Pulsed-wire Technique Research Articles

Requirements and limitations of the pulsed wire technique for measuring short-period long undulators’ magnetic field

The pulsed wire method is an attractive option to measure the magnetic field in insertion devices, mainly for those with restricted access (e.g., small gaps, in-vacuum/cryogenic environments, etc.). Besides first and second field integrals, experiments have proved the feasibility of reconstructing the magnetic field profile. Undulators with a small gap and short period are — and are planned to be — used at diffraction-limited storage rings and free-electron lasers. This contribution outlines the pulsed wire system’s requirements to perform magnetic field reconstruction in such undulators. We examine the main expected limitations, particularly the dispersive, finite pulse-width, discretization error, and sag effects. Furthermore, we present the current status of developing the pulsed wire system at the European XFEL.

Fast Magnetic Measurements Of 8.6 m Undulator

The design of the large-aperture variable-period undulator (VPU) developed for the Novosibirsk free electron laser (FEL) is shortly described. High amplitude of on-axis field is achieved due to the arc shape of the magnet blocks and poles of the undulator. To conduct magnetic measurements and fine tuning of the undulator as well as to perform real time position tracking of magnet blocks, pulsed wire technique was adopted.

A dispersion and pulse width correction algorithm for the pulsed wire method

The pulsed wire technique is an attractive option for the measurement of undulators where the measurement access is restricted due to, for example, narrow undulator gaps or cryogenic environments in the case of superconducting undulators. Using the pulsed wire technique, direct measurements of the first and second integrals of the magnetic field can be obtained. However, one of the main limitations of this technique is the error introduced by dispersive wave motion, due to the finite flexural rigidity of the wire. For the measurement of the first integral of the magnetic field, an error is also introduced by the use of a current pulse with finite pulse width. In this paper, a general solution is presented for dispersive wave motion in pulsed wire measurements. A method for the measurement of the dispersive wave speed is presented and demonstrated through experimental examples. An algorithm is derived which corrects the dispersion and finite pulse-width errors in the measurement of first magnetic field integrals and the dispersion error in the measurement of second magnetic field integrals. The effectiveness of the correction algorithms is demonstrated through experimental measurements, and the results are compared with Hall probe measurements on a short undulator.

Analysis of pulsed wire method for field integral measurements in undulators

Pulsed wire technique is a fast and accurate method for the measurement of first and second field integrals of undulators used in free-electron lasers and synchrotron light sources. In this paper, we present a theoretical analysis of this technique by finding out the analytic solution of the differential equation for the forced vibration of the wire taking dispersion due to stiffness into account. Method of images is used to extend these solutions to include reflections at the ends. For long undulators, the effect of dispersion of the acoustic wave in the wire could be significant and our analysis provides a method for the evaluation of the magnetic field profile even in such cases taking the effect due to dispersion into account in an exact way.

Pulsed wire magnetic field measurement on the permanent magnet focusing device of millimeter-wave traveling wave tube

In our case, the pulsed wire method is applied to the magnetic field measurement of the permanent magnets in focusing device of millimeter-wave traveling wave tube (TWT). The focusing device using permanent magnets is used to help the electron beam keep shape and make it possible go through the slow-wave circuit, effectively interact with the microwave field and well received by collectors, which makes the measurement become a key factor for the development of the millimeter-wave TWT technique. Furthermore, the wire method can be extended used to magnetic field measurement of the magnet with small via hole. The pulsed wire technique is based on the excitation of the harmonics of a wire vibration by Lorenz forces between current in a wire and the surrounding magnetic field. Knowing the amplitude and the phase of the various harmonics of the wire vibration, one can easily reconstruct the surrounding magnetic field. As sensitive the wire system is, ultrastable supporters, high accurate mechanical adjusters, precise light and electric centering device are installed and relevant parameters are properly chosen. In addition, shorter wire is used to reduce the sag, and a simple and effective wire-position detector providing accuracy up to 1μm is designed and tested. Experiment confirms the theoretical conclusions and shows a positive future of application. The prototype system can show several relevant results, e.g., three-dimensional magnetic intensity distribution, maximum, minimum, and medium magnetic inductions, pole pitch error, etc.

