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| MOP001 | Pressurized Hydrogen-Filled Linacs for Muon Cooling | emittance, linac, radiation, scattering | 28 | ||
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New techniques for muon ionization cooling require low-Z energy absorber, strong magnetic fields for focusing and emittance exchange, and high gradient RF cavities to replace the energy lost in the absorber. RF cavities pressurized with hydrogen gas are being developed to provide the most muon beam cooling possible in the short lifetime of the muon. We report the status of the cavity development, including the breakdown suppression due to the gas and new results showing that pressurized cavities show no degradation of performance in strong magnetic fields. We also comment on the development of the designs of the associated muon cooling linacs.
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| MOP003 | A Shared Superconducting Linac for Protons and Muons | linac, proton, factory, target | 34 | ||
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A future Fermilab proton driver* based on TESLA superconducting linac modules can provide protons to produce the muons and also accelerate the muons to be used for a neutrino factory or muon collider. Recent advances in muon cooling** imply muon emittances that are compatible with the 1300 MHz accelerating structures that are the basis for the ILC design. In the example discussed here, H- ions are accelerated to 8 GeV in the superconducting linac, then stripped, stored and bunched in a ring while the linac cavities are rephased for muon acceleration. Then the protons are extracted from the ring to produce pions and muons which are cooled in about six hundred meters, accelerated to a few GeV and injected into the linac at the point for acceleration to add 7 GeV. By recirculating the muons in the constant frequency section of such a proton driver linac, even higher energies can be achieved quickly so that losses from muon decay are minimized. By adding additional refrigeration and RF power, the repetition rate of the linac can be increased to make large increases in the average flux of a neutrino factory and the average luminosity of a muon collider.
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*G. W. Foster and J. A. MacLachlan, Proceedings of LINAC 2002, Gyeongju, Korea |
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| TUP033 | Performances of High-Purity Niobium Cavities with Different Grain Sizes | vacuum, pick-up, superconductivity, linear-collider | 318 | ||
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Grain boundaries have for some time been suspected of influencing the performance of rf cavities made from high purity niobium by limiting the temperature dependent BCS surface resistance to a residual resistance because of impurity segregation and by causing field limitations due to flux penetration. We have carried out a comparative study of the rf behavior of 2.2 GHz TM010 cavities of identical shape, fabricated from single crystal niobium, niobium of grain sizes of the order of several cm2 and standard poly-crystalline material. This contribution reports about the results of the measurements of the temperature dependence of the surface resistance Rs(T) and the Q0 vs Eacc behavior at 2 K. From the analysis of the Rs(T) data at low rf fields material parameters such as gap value, mean free path and residual resistance could be extracted. The dependence of the Q-value on rf field was analyzed with respect to the medium field Q-slope, “Q-drop” at high fields and the “quench” fields. The best performance resulted in a breakdown field of ~ 165 mT, corresponding to an accelerating gradient of Eacc ~ 45 MV/m.
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| TH2003 | Recent Developments in Pulsed High-Power Systems | pulsed-power, kicker, klystron, linear-collider | 541 | ||
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Pulsed power systems are inherent in any high power accelerator system. Applications include, among others, modulators for powering high power klystrons, pulsed power systems to drive linear induction accelerating cells, kicker magnet drivers for storage rings, and a wide variety of beam deflection and pulsed focusing systems. As with many enabling technologies, component limitations and materials properties dominate the engineering tradeoffs that must be made during the system design. An overview of the state-of-the-art in major components of pulsed power systems will be presented. An examination of how those components are being integrated into linac systems will also be performed and an overview of these systems shall be given. The relatively recent shift toward solid-state power electronics solutions to pulsed power engineering problems will be emphasized. Finally, some future trends in the field will be examined.
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| THP025 | R&D of the Long-Life Thyratron Tube | cathode, controls, feedback, pick-up | 622 | ||
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Long lifetime over 50k hours for the thyratron is essential to provide the reasonable availability of the accelerator such as X-FEL and future e+e- linear collider. The lifetime and reliability of a solid-state device are not well confirmed yet. There are some examples that show long life of a thyratron. Many thyratrons were dead due to several common causes related to circuits and operation environment rather than intrinsic problems of a device itself. Several valuable feedback systems are easily adopted to enhance the lifetime. There are still unidentified questions to be verified in the thyratron. Close collaboration between laboratories and companies is strongly requested in order to improve the lifetime and performance of a thyratron.
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| THP026 | HIGH POWER TEST OF COUPLER WITH CAPACITIVE WINDOW | vacuum, coupling, linac, linear-collider | 625 | ||
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New type of coupler with capacitive-coupling inner conductor is designed in KEK. This coupler has a module structure, which is convenient for mass-production, assembling and repairing. Four samples of couplers were made and two of them were tested at high power level. The main parameters of the couplers and test results are presented in this paper.
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| THP027 | Study of PPM-Focused X-band Pulse Klystron | klystron, cathode, linear-collider, simulation | 628 | ||
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The R&D of PPM (Periodic Permanent Magnet)-focused X-band pulse klystrons has been conducted since 1999, originally for Global Linear Collider (GLC) project. So far six prototype tubes have been tested. Some of them successfully produce the power required in GLC (75MW, 1.6μsec pulse width). However their performance was not perfect as a GLC tube. The problems are the stability of RF output and the gun performance. Since GLC programs were terminated in 2004, some limited work on the improvement of the PPM tubes continues at X-Band Test Facility (XTF) in KEK. The work includes the test to evaluate the performance of revised (rebuilt) tubes as well as disassembling these tubes after the test for further inspection. Recent results are reported.
