EPAC 2006 - Classification:05 Beam Dynamics and Electromagnetic Fields/D04 Instabilities

05 Beam Dynamics and Electromagnetic Fields

D04 Instabilities - Processes, Impedances, Countermeasures

Paper	Title	Page
THOAFI02	Ion Instability Observed in PLS Revolver In-vacuum Undulator	2771
	H.-S. Kang, J. Choi, M. Kim, T.-Y. Koo, T.-Y. Lee, P.C.D. Park PAL, Pohang, Kyungbuk
	Revolver In-Vacuum X-ray Undulator which was designed and fabricated at Spring-8 is under commissioning at PLS. This planar undulator whose permanent magnet array structure is a revolving type with 90-degree step provides 4 different undulator wavelengths of 10, 15, 20, and 24 mm. The minimum gap of the undulator is as small as 5 mm. It was observed that the trailing part of a long bunch train was scraped due to ion instability when the undulator gap was closed below 6 mm. At that time the vacuum pressure in the undulator, which is estimated to be about one order of magnitude lower than that of the undulator gap, increased from 1.4 x 10^-10 (gap 20 mm) to 7.9 x 10^-10 Torr (gap 6 mm) at the stored beam current of 100 mA. This high vacuum pressure causes fast beam-ion instability: trailing part of a long bunch train oscillates vertically. It was also confirmed that adjusting the orbit along the undulator has improved the situation to some extent. The ion instability measured with a pico-second streak camera and a one-turn BPM as well as the result of orbit adjustment and chromaticity control will be described in this paper.
	Transparencies
THPCH031	Impedance and Beam Stability Study at the Australian Synchrotron	2844
	R.T. Dowd, M.J. Boland, G. LeBlanc, M.J. Spencer, Y.E. Tan ASP, Clayton, Victoria
	We present the preliminary results of an impedance study of the Australian Synchrotron storage ring. Beam stability thresholds have been determined and an overall impedance budget set. Broad-band impedance has been evaluted for various components of the vacuum chamber, using both analytical formulae and results from MAFIA simulations. Narrow band resonances have also been investigated, with particular attention paid to higher order modes in the RF cavities and their effect on multi-bunch instabilities.
THPCH032	Instability Studies Using Evaluated Wake Fields and Comparison with Observations at SOLEIL	2847
	R. Nagaoka SOLEIL, Gif-sur-Yvette
	Beam instability is predicted for SOLEIL using the impedance data obtained through component-wise numerical evaluations. The paper also attempts to make the first comparison with measurements. A key issue for SOLEIL has been to acquire precise knowledge of impedance up to a few tens of GHz, due to short bunches, of chambers which are essentially 3D and additionally NEG coated to a large part, which is expected to enhance the reactive part of the resistive-wall impedance. The predictability of instabilities with the data attained thus becomes a large concern. Wake potentials computed with a 3D code are transformed to impedances and decomposed into a series of resonators, inductive and resistive components, to deduce the wake functions, while for NEG coated chambers, they are numerically Fourier transformed from analytically derived impedances. Both time and frequency domain simulations are performed to predict the longitudinal and transverse instabilities in single bunch, as well as resistive-wall instabilities in multibunch as a function of chromaticity. A multibunch tracking is also performed to investigate the filling pattern dependence of the latter.
THPCH033	Recent Studies of Geometric and Resistive-wall Impedance at SOLEIL	2850
	R. Nagaoka, J.-C. Denard, M.-P. Level SOLEIL, Gif-sur-Yvette
	Coupling impedance studies are of great importance for SOLEIL not only to avoid beam instability, but also to ensure protection of a concerned chamber from EM fields excited by the beam. This paper deals with components that required particularly such efforts, which include BPMs, ceramic chambers, and a vertical scraper. The heat deposited in the BPM buttons is investigated as a function of the gap between a button and an electrode, button diameter and thickness. High temperatures on a vacuum tight feed-through would be a problem, affecting the measurement stability at high currents. Coupling of a trapped mode among successively passing bunches is also investigated. To evaluate the heat deposited in a titanium coated ceramic chamber, its impedance is analytically solved using the field matching technique. The solution obtained justifies the image current model that assumes a constant image current density across the coating when the skin depth is greater than the coating thickness. The azimuthal image current distribution is pursued with Piwinski's formalism for flat chambers. The paper also deals with components not treated earlier such as a stripline.
THPCH034	Transverse Coupling Impedances From Field Matching in a Smooth Resistive Cylindrical Pipe for Arbitrary Beam Energies	2853
	A.M. Al-Khateeb, A.M. Al-Khateeb, W.M. Daqa Yarmouk, Irbid O. Boine-Frankenheim, R.W. Hasse, I. Hofmann GSI, Darmstadt
	The transverse coupling impedance is investigated analytically. For an off-axis motion of the beam, the perturbed charge distribution of the beam becomes a function of the azimuthal angle, resulting to first order in the beam displacement in a dipole term which is the source of the transverse impedance. All six components of the electromagnetic field are different from zero and, therefore, both TM and TE modes will be excited in the beam-pipe and coupled to the beam at the inner surface of the resistive wall. Using the dipole source term, a linear combination of TM and TE modes is used to get closed form expressions for the transverse electromagnetic field components excited in the beam-pipe, and a generalized analytic expression for the corresponding transverse coupling impedance. It has been found that the contributions of the TM and the TE modes to the real part of the transverse resistive-wall impedance have similar dependence on the relativistic parameter but with opposite signs, the sum of both always being positive. Some approximate simple formulas for three important regions corresponding to small, intermediate and large frequencies in the ultrarelativistic limit were also obtained analytically.