Commissioning of OPHELIE in the DC mode: an electromagnetic planar/helical crossed VUV undulator at Super-ACO

OPHELIE is a crossed Onuki-type fixed-gap electromagnetic undulator capable of producing any polarization state of the emitted light in the VUV by setting its three main parameters: the vertical and horizontal driving currents and the longitudinal dephasing φ. After its construction in accordance with strict mechanical tolerances, the undulator was magnetically checked and calibrated with both a Hall probe and the pulsed wire technique showing a very satisfactory behavior with, in particular, a high horizontal/vertical symmetry. OPHELIE has been then the object of a full on-beam commissioning in the DC mode. The effects on the positron beam, in terms of closed orbit distortion and betatron tune shifts, have been calibrated and fully analyzed. The promising results gained during the magnetic measurement campaign have been confirmed by the spectral characteristics of the undulator which are very satisfactory: full tunability of the fundamental between 4.7 and 20 eV, validation of the superimposition principle of the magnetic fields, and stability of the spectra with respect to a polarity/dephasing switch. Besides, an unexpected φ-dependence of the emitted wavelength has been unravelled and will be taken into account in the computer control of the undulator.

Improvements of the pulsed-wire method to measure undulators

We present the Pulsed-Wire Method (PWM) for undulator measurements used for the first crossed overlapped electromagnetic undulator OPHeLIE. This method gives directly the first integral of the magnetic field using a short current pulse and the second integral using a step current pulse through the wire. We characterized OPHeLIE using the second integral mode to optimize the beam trajectory in the respect of specifications. There was a good correlation between these results and those obtained from the on-beam test. We were therefore encouraged to further develop this method to determine optical performances of undulators. In order to validate this approach we compared the spectrum determined by the pulsed-wire technique and Hall probe scans, used as reference, applied both to the OPHeLIE prototype, which is a shorter planar version. For the PWM, we used the short current pulse according to the undulator's period (the first integral mode). Digital data processing techniques have been used to improve the signal-to-noise ratio, to minimize the effect of the background vibrations and to characterize the wire response.

Pulsed-wire technique for velocity measurements in natural convection flows—a numerical optimisation tool

A computer model for improving the potentialities of the parallel wires anemometers is presented. A new calculation technique has been developed, taking into account the diffusion effects in the transfer phenomena that occur during the measurement procedure. A numerical model which represents the behaviour of a unique wire in relaxation has been developed. It has been shown that, for the range of speeds encountered in natural convection, the phase of relaxation could be separated into two periods. During the first period the diffusion transfer is dominant, whereas in the second period the convection transfer becomes more significant. Moreover, the energy of the emitter wire is introduced as a source term in a transport model built upon the Patankar's method. The validity of the prediction is illustrated by comparison with measurements and very good agreement has been achieved.

Further development of the pulsed wire technique for magnetic field and focusing strength measurements in long undulators

Improvements of pulsed wire method are made with the purpose to make it applicable for accurate field measurements in long undulators. It is shown that the focusing strength of an undulator can be measured with rather high accuracy for both transverse directions. A new approach for solution of the sag problem is suggested enabling to resolve this problem.

Detailed analysis of pulsed-wire technique accuracy

Undulator magnetic fields were carefully investigated by two known methods. The Hall probe was used for direct field measurement and pulsed-wire method was used for direct mesurements of z-integrated fields. Error analyses and comparison of the accuracies of both the methods are made.

Experience on the operation of the 2-in-1 electromagnetic undulator of FELICITA-I

The 5m-Undulator of FELICITA I has been installed at the final position in the storage ring DELTA in spring 95. Detailed magnetic measurements have been performed in place to get a complete characterization of this device in both possible modes, pure undulator and optical klystron mode, respectively. The undulator consists of 38 identic poles. Four power supplies are connected to the main coils. Three of them drive the central six poles to get the dispersive section with its matching to the outer undulator sections. This setup allows a change between the optical klystron and the pure undulator mode without changing the hardware. For measuring the magnetic field components a hall-probe was used. It was found that correction coils for compensating peak field variations were not necessary, because of errors less then 0.2 %. Only for the purpose of steering correction coils had to be used. The field integrals were also measured with the pulsed-wire technique. For fast response concerning the matching, in particular of the dispersive section this technique was found to be very useful. Because of the large period length of 25 cm the wire sag in the rather long undulator could be neglected.

