coupled via rigid low-mass short-distance connections. At least one of these sensors is employed to always provide a continuous and independent feedback signal. Using this, the distance between any other sensor of the array and the sample surface can be controlled, under ambient and in situ conditions, with the resolution and range of the designated sensors on time scales of up to hours. The concept of MSS is applicable to the whole range and any conceivable combination of local probe techniques, especially all other scanning near field probes. MSS offers particularly large advantages for spectroscopic applications. We demonstrate its utility by the example of an atomic force microscope using a commercially available array of cantilevers. By using two cantilever sensors for position control it is possible to eliminate all drift between the sample and the tip’s position. The high potential of the MSS is illustrated by two applications: a thermal noise-reduction based approach with minimal contact forces and the first pN-“force clamp” for single molecule force spectroscopy.
Lock-in technique for concurrent measurement of adhesion and friction with the scanning force microscope72(2001); http://dx.doi.org/10.1063/1.1329898View Description Hide Description
Regardless of all the great progress in new scanning probe microscopy techniques, the concurrent measurement of adhesive and frictional forces with local resolution using scanning force microscopy (SFM) has not been possible until now. In this paper, we present a novel scanning probe microscopy mode, called combined dynamic x mode or CODYMode®. In CODYMode® SFM at least two oscillations with sufficiently different frequencies and amplitudes are superimposed and interact with the sample surface. This enables the concurrent measurement of the topography,adhesive and frictional forces beside further mechanical surfaceproperties of the sample. By means of the characterization of plasmatreated biaxially oriented polypropylene foils the benefits of the new modulation technique are pointed out where common SFM techniques are not adequate. As second application high-velocity friction experiments (in the range of several centimeters per second) on silicon under controlled environmental conditions are introduced and the role of the native water film on it is discussed under friction and viscoelastic aspects.