LINAC 2004 - List of Keywords

A B C D E F G H I K L M O P Q R S T U V

pick-up

Paper	Title	Other Keywords	Page
MOP84	First Cryogenic Tests with JLab's new Upgrade Cavities*	damping, coupling, impedance, higher-order-mode	216
	P. Kneisel, G. Ciovati, G. Myneni, G. Wu Jefferson Lab, Newport News, Virginia J. Halbritter FZ Karlsruhe, Karlsruhe J. Sekutowicz DESY, Hamburg
	For the upgrade of CEBAF to 12 GeV two types of 7-cell cavities have been developed: the High Gradient type (HG) has been optimized with respect to the ratio of E_peak/E_acc and for the Low Loss (LL) type the shunt impedance has been maximized. Each cavity type features four DESY type coaxial HOM couplers and a waveguide input coupler. Design goals for these cavities have been set to E_acc = 20 MV/m with a Q-value at 2.05 K of Q₀ = 8·10⁹. A niobium prototype of each cavity has been fabricated at JLab and in a first test the HG cavity has been evaluated at cryogenic temperatures after appropriate buffered chemical polishing. Data for Q(E) were taken at several temperatures after R(T) was measured during initial pump down. In addition the pressure sensitivity as well as the Lorentz force detuning were evaluated. The damping of approximately 20 High Order Modes was measured to verify the room temperature data. Measurements on the LL prototype are in progress. We present in this contribution a summary of measured results of tests we performed on the new proposed shapes of the upgrade cavities.

TUP52	Methods for Measuring and Controlling Beam Breakup in High Current ERLs	feedback, damping, linac, electron	387
	C. Tennant, K. Jordan, E. Pozdeyev, R.A. Rimmer, H. Wang Jefferson Lab, Newport News, Virginia S. Simrock DESY, Hamburg
	It is well known that high current Energy Recovery Linacs (ERL) utilizing superconducting cavities are susceptible to a regenerative type of beam breakup (BBU). The BBU instability is caused by the transverse deflecting higher-order modes (HOMs) of the cavities which can have high impedance. We present MATLab simulation results for the BBU stability using the analysis tools of control theory. In this framework, methods of experimentally determining the threshold current and the means of suppressing the onset of the instability become more transparent. A scheme was developed to determine the threshold current due to a particular HOM by measuring the decay and rise times of the mode's field in response to an amplitude modulated beam as a function of the average electron beam current. To combat the harmful effects of a particularly dangerous mode, two methods of directly damping HOMs through the cavity HOM couplers were demonstrated. In an effort to suppress the BBU in the presence of multiple, dangerous HOMs, a conceptual design for a bunch-by-bunch transverse feedback system has been developed. By implementing beam feedback, the threshold for instability can be increased substantially.

TUP69	Precision Alignments of Stripline BPMs with Quadrupole Magnets for TTF2	quadrupole, alignment, linac, synchrotron	426
	G. Priebe, D. Nölle, M. Wendt, M. Werner DESY, Hamburg
	We report on our alignment setup to calibrate beam position monitors (BPM) with respect to the magnetic axis of the quadrupole magnets used in the warm sections of the TESLA Test Facility (TTF2). The Stripline BPM's are fixed inside the quadrupole magnets. A streched wire measurement was used to calibrate the electrical axis of the BPM wrt. to the magnetic axis of the quadrupole.

TUP70	Systematic Calibration of Beam Position Monitor in the High Intensity Proton Accelerator (J-PARC) LINAC	quadrupole, linac, simulation, proton	429
	S. Sato, K. Hasegawa, F. Hiroki, J. Kishiro, Y. Kondo, M. Tanaka, T. Tomisawa, A. Ueno, H. Yoshikawa JAERI, Ibaraki-ken Z. Igarashi, M. Ikegami, N. Kamikubota, S. Lee, K. Nigorikawa, T. Toyama KEK, Ibaraki
	In J-PARC, a MW class of proton accelerator is under construction. Improperly- tuned beam would critically result in unacceptable (>0.1%) energy loss. Systematic strategy of fine calibrations of the beam position monitor (BPM) detectors, is therefore required. First, Off-beam-line calibrations of BPMs are taken, with a dedicatedly- designed bench, which has a beam-simulating electric wire carrying 324 MHz. And then discrepancies are calibrated for each BPM between reconstructed electrical center of pick-up plates and measured mechanical center, before the installation of BPM on the beam line. Secondly, after BPMs are installed on the beam line, real beam is used for systematic calibrations (Beam Based Calibration (BBC)). The discrepancies are calibrated between electromagnetic center of Q-magnets and reconstructed beam position. In KEK we have the first stage of J-Parc LINAC with Ion source, RFQ, DTL, Q- and steering-magnets, and lots of BPMs. Implementation of BBC is going with SAD-language, which can also be used for beam steering and beam trajectory simulations, e.g. TRACE-3D. In this presentation, such strategic BPM calibration system will be intensively described.

