The excitation of wake waves by a relativistic homogeneous electron bunch passing through cold magnetized plasma at equilibrium is studied for arbitrary values of the ratio of bunch density to plasma density. The analysis is based on the assumption that the magnetic field is sufficiently strong (omega(B) much greater than omega(p), where omega(p) and omega(B) are the electron plasma and cyclotron frequencies respectively). The periodic and nonperiodic solutions for the momentum of plasma electrons inside the bunch are analyzed. It was shown that the presence of strong external magnetic field may increase the amplitude of wake waves and the maximum transformer ratio, the latter being reached at densities of bunch power than that in the absence of magnetic field. The optimum conditions for obtaining the maximum values of the wake field amplitude and of the transformer ratio are found.
The contribution of Coulomb elastic scattering of beam particles on the residual gas to the beam halo development in a linear accelerator is studied. Starting from the basic differential cross section formula giving the probability of a particle to scatter at a given angle, we analyse the behaviour of those few particles which scatter at rather large angles and we estimate their proportion relative to the beam intensity. We present a transport code dedicated to the scattering of a beam on the residual gas, which we validate with an experiment in which the beam is transported through a drift space. We use then our code to simulate the transport of a beam matched to a uniform focusing channel and we show how the collisional halo develops. The code is finally used in order to establish the loss rate per meter in a high intensity linac. This loss rate is compared to the acceptable one.
Global compensation of nonlinear field errors based on the minimization of nonlinearities in a one-turn map was found to be very effective in reducing the detrimental effects of magnetic field errors in the LHC collision lattice. With a few groups of low-order correctors, nonlinear terms in the one-turn map can be minimized order-by-order and, consequently, the dynamic aperture is substantially increased and the phase-space region occupied by beams becomes much more linear. One advantage of the global correction is the possibility of further optimization of the correction based on a direct measurement of a one-turn map with beam-dynamics experiments.
It is important to have a clear understanding of the transverse emittance in a circular accelerator in order to achieve optimum brilliance. Experience with comparing emittance data From different instruments has shown that systematic errors can be important. In an attempt to detect such errors in the PS Booster, the emittance measurements are made according to two different principles: measurement of density distribution and measurement of amplitude distribution. In this paper we (i) discuss these two principles and the theory behind them; (ii) show how the data can be compared; (iii) describe the instrumentation used For these measurements; and (iv) present results for typical PS Booster beams.
Guiding the laser pulse in a metal waveguide filled with a rare plasma is proposed as a tool for overcoming driver diffraction and controlling the driver trajectory in the laser wake-field accelerator. The wakefield wavelength in this case is determined by the density of the inner plasma and can be made large enough to provide effective wave generation. For the circular waveguide of the radius about the plasma skin-depth, most of the driver energy is contained in TE11 mode, and the waveguide does not drastically diminish the dephasing length.
Electron multipacting is a serious problem in many rf components operating in vacuum. The knowledge of the possible multipacting resonances is valuable when planning new rf designs and when struggling against multipacting in already existing structures. In this paper a detailed numerical analysis of electron multipacting in the TeV Energy Super-conducting Linear Accelerator (TESLA) cavities and coaxial input couplers with ceramic windows is presented. TESLA is an international linear collider R&D effort based on superconducting components. The input coupler design for TESLA has been developed at both DESY and FNAL. In the couplers (both DESY and FNAL type) the discussion is confined to the cylindrically symmetric cold window sections. In the cavities the well-known (two-point) multipacting phenomenon is verified and the computed values agree with the measured ones. In the couplers the situation is more involved, since the rf operation conditions vary and a large number of simulations are required for a complete multipacting analysis. It is found that the cylindrical DESY window design is less sensitive to multipacting than the conical FNAL window design. With the present analysis method it is possible to optimize the rf operation conditions in the DESY design so that multipacting can be avoided.
The comprehension of the mechanism that leads to small beam losses is one of the key points for the feasibility of the next generation of high power linacs. In this paper we study the nonlinear dynamics of the beam halo particles in a FODO channel, by using the Frequency Map analysis. This tool provides a picture which allows to detect the regular, resonant or chaotic regions also in the phase space for a mismatched beam in two degrees of freedom. Moreover we introduce a criterion for single particle stability and we make comparisons with tracking results.