Measurement of instantaneous flow reversals and velocity field in a conical diffuser

An 8° total divergence angle conical diffuser fed with a fully developed turbulent pipe flow at its inlet was experimentally investigated. Through effective use of pulsed-wire anemometry, some quantitative information about instantaneous reversals was obtained in conical diffuser flow. Profiles of streamwise mean and fluctuating velocities, skewness factor, and flatness factor are presented for the stations in the final stages of the diffuser flow, where appreciable instantaneous reversals were detected in the wall region. Results are also presented for the stations in the earlier stages of the diffuser, where smaller instantaneous backflow was present. Comparative results of conventional pressure measurements and hot-wire anemometer measurements were obtained to determine quantitatively the effects of instantaneous flow reversals on these techniques. In the regions of instantaneous backflow, the pulsed-wire technique was found most accurate for measuring the mean and fluctuating velocities. The hot-wire and pulsed-wire anemometer results were also used to obtain guidelines for hot-wire use in highly turbulent instantaneously reversing flows. The integral constraint of continuity was used at the diffuser stations to evaluate measurement techniques for obtaining mean velocity in a conical diffuser flow. The distributions of streamwise mean and fluctuating wall shear stress were measured with a pulsed-wire skin friction probe. Pulsed-wire wall shear stress probe results indicated serious errors in the values of the mean wall shear stress obtained by the conventional Preston tube in the region of appreciable instantaneous backflow. Some of the separation prediction criteria reported in the literature were evaluated for their validity using the present results of quantitative instantaneous backflow. These measurements illustrate important characteristics of instantaneous flow reversals and the velocity field in the highly turbulent axisymmetric flow developing under a severe adverse pressure gradient.

A pulsed-wire technique for velocity and temperature measurements in natural convection flows

A pulsed-wire probe based on the use of one or two parallel wires, capable of measuring the velocity and the temperature in natural convection flows is described. These measurements are based on the analysis of the relaxation response of a pulsing wire, submitted to a very short electrical pulse. The analysis of the temperature variation on an optional second receiver wire, gives information about the velocity direction. The implementation simplicity of this probe, its good spatial precision, the lack of thermal contamination of the flow, as well as the possibility of obtaining simultaneous velocity and temperature measurements, allow the integration of the device in a multi-point measurement network, capable to deliver thermal and dynamic cartographies of unsteady convection flows.

A staggered-array wiggler for far-infrared, free-electron laser operation

A staggered-array wiggler for a far-infrared free-electron laser (FEL) has been built at Stanford, and its magnetic properties have been tested. This type of wiggler has several desirable features: high wiggler field at short wiggler periods, wavelength tuning by a solenoid current, electron beam confinement by a solenoid field, and looser machining tolerances. A 10.8-kilogauss peak wiggler field has been measured at a 7.0-kilogauss solenoid field for a 1.0-cm wiggler period and a 2.0-mm gap. The small-signal gain has been calculated analytically and by computer simulation for a 0.5-m long wiggler. For an 8-A, 9-ps current pulse and a 3.3-MeV electron beam, 5-dB gain is predicted. Twenty- to thirty-percent wavelength tuning can be achieved by adjusting the solenoid field and still maintain reasonable small-signal gain. The pulsed-wire technique was employed to test the field uniformity of this novel wiggler, and the measured field variation was about 1%. >

Performance characterization of a far-infrared, staggered wiggler

The performance of a staggered-array wiggler for the far infrared free-electron laser (FIRFEL) project at Stanford has been characterized analytically and experimentally. A 10.8 kG peak wiggler field was measured for a 2.0 mm gap and a 1.0 cm wiggler period at a 7.0 kG solenoid field. The wiggler field uniformity was investigated by the pulsed-wire technique, and measurements showed a 1.2% rms field variation over a 50 cm wiggler section. With fairly simple design schemes, studies indicate the velocity drift in a staggered wiggler can be eliminated.

Wiggler field measurements and corrections using the pulsed wire technique

We have performed magnetic field ,easurements on permanent-magnet wigglers using the pulsed wire technique. The first and second integrals of the field were directly measured, and the field was determined by differentiating the first integral. The pulsed wire technique permits the rapid accumulation of data and the identification of defective wiggler magnets. Magnet defects of different types can be distinguished and compensated for by attaching small, appropriately oriented magnets to each defective magnet. We have made major improvements in the fields using these techniques.

A pulsed-wire technique for velocity measurements in highly turbulent flows

The velocity measuring technique described in this paper consists of measuring the time of flight of a tracer of heated air from an electrically pulsed wire to one of two sensor wires which are operated as resistance thermometers. These sensor wires are at right angles to the pulsed wire and are placed one on either side of the pulsed wire. The instrument may be used in highly turbulent flows including regions in which flow reversals occur. The paper discusses the theoretical behaviour of the probe and the results of some calibration experiments.