THPCH035	Characterisation of the EU-HOM-damped Normal Conducting 500 MHz Cavity from the Beam Power Spectrum at DELTA	2856
	R.G. Heine, P. Hartmann, T. Weis DELTA, Dortmund
	A HOM-damped prototype cavity developed in the framework of an EC collaboration has been installed into the Dortmund synchrotron light source DELTA. This paper reports on beam studies performed at beam energies of 1.5 GeV and 542 MeV in an attempt to get information on coupled bunch instability thresholds. In addition an evaluation of the longitudinal cavity impedance is presented, based on beam power spectra up to 3 GHz for different filling patterns of the storage ring by analysing the RF signal from the HOM-dampers.
THPCH036	Wakefield Calculations for 3D Collimators	2859
	I. Zagorodnov DESY, Hamburg K.L.F. Bane SLAC, Menlo Park, California
	The wakefield effects of the collimators is of concern for future projects. To relax the wakefield effects a gradual transition from a large to a small aperture is used. The impedance of a smooth round collimator is understood well and a good agreement between measurements, theory and simulations is achieved. However, for rectangular flat collimators there is noticeable difference between theory and experiment. Using recently developed time domain numerical approach, which is able to model curved boundaries and does not suffer from dispersion in longitudinal direction, we calculate the short-range geometric wakefields of 3D collimators. This method together with developed by us recently indirect 3D integration algorithm allows to obtain accurate numerical estimations, which are compared to measurements and to analytical results. The applicability range for the analytical formulas is highlighted.
THPCH037	Wakefields Effects of New ILC Cavity Shapes	2862
	I. Zagorodnov DESY, Hamburg N. Solyak Fermilab, Batavia, Illinois
	The operation of International Linear Collider (ILC) requires high gradients and quality factors in accelerating structure. One way to reach it is to modify the cavity shape to reduce the ratio of peak surface magnetic to accelerating field. Two candidate shapes are suggested recently: the Re-entrant shape and the Low-Loss shape. In this paper we estimate numerically longitudinal and transverse short range wake functions for the new shapes. The obtained analytical expressions are used in beam dynamic simulations for ILC lattice. We show that ILC will tolerate the cavities with the new shape and the smaller iris diameter.
THPCH038	The PANDA Insertion Impedance in High Energy Storage Ring of FAIR	2865
	E. Senicheva, A. Lehrach, D. Prasuhn FZJ, Jülich
	The PANDA insertion due to the special shape of the vacuum pipe creates a discontinuity. This was expected to be the main contribution in the impedance of the vacuum chamber. In this paper we present the results of computations dealing with this problem. From many published articles it is known that the reliability of the results depends on many factors and some time they differ from each other significantly. Therefore we have investigated the impedance of the PANDA insertion using different codes and methods, in particular, MAFIA, ABCI and the analytical estimation with the formula Yakoya recognized as a most successful theoretical estimation of the tapers. Besides, PANDA has two symmetrical T-shape insertions, which have been calculated by 3D MAFIA and compared with the results given by the diffraction theory. We have analysed the longitudinal and the transverse impedance.
THPCH039	Beam Studies with Coherent Synchrotron Radiation from Short Bunches in the ANKA Storage Ring	2868
	A.-S. Müller, I. Birkel, S. Casalbuoni, B. Gasharova, E. Huttel, Y.-L. Mathis, D.A. Moss, P. Wesolowski FZK, Karlsruhe C. J. Hirschmugl UWM, Milwaukee, Wisconsin
	In the ANKA storage ring it is possible to store bunches with RMS lengths of the order of 1 ps using a dedicated optics with reduced momentum compaction factor. For short bunch operation a beam energy of 1.3 GeV is chosen as a trade-off between low energy longitudinal instabilities and the increase in natural bunch length with energy. At this medium energy (the energy range of the ANKA storage ring is 0.5 to 2.5 GeV) steady state emission of coherent synchrotron radiation is observed by the ANKA-IR beamline below the threshold current defined by the micro-bunching instability. At lower beam energies where the natural bunch length is significantly shorter, bursts of coherent synchrotron radiation are detected in spite of the longitudinal oscillation. The far infrared spectrum is sensitive to the dynamics of the charge distribution generating the radiation. Measurements of the frequency spectrum of the infrared detector signal add information on bunch dynamics. This paper gives an overview of the studies performed at the ANKA storage ring.
