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Space-charge limits on the transport of ion beams in a long alternating gradient system

Description: We have experimentally studied the space-charge-dominated transport of ion beams in an alternating-gradient channel, without acceleration. We parameterize the focusing strength in terms of the zero-current ''betatron'' oscillation phase advance rate, sigma/sub 0/ (degrees per focusing period). We have investigated the conditions for ''stability'', defined as the constancy of the total current and phase space area of the beam during transport. We find that the beam may be transported with neither loss of current nor growth in phase area if sigma/sub 0/ < 90/sup 0/. In this regime, the space-charge repulsive force can counter 98-99% of the externally applied focusing field, and the oscillation frequency of the beam particles can be depressed by self-forces to almost a factor of 10 below the zero-current value, limited only by the optical quality of our ion source. For sigma/sub 0/ > 90/sup 0/, we find that collective interactions bound the maintainable density of the beam, and we present a simple, semi-empirical characterization for stability, within our ability to distinguish the growth rate from zero in our apparatus. Our channel comprises 87 quadrupole lenses, 5 of which are used to prepare the beam for injection into the non-azimuthally-symmetric focusing channel.
Date: November 1, 1986
Creator: Tiefenback, M.G.
Partner: UNT Libraries Government Documents Department

Experimental measurement of emittance growth in mismatched space-charge-dominated beams

Description: Using the Single Beam Transport Experiment (SBTE) at LBL, we have measured the emittance of a well-matched 4.6-mA beam of 122-keV Cs/sup +/ to be conserved from injection into through exit from an 80-lens segment of the AG focussing channel. We then mismatched the beam into the same channel such that the maximum (minimum) radius of the beam at the midplane between lenses was about 1.5 (0.5) times the former value. We caused mismatches in the envelope of the beam in both transverse dimensions (labeled a and b) in modes both symmetric (deltaa = deltab) and antisymmetric (deltaa = -deltab). We found the mismatch amplitude to decay during the beam transit through the channel for both modes of mismatch, although more so for the antisymmetric mode. We also found the emittance of the symmetrically mismatched beam to be the same as for the matched beam, while the emittance of the antisymmetrically mismatched beam grew by as much as a factor of four over that for the matched beam.
Date: March 1, 1987
Creator: Tiefenback, M.G.
Partner: UNT Libraries Government Documents Department

Measurements of stability limits for a space-charge-dominated ion beam in a long A. G. transport channel

Description: The Single Beam Transport Experiment at LBL consists of 82 electrostatic quadrupole lenses arranged in a FODO lattice. Five further lenses provide a matched beam from a high-current high-brightness cesium source for injection into the FODO channel. We call the transport conditions stable if both the emittance and current remain unchanged between the beginning and end of the channel, and unstable if either the emittance grows or the current decreases because of collective effects. We have explored the range of single-particle betatron phase advance per period from sigma/sub 0/ = 45/sup 0/ to 150/sup 0/ to determine the stability limits for the space-charge depressed phase advance, sigma. No lower limit for sigma (down to 7/sup 0/) has been found at sigma/sub 0/ = 60/sup 0/, whereas limits have clearly been identified and mapped in the region of sigma/sub 0/ above 90/sup 0/.
Date: May 1, 1985
Creator: Tiefenback, M.G. & Keefe, D.
Partner: UNT Libraries Government Documents Department

Survey and analysis of line-frequency interference in the CEBAF accelerator

Description: Feedthrough of interference from the AC power line into accelerator components is a problem which in pulsed accelerators can be reduced by operation synchronous with the AC line. This means of avoiding line-frequency effects is ineffective for continuous wave machines such as the CEBAF accelerator. We have measured line-frequency perturbations at CEBAF both in beam position and energy by using the beam position monitor system as a multiple-channel sampling oscilloscope. Comparing these data against the measured static optics (taken synchronously with the AC line) we have been able to identify point sources of interference, and resolve line-synchronous variations in the beam energy at a level near 0.001%. 3 refs., 2 figs., 1 tab.
Date: December 31, 1995
Creator: Tiefenback, M.G. & Li, Rui
Partner: UNT Libraries Government Documents Department

Compensation of RF-Induced Energy Spread in the CEBAF Injector Chopping System

Description: The CEBAF injector chopping system must generate three interleaved 499 MHz pulse trains of independently variable current from a DC input beam prior to axial compression. The chopper consists of two deflection cavities with an aperture midway between them. Lenses flanking the aperture focus the beam from the first cavity into the center of the second, where the RF deflection from the first cavity is removed. The symmetry of the RF energy spread across any time-slice of the beam is dominantly odd. The inverting optics used to focus the beam into the second cavity causes near cancellation of the energy spread from the two cavities. We present experimental measurements of the energy spread effects from a fundamental frequency (1497 MHz) chopper prototype producing a beam of suitable transverse emittance and energy spread, and discuss the expected performance of the subharmonic chopper system to be used for commissioning starting in January 1994.
Date: May 1, 1993
Creator: Tiefenback, M.G. & Krafft, G.A.
Partner: UNT Libraries Government Documents Department