TUP71	Highly Sensitive Measurements of the Dark Current of Superconducting Cavities for TESLA Using a SQUID Based Cryogenic Current Comparator	electron, shielding, feedback, cryogenics	432
	W. Vodel, R. Neubert, S. Nietzsche FSU, Jena K. Knaack, M. Wendt, K. Wittenburg DESY, Hamburg A. Peters GSI, Darmstadt
	This contribution presents a Cryogenic Current Comparator (CCC) as an excellent tool for detecting dark currents generated, e.g. by superconducting cavities for the upcoming TESLA project (X-FEL) at DESY. To achieve the maximum possible energy the gradient of the superconducting RF cavities should be pushed close to the physical limit of 50 MV/m. The undesired field emission of electrons (so-called dark current) of the superconducting RF cavities at strong fields may limit the maximum gradient. The absolute measurement of the dark current in correlation with the gradient will give a proper value to compare and classify the cavities. The main component of the CCC is a highly sensitive LTS-DC SQUID system which is able to measure extremely low magnetic fields, e.g. caused by the dark current. For this reason the input coil of the SQUID is connected across a special designed toroidal niobium pick-up coil for the passing electron beam. A noise limited current resolution of nearly 2 pA/√(Hz) with a measurement bandwidth of up to 70 kHz was achieved in the laboratory. Design issues of the CCC and the application in the CHECHIA cavity test stand at DESY as well as experimental results will be discussed.

TUP72	TTF II Beam Monitors for Beam Position, Bunch Charge and Phase Measurements	vacuum, undulator, instrumentation, single-bunch	435
	M. Wendt, D. Nölle DESY, Hamburg
	An overview of the basic beam instrumentation with regard to elecromagnetic beam monitors for the TESLA Test Facility phase II (TTF II) is given. Emphasis is put on beam position monitor (BPM) and toroid transformer systems for beam orbit and bunch charge observations. Furthermore broadband monitors, i.e. wall current and bunch phase monitors, are briefly presented.

THP32	New Accelerating Modules RF Test at TTF	linac, klystron, superconductivity, radiation	672
	D. Kostin DESY, Hamburg
	Five new accelerating modules were installed into the TTF tunnel as a part of the VUV FEL Linac. They are tested prior to the linac operation. The RF test includes processing of the superconducting cavities, as well as maximum module performance tests. The test procedure and the achieved performance together with the test statistical analysis are presented.

THP33	Progress toward NLC/GLC Prototype Accelerator Structures	dipole, simulation, impedance, linac	675
	J. Wang, G. Bowden, V.A. Dolgashev, R.M. Jones, J. Lewandowski, C.D. Nantista, S.G. Tantawi SLAC/ARDA, Menlo Park, California C. Adolphsen, D.L. Burke, J.Q. Chan, J. Cornuelle, S. Döbert SLAC/NLC, Menlo Park, California T. Arkan, C. Boffo, H. Carter, N. Khabiboulline FNAL, Batavia, Illinois N. Baboi DESY, Hamburg D. Finley, I. Gonin, S. Mishra, G. Romanov, N. Solyak Fermilab, Batavia, Illinois Y. Higashi, T. Higo, T. Kumi, Y. Morozumi, N. Toge, K. Ueno KEK, Ibaraki Z. Li, R. Miller, C. Pearson, R.D. Ruth, P.B. Wilson, L. Xiao SLAC, Menlo Park, California
	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).

THP65	Low-Power RF Tuning of the Spallation Neutron Source Warm LINAC Structures	coupling, linac, target, vacuum	760
	C. Deibele, G. Johnson ORNL, Oak Ridge J. Billen, N.K. Bultman, J. Stovall LANL, Los Alamos, New Mexico J. Error, P. Gibson ORNL/SNS, Oak Ridge, Tennessee J. Manolitsas, D. Trompetter ACCEL, Bergisch Gladbach A. Vasyuchenko RAS/INR, Moscow L. Young TechSource, Santa Fe, NM
	The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built at Oak Ridge National Laboratory. A conventional 402.5 MHz drift-tube linac (DTL) accelerates the beam from 2.5 to 86 MeV, and the 805 MHz coupled-cavity linac (CCL) continues acceleration to 186 MeV. Tuning the six DTL tanks involves adjusting post-coupler lengths and slug tuners to achieve the design resonant frequency and stabilized field distribution. A 2.5 MW klystron feeds RF power into each DTL tank through a ridge-loaded waveguide that does not perturb either the frequency or field distribution in the tank. The CCL consists of 4 RF modules operating in the βλ/2 mode. Each module contains 96 accelerating cavities in 12 segments of 8 cavities each, 11 active bridge coupler cavities, and 106 nominally unexcited coupling cavities. For each RF module, power from a single 5 MW klystron splits once and drives bridge couplers 3 and 9. We will discuss the special tools and measurement techniques developed for the low-power tuning activities.

THP92	Effect of the Tuner on the Field Flatness of SNS Superconducting RF Cavities	simulation, coupling, resonance, superconducting-RF	815
	A. Sun ORNL/SNS, Oak Ridge, Tennessee H. Wang, G. Wu Jefferson Lab, Newport News, Virginia
	Field flatness in a multi-cell superconducting cavity affects not only the net accelerating voltage, but also the peak surface field and the Lorenz Force detuning coefficient. Our measurement indicates that the field flatness changes both external Q of the Fundamental Power Coupler (FPC) and external Q of the Field Probe (FP). The field amplitude tilts linearly to the distance between the cell center and the cavity’s geometry center (pivot point). The tilt rate has been measured in a cryomodule cold (2 K) test, being about 2%/100 kHz, relative the field flatness at the cavity’s center frequency of 805 MHz. Bead-pull measurements confirmed that the field flatness change is 2.0%/100 kHz for a medium β cavity with helium vessel, and 1.72%/100 kHz without helium vessel. These results matched the predictions of simulations using ANSYS and SUPERFISH. A detailed analysis reveals that longitudinal capacitive gap deformation is the main cause of the frequency change. Field flatness change was not only due to the uneven stored energy change within the cell, but also due to cell-to-cell coupling.