THPCH040	Linac Focusing and Beam Break Up for 4GLS	2871
	E. Wooldridge, B.D. Muratori CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire
	As part of the design for 4GLS the linac focusing and its effect on the beam break up (BBU) threshold have been studied. The choice of graded gradient focusing scheme is discussed and initial models of the focusing, using a triplet of quadrupoles between each of the modules within the linac, are presented. The quadrupoles were set-up in a defocusing - focusing - defocusing format with strengths of -1/2k, k, -1/2k. Using these models the BBU threshold was computed using available codes assuming a 9-cell TESLA cavity within the linac and a 7-cell design with HOM dampers. A sweep of the magnet strength with respect to the BBU threshold showed that there is an optimum setting.
THPCH041	Alternate Cavity Designs to Reduce BBU	2874
	E. Wooldridge CCLRC/DL/ASTeC, Daresbury, Warrington, Cheshire
	An investigation was carried out on alternate cavity designs to decrease the effect of the higher order modes (HOMs) whilst maintaining the cavity accelerating gradient. The cavities were modelled in Microwave Studio and the number of cells per cavity and the number of cells per unit length were examined. HOM data from these models was used in beam break up (BBU) codes to calculate the threshold and the results are presented here. The cells of each cavity were slightly deformed alternately in the x and y planes so that the fundamental frequency of the cavity remained unperturbed, whilst minimising the HOM coupling between consecutive cells. Other patterns, such as alternating each deformed cell by several degrees, were also investigated. The data from these e-m models was also used in BBU calculations and their thresholds calculated.
THPCH042	Numerical Estimations of Wakefields and Impedances for Diamond Collimators	2877
	S.A. Pande, R. Bartolini, R. T. Fielder, M. Jensen Diamond, Oxfordshire
	The storage ring of the Diamond light source will use two collimators, one in horizontal and one in the vertical planes in the injection straight to protect the IDs from the injection and Touschek losses. These collimators, in the form of tapered metallic intrusions in to the vacuum chamber, will generate considerable wake fields and will contribute to the overall machine impedance. In this paper we present the results of ABCI and MAFIA numerical simulations to estimate these effects.
THPCH043	Jitter Studies for the FERMI@ELETTRA Linac	2880
	P. Craievich, S. Di Mitri ELETTRA, Basovizza, Trieste A. Zholents LBNL, Berkeley, California
	The FEL project FERMI@ELETTRA* will use the existing linac upgraded to 1.2GeV to produce photon pulses in the wavelength range between 100-10 nm by means of harmonic generation in a seeded scheme. FEL operations foresee stringent requirements for the stability of the global linac output parameters, such as the electron bunch arrival time, peak current, average energy and the slice electron bunch parameters, such as the slice peak current and slice average energy. In order to understand the sensitivity of these parameters to jitters of various error sources along the linac an elaborate study using tracking codes has been performed. As a result, we created a tolerance budget to be used as guidance in the design of the linac upgrade. In this paper we give a detailed description of the applied procedures and present the obtained results. *C. Bocchetta et al. "FERMI@ELETTRA - Conceptual Design for a Seeded Harmonic Cascade FEL for EUV and Soft X-rays", this conference.
THPCH044	Beam Break-up Instability in the FERMI@ELETTRA Linac	2883
	S. Di Mitri, P. Craievich ELETTRA, Basovizza, Trieste
	The beam break-up instability is studied for the 1.2 GeV linac of FERMI @ ELETTRA FEL. This instability is driven by transverse wake fields acting on an electron beam travelling off-axis in the accelerating structures due to the launching errors in positions, angles, energy and misalignment of various lattice elements. Two operational scenarios are considered: one with a relatively long electron bunch of 1.5 ps and a moderate peak current of 500 A and one with a shorter bunch of 0.7 ps and a higher peak current of 800 A. Attention is given to the correction of the "banana" shape of the electron bunch caused by the instability. The simulation results are compared with the analytical predictions. C. Bocchetta et al. “FERMI@Elettra – Conceptual Design for a Seeded Harmonic Cascade FEL for EUV and Soft X-rays”, this conference.
THPCH045	Transverse Head-tail Modes Elimination with Negative Chromaticity and the Transverse Multi-bunch Feedback System at ELETTRA	2886
	E. Karantzoulis, M. Lonza ELETTRA, Basovizza, Trieste
	The rigid dipole head-tail mode threshold at ELETTRA is by now quite low and increasing positively the chromaticity does not bring a much overall advantage in the machine performance. Using the bunch-by-bunch transverse feedback (TMFB), a threshold increase has been observed until the onset of the higher modes, which being bunch shape modes cannot be detected and therefore eliminated by the feedback system. To overcome this problem the machine has been set to a small but negative chromaticity. In this case the m=0 mode is unstable with a very low (<1 mA/bunch) threshold but the higher modes should be stable, especially when the main source of the transverse impedance comes from the resistive wall as in our case. Indeed when activating the TMFB no onset of any modes was observed within reasonable current limits (15 mA/bunch) that we plan to further investigate. In the paper after a theoretical discussion on the role of chromaticity and various types of impedances in the head-tail onset mechanism, the experimental results are presented and discussed.