Online Measurement and Tuning of Multipass Recirculation Time in the CEBAF Linac

Description: CEBAF is a recirculating electron accelerator with negligible synchrotron motion of the particles within each bunch. On-crest RF acceleration is used to minimize the time-averaged energy spread of the beam. Previously installed diagnostics** allow us to maintain the relative timing of the beam and the RF, noninvasively compensating for residual drifts of the RF timing system on the first acceleration pass. However, residual setup errors for the path length (recirculation time) and variable path length drift between passes result in relative drifts of beam energy at the 10-4 scale for some passes, even when the energy of any one of the recirculation passes is held fixed by adjusting the RF gradient. We have extended the diagnostic system to the higher acceleration passes and can correct the recirculation timing at the scale of hundreds of femtoseconds (tenths of a degree of 1497 MHz RF phase). Variation in the higher-pass beam to RF timing indicates drift in the beam recirculation time. The previous procedure for measuring and tuning the path length required suspension of beam delivery to the users. These actions can now be done without interruption of beam to the experimenters.
Date: June 1, 2001
Creator: Tiefenback, M.G.
Partner: UNT Libraries Government Documents Department

Beam-based phase monitoring and gradient calibration of Jefferson Laboratory RF systems

Description: Beam-based monitoring was instituted at Jefferson Lab in December of 1994 to correct the RF system phase setpoints against drifts of the master oscillator system, while running transparently to normal beam setup and delivery operations. As then implemented, the master oscillator distribution was subject to large thermal drifts, but no means existed of tracking the distribution of drifts, nor of identifying phase errors for individual cavities following maintenance activities. The background process implemented is capable of cresting each of the 312 cavities in the linacs to within approximately {+-} 2 degrees. In addition, during dedicated system measurement operations, the RF cavity gradients have been calibrated with beam to within a tolerance of approximately {+-} 5%. These beam-based results are in many cases more than 20% different from commissioning results using RF measurement techniques.
Date: July 1997
Creator: Tiefenback, M. G. & Brown, K.
Partner: UNT Libraries Government Documents Department

Runtime accelerator configuration tools at Jefferson Laboratory

Description: RF and magnet system configuration and monitoring tools are being implemented at Jefferson Lab to improve system reliability and reduce operating costs. They are prototype components of the Momentum Management System being developed. The RF is of special interest because it affects the momentum and momentum spread of the beam, and because of the immediate financial benefit of managing the klystron DC supply power. The authors describe present and planned monitoring of accelerating system parameters, use of these data, RF system performance calculations, and procedures for magnet configuration for handling beam of any of five beam energies to any of three targets.
Date: July 1, 1997
Creator: Tiefenback, M.G.; Doolittle, L. & Benesch, J.F.
Partner: UNT Libraries Government Documents Department

Effect of induced charge at boundaries on transverse dynamics of a space-charge-dominated beam

Description: A particle simulation code has been used to study the effect of transverse beam dynamics of charge induced on focusing electrodes. A linear transport system was assumed. The initial particle distribution was taken to be that of a uniform elliptical beam with a Gaussian velocity distribution. For misaligned, highly space-charge-dominated beams (betatron phase advance per lattice period less than or equal to 10/sup 0/), a large oscillation of the rms emittance occurred in a beat pattern. Linearized Vlasov analysis shows the oscillation to be a sextupole oscillation, driven by the beam coherent betatron motion. Emittance growth accompanied the oscillation. Preliminary experimental results from the Single Beam Transport Experiment (SBTE) are consistent with the code results. Addition of a dodecapole nonlinearity to the computational focusing field greatly reduces the oscillation amplitude.
Date: May 1, 1985
Creator: Celata, C.M.; Haber, I.; Laslett, L.J.; Smith, L. & Tiefenback, M.G.
Partner: UNT Libraries Government Documents Department

Research in the US on heavy ion drivers for inertial confinement fusion

Description: The US study of high-energy multigap accelerators to produce large currents of heavy ions for inertial fusion is centered on the single-pass induction linac method. The large technology base associated with multigap accelerators for high-energy physics gives confidence that high efficiency, high repetition rate, and good availability can be achieved, and that the path from scientific demonstration to commercial realization can be a smooth one. In an induction linac driver, multiple (parallel) ion beams are accelerated through a sequence of pulsed transformers. Crucial to the design is the manipulation of electric fields to amplify the beam current during acceleration. A proof-of-principle induction linac experiment (MBE-4) is underway and has begun the first demonstration of current amplification, control of the bunch ends, and the acceleration of multiple beams. A recently completed experiment, called the Single Beam Transport Experiment has shown that we can now count on more freedom to design an alternating-gradient quadrupole focusing channel to transport much higher ion-beam currents than formerly believed possible. A recent Heavy Ion Fusion System Assessment (HIFSA) has shown that a substantial cost saving results from use of multiply-charged ions, and that a remarkably broad range of options exist for viable power-plant designs. The driver cost at 3 to 4 MJ could be $200/joule or less, and the cost of electricity in the range of 50 to 55 mills/kWhr.
Date: October 1, 1986
Creator: Celata, C.; Faltens, A.; Fessenden, T.J.; Judd, D.L.; Keefe, D.; Kim, C.H. et al.
Partner: UNT Libraries Government Documents Department