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| THP038 | Normal Conducting High-Gradient Studies at KEK | linear-collider, acceleration, extraction, vacuum | 661 | ||
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Normal-conducting high field studies have been pursued at XTF, a high power X-band RF facility of KEK developed for linear collider. Three traveling-wave structures developed for X-band linear collider were studied in high field of more than 70MV/m level. High-field characteristic such as field emission properties and trip rate, etc. are studied carefully as the processing proceeds. Operation at 50MV/m level was found very stable while breakdowns happened once an hour or so at more than 70MV/m, indicating the approach to some critical point. This characteristics is discussed in conjunction with various author’s trials to make a scaling law of severe breakdowns among power, pulse width and so on. Further basic studies on field/power limitation or robustness against breakdowns in various materials are planned using narrowed waveguide configuration. Unique features related to this study is also described.
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| THP066 | Lorentz-Force Detuning Analysis for Low-Loss, Re-entrant and Half-Reentrant Superconducting RF Cavities | simulation, coupling, linear-collider, superconducting-RF | 734 | ||
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The RF design of a superconducting elliptical cavity requires a trade-off in the optimization of the cell shape between the region of high electric field and the region of high magnetic field. In practice, the cavity performance may be limited not by the RF characteristics, but by detuning due to the Lorentz force, bath pressure fluctuations, or microphonics; Lorentz force detuning is of concern primarily for pulsed accelerators such as the proposed International Linear Collider. Hence the structural properties must also be taken into account in the cavity design. Several new cavity shapes are being developed in which the surface magnetic field is decreased relative to the TeSLA cavity shape, with the goal of reaching a higher accelerating gradient. This study will compare the Lorentz force detuning characteristics of the TeSLA, "low-loss", "reentrant", and "half-reentrant" cavity middle cells, and explore possible methods for stiffening the structures.
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| THP078 | High-Gradient Test of a Tungsten-Iris X-Band Accelerator Structure at NLCTA | vacuum, linear-collider, RF-structure, impedance | 764 | ||
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The CLIC study group at CERN has built two X-band accelerating structures to be tested at SLAC in NLCTA. The structures consist of copper cells with insert irises made out of Molybdenum and Tungsten, clamped together and installed in a vacuum tank. These structures are exactly scaled versions from structures tested previously at 30 GHz and with short pulses (16 ns) in the CLIC Test Facility at CERN. At 30 GHz these structures reached gradients of 150 MV/m for Tungsten and 195 MV/m for Molybdenum. These experiments were designed to provide data on the dependence of rf breakdown on pulse length and frequency. This paper reports in particular on the high-gradient test of the tungsten-iris structure. At a pulse length of 16 ns a gradient of 125 MV/m was reached at X-band, 20 % lower than the 150 MV/m measured at 30 GHz in the CLIC Test Facility. The pulse length dependence and the dependence of the break down rate as a function of gradient were measured in detail. The results are compared to data obtained from the Molybdenum-Iris experiment at X-band which took place earlier as well as to 30 GHz data.
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| THP093 | Polyhedral Cavity for Superconducting Linacs | linac, coupling, emittance, brightness | 803 | ||
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A polyhedral cavity structure has been devised for use in superconducting linacs. It has the same ellipsoidal side contour as a TESLA cavity but is configured as a polyhedron in its end view. Each segment of the polyhedron consists of a Nb foil bonded to a Cu wedge that has been machined to the desired ellipsoidal inner contour. There are no welds, and the seams between adjacent segments do not affect the high Q of the accelerating mode but block the azimuthal currents of deflecting modes. The power coupled into deflecting modes can be slot-coupled at the seams into dielectric-loaded waveguides integrated in the copper segments and conveyed to warm termination. The inner surface of each segment is accessible for polishing and characterization. It accommodates application of improved superconducting surfaces, such as the multi-layer thin-film Nb3Sn proposed by Gurevich. Refrigeration can be provided by gun-bored channels within the copper segments. The copper segments provide a rigid assembly that eliminates Lorentz detuning. The talk will discuss the mode properties and coupling strategies, the strategy for Nb/Cu bonding, and plans for building and testing of prototype cavities.
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| FR1004 | Recent Developments in SRF Cavity Science and Performance | superconductivity, linear-collider, controls, cathode | 818 | ||
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The performances of SRF cavities made of high purity bulk niobium have been improving in the last few years and surface magnetic fields (Bp) close to the thermodynamic critical field of niobium have been achieved in a few cases. The recommendation made in 2004 in favor of SRF as the technology of choice for the International Linear Collider (ILC), requires to improve the reliability of multi-cell cavities operating at accelerating gradients (Eacc) of the order of 35 MV/m . Additionally, a better understanding of the present limitations to cavity performance, such as the high-field Q-drop is needed. This contribution presents some recent developments in SRF cavity science and performance. Among the most significant advances of the last few years, new cavity shapes with lower ratio Bp/Eacc were designed and tested. Cavities made of large-grain niobium became available, promising lower cost at comparable performance to standard fine-grain ones and several tests on single-cell cavities were done to gain a better understanding of high-field losses. In addition, studies to improve the reliability of electropolishing are being carried out by several research groups.
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