THPCH047	Maps for Electron Clouds: Application to LHC	2889
	T. Demma, S. Petracca U. Sannio, Benevento F. Ruggiero, G. Rumolo, F. Zimmermann CERN, Geneva
	Electron Cloud studies performed so far were based on very heavy computer simulations taking into account photoelectron production, secondary electron emission, electron dynamics, and space charge effects providing a very detailed description of the electron cloud evolution. In a recent paper* it has been shown that, for the typical parameters of RHIC, the bunch-to-bunch evolution of the electron cloud density can be represented by a cubic map. Simulations based on this map formalism are orders of magnitude faster than those based on usual codes. In this communication we show that the map formalism is also reliable in the range of typical LHC parameters, and discuss the dependence of the polynomial map coefficients on the physical parameters affecting the electron cloud (SEY, chamber dimensions, bunch spacing, bunch charge, etc.). *U. Iriso and S. Peggs. "Maps for Electron Clouds", Phys. Rev. ST-AB 8, 024403, 2005.
THPCH048	Transverse Coupled Bunch Instability Driven by 792-MHz Cavity HOM in NewSUBARU Electron Storage Ring	2892
	S.H. Hisao, A. Ando, S. Hashimoto, T. Matsubara, Y. Miyahara, Y. Shoji NewSUBARU/SPring-8, Laboratory of Advanced Science and Technology for Industry (LASTI), Hyogo
	The 792-MHz HOM of the RF cavity can drive horizontal coupled bunch instability in the NewSUBARU electron storage ring. This instability is now avoided by tuning the HOM frequency with an additional tuner (HOM tuner). Detailed characteristics of this instability were investigated by changing the HOM frequency, betatron tune, chromaticity and magnitude of the stored current at the energy of 1 GeV. The experiments were performed with 6-bunch equi-space filling to clarify the mode number. Bunch oscillations show saw-tooth patterns when the stored current is not so large. The measured results are compared with an analytical calculation using a rigid bunch model and Sacherer's formalism. The fundamental aspects can be well explained by the calculation, but there exist many problems that cannot be explained by the rigid bunch model.
THPCH049	Simulation Study of Transverse Coupled-bunch Instabilities due to Electron Cloud in KEKB LER	2895
	X.W. Dong, S.-I. Kurokawa, K. Ohmi KEK, Ibaraki
	In this paper we report simulation results on the transverse coupled-bunch instabilities (TCBI) due to electron cloud at the KEKB Low Energy Ring (LER). The formation of electron cloud and related TCBI is investigated based on realistic solenoid field model. Studies on electron cloud in Quadrupole which could induce TCBI are also presented in this paper.
THPCH050	Further Studies on Betatron Sidebands due to Electron Clouds	2898
	J.W. Flanagan, H. Fukuma, Y. Funakoshi, S. Hiramatsu, T. Ieiri, H. Ikeda, K. Ohmi, K. Oide, M. Tobiyama KEK, Ibaraki
	We have observed vertical betatron sidebands in the transverse beam spectra of positron bunches at the KEKB LER which are associated with the presence of electron clouds in single-beam studies, and which are also associated with a loss of luminosity when the KEKB beams are in collision. The sidebands may be signals of a fast head-tail instability due to short-range wakes within the electron cloud, providing a diagnostic for exploring the mechanism for transverse beam blow-up due to electron clouds. We report here on further studies on the behavior of the sidebands under varying beam conditions, including varying initial beam size below the beam blow-up threshold, chromaticity, RF voltage and fill pattern. J. W. Flanagan et al. PRL 94, 054801 (2005).**J. W. Flanagan et al. Proc. PAC05, p. 680 (2005).
THPCH051	The Effect of the Solenoid Field in Quadrupole Magnets on the Electron Cloud Instability in the KEKB LER	2901
	H. Fukuma, J.W. Flanagan, T. Kawamoto, T. Morimoto, K. Oide, M. Tobiyama KEK, Ibaraki F. Zimmermann CERN, Geneva
	The electron cloud instability which causes vertical beam blowup in the KEKB LER is largely suppressed by applying a weak solenoid field to vacuum chambers in the drift region. However the blowup is still observed when the bunch spacing is reduced to 3.27 rf buckets or shorter. A question is where the remaining electron clouds are. To investigate the electron clouds in the quadrupole magnets, solenoids made of flat cables were developed and installed in 88 quadrupole magnets. The field strength of the solenoid is 17 Gauss. The effect of the solenoid field on the blowup is now under beam study. So far no clear effect of the solenoids on the luminosity and the sideband accompanied by the blowup is found. We report on the solenoid system, the results of the experiments and comparison of the experimental results with simulations.
THPCH052	Dependence of Transverse Instabilities on Amplitude Dependent Tune Shifts	2904
	T. Miyajima, K. Harada, Y. Kobayashi, S. Nagahashi KEK, Ibaraki
	In the Photon Factory electron storage ring, transverse instabilities have been observed in multi-bunch operation mode. The instabilities can be suppressed by amplitude dependent tune shifts, which are induced by the sextupole, octupole and higher order magnetic field. Since four octupole magnets have been installed in the PF ring, we can control the tune shifts, which is caused by the octupole magnetic field, independently of chromaticities, which is caused by sextupole magnetic field. In order to research the suppression mechanism of the instabilities, we measured the dependence of the instabilities on the tune shifts, which are induced by the octupole field. The threshold of the tune shifts, which suppress the instabilities, were observed in the measurement, and it depended on the filling pattern of the bunch train and the beam current per bunch. In addition, we will present the results of the measurement before and after the reconstruction for the straight-sections upgrade at the PF ring, which was carried out in 2005.