A simulation that uses a new method to obtain tail distributions in electron-positron storage rings has been applied to KEKB. This program makes it possible to investigate the tail distribution using a simple and fast simulation method and shows good agreement with solvable cases. The simulation shows beam-tail distributions due to transverse and longitudinal rare random processes. A six-dimensional beam-beam interaction is considered. The lifetime of the electron beam in KEKB due to rare random processes and beam-beam interaction is estimated to be 5 h.
Propert ies of .ynchrolron radlution (SR :ue addressed in terms of the behnvionr 1 f the r:idiatcd wave eh:ctric field . Bused n this method a simple theory of low l'rcq11cncy farlicld SR is established lmblc to rcuch up lo X-ray energy rungc in suffi kntly l1igh cnerg lcpt n mach ines; spectral :1 11gular cl istribu,.uons and othor pnip ' rtics urc derived . As 1 direct outcome. it is shown thul n (2h)-dcvi;iti n dipole produces hi •hcst brightness i11 the low frequency spcctrnl rongo with a gu.in r lhur w.r.t. edge S R um! up to scveml order l'rnagnitudc w.r.l. body SR from a long djpo le. Numerical ex.amplcs i11 given .
We present an effective numerical technique for computing the electrical image coefficients for rounded rectangular pipes with perfectly conducting walls. The method of moments is used to solve the involved boundary-value problem in integral form, by means of(a rapidly converging representation of) the rectangular-domain Green's function, together with a set of piecewise parabolic subdomain basis functions, yielding high speed and accuracy, with minimum storage budget (no prior meshing required). As a distinctive feature of the proposed method no numerical differentiation is required, resulting into far better accuracy as compared, e.g., to finite-element and finite-difference methods. Application to some simplified cross section geometries relevant to LHC (square with rounded corners, stadium and cut-circle) are presented. As a check of accuracy of the proposed approach, a comparison with available exact (analytical) results for the circular pipe (hardest possible benchmark), shows an excellent agreement.
The interest in using optical transition radiation (OTR) in high energy (multiGeV) beam diagnostics has motivated theoretical and experimental investigations on the limitations brought by diffraction on the attainable resolution. This paper presents calculations of the diffraction effects in an optical set-up using OTR. The OTR diffraction pattern in a telescopic system is calculated taking into account the radial polarization of OTR. The obtained diffraction pattern is compared to the patterns obtained by other authors and the effects of different parameters on the shape and on the size of the OTR diffraction pattern are studied. The major role played by the radial polarization on the shape of the diffraction pattern is outlined. An alternative method to calculate the OTR diffraction pattern is also sketched
For precise measurements of the magnetic field in ramped machines, different magnetic markers are in use. The best known are peaking strips, Nuclear Magnetic Resonance (NMR) probes and Electron Spin Resonance (ESR) probes. Their operational principles and limitations are explained and some examples of recent and new applications are given. A fuller theoretical description is given of the lesser-known Ferrimagnetic Resonance (FMR) probe and its practical application. The essential purpose of these magnetic markers is the in situ calibration of either on-line magnetic field measurements (e.g. via a magnetic pick-up coil) or field predictions (e.g. using a magnet model).
This report gives the results of a programme of experimental investigations, which were carried out to test stacking of lead ions in a storage ring (the former Low Energy Anti proton Ring, LEAR) at 4.2 MeV per nucleon. The motivation was to demonstrate the feasibility of gaining the large factor in the phase-space density required for injection into the Large Hadron Collider (LHC). In the first part of the report, the layout of the experiments is described, the choice of the parameters of the electron cooling system used for stacking is reported and the multiturn injection using horizontal- and longitudinal (and in the final project also vertical-) phase space is discussed. In the second part the experimental results are presented. Factors ofvital importance are the stacking efficiency, the beam life-time and the cooling time of the ions. The beam decay owing to charge exchange with the residual gas and to recombination by the capture of cooling electrons was intensively studied. Beam instabilities and space-charge effects in the ion beamturned out to be additional, although less serious, limitations of the accumulation rate. The cooling speed as a function of cooler and storage-ring properties was investigated over a wide range of parameters. Among the 'surprises' encountered are an anomalously fast recombination rate for certain ion charge states (Pb 53 +), a strong dependence of the cooling time on the dispersion function of the storage ring, and an intensity-dependent outgassing ofequipment in the vacuum chamber. After a careful choice ofparameters and antidotes, an overall factor of 120 in intensity could be gained, by multi-turn injection and stacking for 4 s. The intensity obtained (6 x 10 8 ions with a length corresponding to four LHC bunches) is only a factor oftwo short of the LHC requirement, and the stacking time (4 s instead of 2 s foreseen for filling each LHC ring in 8 min) is another factor of 2 off.
