| Paper | Title | Page |
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| MOP28 | A Study of Higher-Band Dipole Wakefields in X-Band Accelerating Structures for the G/NLC | 99 |
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The X-band linacs for the GLC/NLC (Global Linear Collider/Next Linear Collider) have evolved from the DDS (Damped Detuned Structure) series [1,2]. The present series of accelerating structures are each 60 cm in length and incorporate both damping and detuning of the dipole modes which comprise the wakefield. In order to adequately damp the wakefield the dipole frequencies of adjacent structures are interleaved. The properties of the first dipole band have been extensively studied. However, limited analysis has been done on the higher order dipole bands. Here, we calculate the contribution of the higher order bands of the interleaved structures to the wakefield using a mode matching computer code [3]. Beam dynamics issues are also studied by tracking the beam through the complete linac using the particle beam tracking code LIAR [4].
[1] R.M Jones et al,1996,Proc. EPAC96 (also SLAC-PUB-7187) [2] J.W. Wang et al, 2000, Proc. LINAC2000 (also SLAC-PUB-8583) [3] V.A. Dolgashev, Ph.D. thesis, Budker INP, Novosibirsk, 2002.[4] R. Assman et al, LIAR, SLAC-PUB AP-103 |
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| MOP40 | A Study Of Coupler-Trapped Modes In X-Band Linacs for the GLC/NLC | 129 |
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| Each of the X-band accelerating structures for the GLC/NLC consist of 55 cells which accelerate a train of charged particles. The cells are carefully designed to ensure that the transverse wakefield left behind each bunch does not disrupt the trailing bunches. However, unless attention is paid to the design of the fundamental mode coupler, then a dipole mode is trapped in the region of the coupler and cells. This mode can give rise to severe emittance dilution if care is not taken to avoid a region of resonant growth in the emittance. Here, we present results on HFSS simulations, cold test experimental measurements and beam dynamics simulations arising as a consequence of the mode trapped in the coupler. The region in which the trapped mode has little influence on the beam is delineated. | ||
| MOP41 | Emittance-Imposed Alignment and Frequency Tolerances for the TESLA Linear Collider | 132 |
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| One option in building a future 500 GeV c.m. collider is to use superconducting 1.3 GHz 9-cell cavities. Wakefields excited by the bunch train in the TESLA linac can resonantly drive the beam into unstable operation such that a BBU (Beam Break Up) mode results or at the very least significant emittance dilution occurs. The largest kick factors (proportional to the transverse fields which transversely kick the beam off axis) are found in the first three dipole bands and hence multi-bunch emittance growth is mainly determined from these bands. These higher order dipole modes are damped by carefully orientating higher order mode couplers at the downstream end of the cavities. We investigate the dilution in the emittance of a beam injected with an initial offset from the axis of the cavities. The dependence of beam emittance on systematic errors in the cell frequencies is investigated. We also vary the bunch spacing in order to simulate a systematic frequency error. While scanning the bunch spacing over a wide range, the emittance presents sharp peaks since only few modes contribute effectively to emittance growth. The locations of these peaks sets the frequency tolerances on the structures. | ||
| MOP42 | Linac Alignment and Frequency Tolerances from the Perspective of Contained Emittances for the G/NLC | 135 |
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We maintain the stable progress of a beam consisting of a train of bunched charges, by a careful design of the geometry of the structures [1]. In practice, the next generation of linear colliders will consist of several tens of thousands of X-band accelerating structures and this will entail inevitable errors in the dimensions and alignments of cells -and groups thereof. These errors result in a dilution of the beam emittance and consequently a loss in overall luminosity of the collider. For this reason it is important to understand the alignment tolerances and frequency tolerances that are imposed for a specified emittance budget. Here we specify an emittance dilution of no more than 10% of the initial value and we track the progress of the beam down the linac whilst accelerating structures (and sub-groups thereof) are misaligned in a random manner and at the same time random frequency are incorporated with structures. This results in tolerances in both frequency errors and sets of alignment errors to be imposed on the structures for a specified emittance dilution.
[1] R.M. Jones, 1997, SLAC NLC-Note 24. |
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| MOP64 | Wire Measurement of Impedance of an X-Band Accelerating Structure | 165 |
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Several tens of thousands of accelerator structures will be needed for the next generation of linear collders known as the GLC/NLC (Global Linear Collider/Next Linear Collider). To prevent the beam being driven into a disruptive BBU (Beam Break Up) mode or at the very least, the emittance being signifcantly diluted, it is important to damp down the wakefield left by driving bunches to a manageable level. Manufacturing errors and errors in design need to be measurable and compared with predictions. We develop a circuit model of wire-loaded X-band accelerator structures. This enables the wakefield (the inverse transform of the beam impedance) to be readily computed and compared with the wire measurement. We apply this circuit model to the latest series of accelerating for the GLC/NLC. This circuit model is based upon the single-cell model developed in [1] extended here to complete, multi-cell structures.
[1] R.M. Jones et al, 2003, Proc. PAC2003 (also SLAC-PUB 9871) |
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| THP33 | Progress toward NLC/GLC Prototype Accelerator Structures | 675 |
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| The accelerator structure groups for NLC (Next Linear Collider) and GLC (Global Linear Colliders) have successfully collaborated on the research and development of a major series of advanced accelerator structures based on room-temperature technology at X-band frequency. The progress in design, simulation, microwave measurement and high gradient tests are summarized in this paper. The recent effort in design and fabrication of the accelerator structure prototype for the main linac is presented in detail including HOM (High Order Mode) suppression and couplers, fundamental mode couplers, optimized accelerator cavities as well as plans for future structures. We emphasize techniques to reduce the field on the surface of the copper structures (in order to achieve high accelerating gradients), limit the dipole wakefields (to relax alignment tolerance and prevent a beam break up instability) and improve shunt impedance (to reduce the RF power required). |