THPCH053	Numerical and Experimental Study of Cooling-stacking Injection in HIMAC Synchrotron	2907
	E. Syresin JINR, Dubna, Moscow Region K. Noda NIRS, Chiba-shi S. Shibuya AEC, Chiba T. Uesugi KEK, Ibaraki
	The cooling-stacking injection at the HIMAC synchrotron was used to increase the intensity of Ar¹⁸⁺ ion beam. The beam stacking was realized in a horizontal free phase-space, which was created by the HIMAC electron cooler. The stack intensity of (1.5~2.5)·10⁹ ppp was accumulated at an injection intensity of (0.3~1.0)e9. The stack intensity was limitted by the ion lifetime. A peculiarity of present cooling-stacking experiments is related to lifetime difference by a factor of 2~3 of the stack and injected ions. The lifetime of stack ions is determined by vacuum pressure. The new injected ions were slowly lost at multiple scattering on residual gas atoms at diffusion heating in the vertical direction caused by the acceptance of 30pi-mm-mrad and a reduction of cooling force at large betatron amplitudes. The results of numerical simulations and experimental study of cooling-stacking injection on the HIMAC synchrotron are presented.
THPCH054	SIMPSONS with Wake Field Effects	2910
	Y. Shobuda, F. Noda JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken Y.H. Chin, K. Takata, T. Toyama KEK, Ibaraki S. Machida CCLRC/RAL/ASTeC, Chilton, Didcot, Oxon
	Simpsons, which is originally developed by S. Machida, is the program which calculates the space charge effect to the beam in the ring. The wake field effect to the beam is also installed in this program, because the emittance growth not only due to the space charge effect, but also due to the wake field effect is the important issue. The results of the simulation in J-PARC case are also represented.
THPCH057	The Fast Vertical Single-bunch Instability after Injection into the CERN Super Proton Synchrotron	2913
	E. Métral, G. Arduini, T. Bohl, H. Burkhardt, G. Rumolo CERN, Geneva B. Spataro INFN/LNF, Frascati (Roma)
	Since 2003, high-intensity single-bunch proton beams with low longitudinal emittance have been affected by heavy losses after less than one synchrotron period after injection. The effects of the resonance frequency of the responsible impedance, longitudinal emittance and chromaticity on the intensity threshold were already discussed in detail in 2004, comparing analytical predictions with simulation results. In this paper the evolution of the instability between injection and the time of beam loss is our main concern. Measurements are compared with HEADTAIL simulations. A travelling-wave pattern propagating along the bunch, which is the signature of a Beam Break-Up or Transverse Mode Coupling Instability (TMCI), is clearly identified. The oscillating frequency, near ~1 GHz, is in good agreement with the usual broad-band impedance model deduced from beam-based measurements like the head-tail growth rate vs. chromaticity.
THPCH058	Simulation Study on the Beneficial Effect of Linear Coupling for the Transverse Mode Coupling Instability in the CERN Super Proton Synchrotron	2916
	E. Métral, G. Rumolo CERN, Geneva
	The intensity threshold of the transverse mode coupling instability in a flat vertical chamber, as in the CERN Super Proton Synchrotron, is much higher in the horizontal plane than in the vertical one. This asymmetry between the transverse planes led us to the idea that linear coupling from skew quadrupoles could be used to increase the intensity threshold. This technique is already applied, for instance, in the CERN Proton Synchrotron, where a slow head-tail horizontal instability due to the resistive-wall impedance is stabilized by linear coupling only, i.e. with neither octupoles nor feedbacks. This paper presents the results of the study of the effect of linear coupling on the transverse mode coupling instability, using the HEADTAIL simulation code.
THPCH059	Kicker Impedance Measurements for the Future Multi-turn Extraction of the CERN Proton Synchrotron	2919
	E. Métral, F. Caspers, M. Giovannozzi, A. Grudiev, T. Kroyer, L. Sermeus CERN, Geneva
	In the context of the novel multi-turn extraction, where charged particles are trapped into stable islands in transverse phase space, the ejection of five beamlets will be performed by means of a set of three new kickers. Before installing them into the machine, a measurement campaign has been launched to evaluate the impedance of such devices. Two measurement techniques were used to try to disentangle the driving and detuning impedances. The first consists in measuring the longitudinal impedance for different transverse offsets using a single displaced wire. The sum of the transverse driving and detuning impedances is then deduced applying Panofsky-Wenzel theorem. The second uses two wires excited in opposite phase and yields the driving transverse impedance only. Finally, the consequences on the beam dynamics are also analyzed.
THPCH060	Simulation Study on the Energy Dependence of the TMCI Threshold in the CERN-SPS	2922
	G. Rumolo, E. Métral, E.N. Shaposhnikova CERN, Geneva
	This paper concentrates on theoretical studies of Transverse Mode Coupling Instability at the SPS. It shows the expected thresholds based on a HEADTAIL tracking model and on impedance values estimated from previous measurements. First, the effect of space charge is addressed as an important ingredient at the low energies. Subsequently, the change of TMCI threshold possibly induced by a higher injection energy into the SPS (plausible according to the upgrade studies) is investigated and a scaling law with energy is derived.