We will present measurements and calculations related to the antisymmetric perturbations, and comparisons with the symmetric ones, of the IFUSP race-track microtron booster accelerator end magnets. These perturbations were measured in planes situated at +/-12 mm of the middle plane, in a gap height of 4 cm, for a field distribution of about 0.1 T. The measurements were done in 1170 points, separated by a distance of 8 mm, using an automated system with a +/-1.5 mu T differential Hall probe. The race-track microtron booster is the second stage of the 30.0 MeV electron accelerator under construction at the Linear Accelerator Laboratory in which the required uniformity for the magnetic field is of about 10(-3). The method of correction employed to homogenize the IFUSP race-track microtron booster accelerator magnets assures uniformity of 10(-5) in an average field of 0.1 T, over an area of 700 cm(2). This method uses the principle of attaching to the pole pieces correction coils produced by etching techniques, with copper leads shaped like the isofield lines of the normal component of the magnetic field measured. The ideal planes, in which these measurements are done, are calculated and depend on the behavior of the magnetic field perturbations: symmetric or antisymmetric with reference to the middle plane of the magnet gap. These calculations are presented in this work and show that for antisymmetric perturbations there is no ideal plane for the correction of the magnetic field; for the symmetric one, these planes are at +/-60% of the half gap height, from the middle plane. So this method of correction is not feasible for antisymmetric perturbations, as will be shown. Besides, the correction of the symmetric portion of the field distribution does not influence the antisymmetric one, which almost does not change, and corroborates the theoretical predictions. We found antisymmetric perturbations of small intensity only in one of the two end magnets. However, they are not detected at +/- 1 mm of the middle plane and will not damage the electron beam.
Electropolishing (EP) offers a smoother surface, and that makes us expect high gradients in superconducting niobium cavities. EP has a little bit complicated procedure and needs annealing to avoid the hydrogen Q-disease. On the other hand, chemical polishing (CP) is easier and cheaper, so widely used in many laboratories. For the conventional field gradients ( ∪ 5 MV/m) in storage rings (TRISTAN-MR, HERA), no difference was seen on the cavity performance between EP and CP. In the high gradients research for TESLA, we have to establish the surface treatment method which has a high reliability on the high gradients over than 25 MV/m and is cheaper in the preparation cost. So far, CP has offered a nice cavity performance, but a concern on "European Headache" was developed recently on the high gradients at Saclay. KEK has intensively investigated the difference between the effect of CP and EP on high gradient performance since the last SRF workshop at Saclay. In this investigation "European headache" was also seen in chemically polished cavities at KEK, in addition these cavities often showed a new Q-degradation without x-ray signals which seriously limited the high gradient. EP offers the high gradient over than 30 MV/m reliably. At the moment, EP seems to have a superiority over CP on the high gradient over than 25 MV/m.
Coherent transverse dipole oscillations in colliding head-on non-rigid bunches are studied using the Vlasov equation. The corresponding eigenvalue problem is solved numerically in the case of round Gaussian bunches of equal size but with not necessarily equal intensities. Transition from the weak-strong to the strong-strong cases is found at the intensity ratio of about 60% when a discrete pi-mode frequency emerges from continuum of eigenfrequencies related to the beam-beam tunespread in the weaker bunch.In the strong-strong case the large coherent beam-beam tuneshift dominates over interchange processes between coherent and incoherent motion: it can switch off Landau damping of dipole transverse oscillations, slows down incoherent emittance growth due to external kicks on the beams. The consequences for the transverse feedback operation in collision are discussed.