THPCH061	Tune Shift Induced by Nonlinear Resistive Wall Wake Field of Flat Collimator	2925
	F. Zimmermann, G. Arduini, R.W. Assmann, H. Burkhardt, F. Caspers, M. Gasior, O.R. Jones, T. Kroyer, E. Métral, S. Redaelli, G. Robert-Demolaize, F. Roncarolo, G. Rumolo, R.J. Steinhagen, J. Wenninger CERN, Geneva
	We present formulae for the coherent and incoherent tune shifts due to the nonlinear resistive wall wake field for a single beam traveling between two parallel plates. In particular, we demonstrate that the nonlinear terms of the resistive wall wake field become important if the gap between the plates is comparable to the transverse rms beam size. We also compare the theoretically predicted tune shift as a function of gap size with measurements for an LHC prototype graphite collimator in the CERN SPS and with simulations.
THPCH062	Collective Effects in the Storage Ring of Taiwan Photon Source	2928
	P.J. Chou, C.H. Kuo, C.-C. Kuo, M.-H. Wang NSRRC, Hsinchu
	A new 3- 3.3 GeV synchrotron light source is proposed and named the Taiwan Photon Source (TPS). The TPS design has a natural horizontal emittance less than 2 nm-rad and low emittance coupling, which results in small beam size. The nominal bunch length in TPS storage ring is much shorter compared to the existing Taiwan Light Source, that makes the issue of parasitic heating more significant. Several small-gap insertion devices are planned to provide extremely bright x-ray photon beam. Those design features have impacts on collective beam instabilities. A preliminary study of collective effects in the TPS storage ring is presented.
THPCH064	Comparison of Three CSR Radiation Powers for Particle Bunches and Line Charges	2931
	K.A. Heinemann, G. Bassi, J.A. Ellison UNM, Albuquerque, New Mexico
	We are studying coherent synchrotron radiation (CSR) from arbitrary planar orbits as discussed in another abstract we submitted to EPAC06. It is important to have one-dimensional approximations. Here we report on work constructing and validating such approximations. As part of our work two well known papers by Saldin, Schneidmiller and Yurkov (SSY* are considered which deal with the CSR via a one-dimensional approximation whereby the electron bunch is modelled by a line density. Their one-dimensional approach is important because it is used in various CSR codes and since it serves to some extent as a role model for higher-dimensional models. The present report deals with some general aspects of the work of SSY. In particular, care is taken of the renormalization procedure and of the statistical description in terms of the line density. SSY use a renormalized retarded field whereas the present work uses the radiation field which is defined as half the difference of the retarded and advanced fields. The radiation field came into prominence when Dirac** introduced the Lorentz-Dirac equation. E. L. Saldin, et al. Nucl. Instr. Meth. Phys. Res. A 398, 373 (1997) and 417, 158 (1998).*P.A.M. Dirac, Proc. Roy. Soc. (London) A167, 148 (1938).
THPCH065	Suppression of Transverse Instability by a Digital Damper	2934
	A.V. Burov, V.A. Lebedev Fermilab, Batavia, Illinois
	When a beam phase space density increases, it makes its motion intrinsically unstable. To suppress the instabilities, dampers are required. With a progress of digital technology, digital dampers are getting to be more and more preferable, compared with analog ones. Conversion of an analog signal into digital one is described by a linear operator with explicit time dependence. Thus, the analog-digital converter (ADC) does not preserve a signal frequency. Instead, a monochromatic input signal is transformed into a mixture of all possible frequencies, combining the input one with multiples of the sampling frequency. Stability analysis has to include a cross-talk between all these combined frequencies. In this paper, we are analyzing a problem of stability for beam transverse microwave oscillations in a presence of digital damper; the impedance and the space charge are taken into account. The developed formalism is applied for antiproton beam in the Recycler Ring at Fermilab.
THPCH066	Transient Beam Loading in the DIAMOND Storage Ring	2937
	S. De Santis, J.M. Byrd LBNL, Berkeley, California R. Bartolini Diamond, Oxfordshire
	Harmonic cavity systems have been installed on several 3rd generation light sources to lengthen the bunches and increase the Touschek lifetime. Apart from this beneficial effect, harmonic cavities are known to increase the transient beam loading in high-current machines, due to the presence of gaps in the fill pattern. The amplitude of this effect, which is substantially larger than that caused by the main RF system, can in turn produce considerable variations in bunch length and phase along the train, which result in a significant reduction of the lifetime increase. We have developed a tracking simulation, which we have applied to the analysis of the beam loading transients in Diamond, for the case of passive superconducting harmonic cavities. The influence of beam current, gap amplitude and harmonic cavity tuning on the final lifetime have been studied, as well as the effects of higher-order modes.
THPCH067	Coherent Synchrotron Radiation Studies at the Accelerator Test Facility	2940
	S. De Santis, J.M. Byrd LBNL, Berkeley, California A. Aryshev, T. Naito, J. Urakawa KEK, Ibaraki M.C. Ross SLAC, Menlo Park, California
	Coherent Synchrotron Radiation (CSR) has been the object of recent experiments and is a topic of great importance for several accelerator currently in their design phase (LCLS, ILC, CIRCE). We present the results of several experimental sessions performed at the Advanced Test Facility - KEK (ATF). An infrared bolometer was used to detect the emitted infrared radiation in the 1-0.05 mm wavelength range as a function of several beam parameters (beam current, RF power, extraction timing, photoinjector laser phase). The beam energy spread was also recorded. We found that the mismatch between injected and equilibrium beam is the source of the coherent signal detected concurrently with the bunch injection.
THPCH069	BBU Calculations for Beam Stability Experiments on DARHT-2	2943
	Y. Tang ATK-MR, Albuquerque, New Mexico K.C.D. Chan, C. Ekdahl LANL, Los Alamos, New Mexico T.P. Hughes Voss Scientific, Albuquerque, New Mexico
	The DARHT-2 (Dual-Axis Radiographic Hydrodynamics Test) facility is expected to produce a 2-kA, 20-MeV, 2-microsecond flattop electron beam pulse. Normal operation requires that the beam exit the accelerator with a normalized transverse emittance of less than 0.15-cm-rad. The beam break up (BBU) instability is a potentially serious effect for a high current linear accelerator. It arises from the interaction between the beam and the cavity modes of the accelerating cells. In support of the beam stability experiments, simulations of BBU for DARHT-2 using the Lamda code have been carried out. The simulations used experimental data for the transverse impedance of the cells. Lamda was benchmarked against results calculated with the LLNL code BREAKUP. For nominal transport parameters, simulation results showed that the BBU growth was not significant in changing the beam spot. For a magnetic field reduced by a factor of 5, BBU growth was over a factor of 100, and the image displacement effect was significant.
THPCH070	Long-pulse Beam Stability in the DARHT-II Linear-induction Accelerator	2946
	C. Ekdahl, E.O. Abeyta, P.A. Aragon, R. Archuleta, R. Bartsch, K.C.D. Chan, D. Dalmas, S. Eversole, R.J. Gallegos, J. Harrison, E. Jacquez, J. Johnson, B.T. McCuistian, N. Montoya, S. Nath, D. Oro, L.J. Rowton, M. Sanchez, R.D. Scarpetti, M. Schauer LANL, Los Alamos, New Mexico H. Bender, W. Broste, C. Carlson, D. Frayer, D. Johnson, A. Tipton, C.-Y. Tom Bechtel Nevada, Los Alamos, New Mexico R.J. Briggs SAIC, Alamo, California T.P. Hughes, C. Mostrom, Y. Tang ATK-MR, Albuquerque, New Mexico M.E. Schulze GA, San Diego, California
	The beam breakup instability has long been a problem for linear induction accelerators (LIAs). Although it is predicted to saturate in the strong focus regime relevant to LIAs most, if not all, LIAs have had pulse-widths too short to observe this effect. We recently completed BBU experiments on a 1.2 kA, 6.7-MeV configuration of the DARHT-II LIA having a 1600-ns pulse length much longer than the saturation time. The saturated growth observed in these experiments when we reduced the magnetic guide-field strength was in agreement with theory. We used these results to deduce that BBU growth will be insignificant in the final 2-kA, 17-MeV DARHT-II configuration with the tunes that will be used. Another problematic instability for long-pulse LIAs such as DARHT-II is the ion-hose. We also performed experiments with the 6.7-MeV long-pulse configuration of DARHT-II in which we deliberately induced ion-hose by raising the background pressure far above its normal value. The results of these experiments were used to show that ion-hose will not be a problem for to the final DARHT-II configuration.
THPCH071	Coupling Impedances of Small Discontinuities for Non-ultrarelativistic Beams	2949
	S.S. Kurennoy LANL, Los Alamos, New Mexico
	The beam coupling impedances of small discontinuities of an accelerator vacuum chamber have been calculated (e.g., * and references therein) for ultrarelativistic beams using Bethe's diffraction theory. Here we extend the results for an arbitrary beam velocity. The vacuum chamber is assumed to have an arbitrary, but fixed, cross section. The longitudinal and transverse coupling impedances are derived in terms of series over cross-section eigenfunctions, while the discontinuity shape enters via its polarizabilities. Simple explicit formulas for the circular and rectangular cross sections are presented. The impedance dependence on the beam velocity exhibits some unusual features. For example, the reactive impedance, which dominates in the ultrarelativistic limit, can vanish at a certain beam velocity, or its magnitude can exceed the ultrarelativistic value many times. *S. S. Kurennoy et al. Phys. Rev. ·10⁵² (1995) 4354.
THPCH072	Wakefields in the LCLS Undulator Transitions	2952
	K.L.F. Bane SLAC, Menlo Park, California I. Zagorodnov DESY, Hamburg
	We have studied longitudinal wakefields of very short bunches in non-cylindrically symmetric (3D) vacuum chamber transitions using analytical models and the computer program ECHO. The wake (for pairs of well-separated, non-smooth transitions) invariably is resistive, with its shape proportional to the bunch distribution. For the example of an elliptical collimator in a round beam pipe we have demonstrated that—as in the cylindrically symmetric (2D) case—the wake can be obtained from the static primary field of the beam alone. We have obtained the wakes of the LCLS rectangular-to-round transitions using indirect (numerical) field integration combined with a primary beam field calculation. For the LCLS 1 nC bunch charge configuration we find that the total variation in wake-induced energy change is small (0.03% in the core of the beam, 0.15% in the horns of the distribution) compared to that due to the resistive wall wakes of the undulator beam pipe (0.6%).
THPCH073	Reflectivity Measurements for Copper and Aluminum in the Far Infrared and the Resistive Wall Impedance in the LCLS Undulator	2955
	K.L.F. Bane, G.V. Stupakov SLAC, Menlo Park, California J. Tu City College of The City University of New York, New York
	Reflectivity measurements in the far infrared, performed on aluminum and copper samples, are presented and analyzed. Over a frequency range of interest for the LCLS bunch, the data is fit to the free-electron model, and to one including the anomalous skin effect. The models fit well, yielding parameters dc conductivity and relaxation times that are within 30-40\% of expected values. We show that the induced energy in the LCLS undulator region is relative insensitive to variations on this order, and thus we can have confidence that the wake effect will be close to what is expected.
THPCH075	Simulation of the Electron Cloud for Various Configurations of a Damping Ring for the ILC	2958
	M.T.F. Pivi, T.O. Raubenheimer, L. Wang SLAC, Menlo Park, California K. Ohmi KEK, Ibaraki R. Wanzenberg DESY, Hamburg A. Wolski Liverpool University, Science Faculty, Liverpool F. Zimmermann CERN, Geneva
	In the beam pipe of the Damping Ring (DR) of the International Linear Collider (ILC), an electron cloud may be first produced by photoelectrons and ionization of residual gasses and then increased by the secondary emission process. This paper reports about the work that has been done by the electron cloud assessment international task force group for the recommendation of the ILC Damping Rings baseline design, made in November 2005. We have carefully estimated the secondary electron yield (SEY) threshold for electron cloud build-up and estimated the related single^- and coupled-bunch instabilities that can be caused by the presence of electron cloud as a function of beam current and surface properties, for a variety of optics designs. The result of these studies was an important consideration in the choice of a 6-km design for the ILC damping rings. On the basis of the theoretical and experimental work, the baseline configuration specifies a pair of damping rings for the positron beam to mitigate the effects of the electron cloud.
THPCH076	Resistive Wall Wake Effect of a Grooved Vacuum Chamber	2961
	G.V. Stupakov, K.L.F. Bane SLAC, Menlo Park, California
	To suppress the emission of secondary electrons in accelerators with positively charged beams (ions or positrons) it has been proposed to use a vacuum chamber that is longitudinally grooved (or, equivalently, one can say finned)/. One consequence of having such a chamber in an accelerator is an increased resistive wall impedance. In this paper, we calculate the resistive wall impedance of one such finned chamber, planned to be used in experimental studies of secondary emission suppression at SLAC. For rectangular fins, we use an analytical method based on a conformal mapping approach; we compare the results with a numerical solution of the field equation. We also numerically compute the impedance for rounded fins (as will be used in the SLAC experiment) and analyse how the impedance depends on geometric properties of the fins. A. A. Krasnov. Vaccum, vol. 73, p. 195, (2004).**G. Stupakov and M. Pivi. Preprint SLAC-TN-04-045, (2004).
THPCH077	Resistive-wall Instability in the Damping Rings of the ILC	2964
	L. Wang, K.L.F. Bane, T.O. Raubenheimer, M.C. Ross SLAC, Menlo Park, California
	In the damping rings of the International Linear Collider (ILC), the resistive-wall instability is one of the dominant transverse instabilities. This instability directly influences the choice of material and aperture of the vacuum pipe, and the parameters of the transverse feedback system. This paper investigates the resistive-wall instabilities in an ILC damping ring under various conditions of beam pipe material, aperture, and fill pattern.
THPCH080	Transverse Impedance of Small-gap Undulators for NSLS-II	2970
	A. Blednykh, S. Krinsky, B. Podobedov, J.-M. Wang BNL, Upton, Long Island, New York
	We discuss the transverse impedance resulting from the use of small-gap undulators in the proposed NSLS-II storage ring. For superconducting undulators, the impedance arises due to the tapered elliptical vacuum chamber. For in-vacuum permanent magnet devices, the impedance results from a more complex geometry. We consider both cases and report results obtained using the electromagnetic simulation program GdfidL.
THPCH081	Transverse Impedance of Elliptical Cross-section Tapers	2973
	B. Podobedov, S. Krinsky BNL, Upton, Long Island, New York
	We investigate the transverse impedance of elliptical cross-section tapers. Analytical estimates for the dipole and quadrupolar components of the impedance at low frequency are obtained by extending a perturbation approach introduced by Stupakov. The perturbation theory results are compared to EM code GdfidL and are found to be in excellent agreement.