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Title: Analysis of measured post-coupler fields in a ramped-gradient drift-tube linac

Abstract

Post couplers longitudinally stabilize the axial electric field in a drift-tube linac (DTL) against tuning errors. Displacing the post couplers toward one end of the structure results in a ramped field that increases toward the other end. Such a ramped-gradient DTL is also stable, but some of the cavity power is dissipated on the post couplers to maintain the ramp. This paper reports bead-perturbation measurements of the post-coupler magnetic fields near the tank well. A simple physical model reproduces the measured fields with an accuracy of about 2% and yields current-density distributions on the post couplers in terms of the current density on the cavity wall. The peak power density occurs on the sides of the post couplers where the TM/sub 010/ magnetic field adds to the post-coupler magnetic field. In the Los Alamos Ramped-Gradient DTL (RGDTL), the peak power densities on post couplers located in the steepest part of the ramp exceed by more than 10 times the maximum power density on the tank wall far from any post couplers. Methods to reduce such power losses to levels comparable to a flat-gradient DTL will be discussed. 2 refs. , 6 figs., 1 tab.

Authors:
Publication Date:
Research Org.:
Los Alamos National Lab., NM (USA)
OSTI Identifier:
6807935
Report Number(s):
LA-UR-88-3248; CONF-881049-13
ON: DE89000343
DOE Contract Number:
AC04-76DP00789
Resource Type:
Conference
Resource Relation:
Conference: Linear accelerator conference, Williamsburg, VA, USA, 3 Oct 1988; Other Information: Portions of this document are illegible in microfiche products
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; LINEAR ACCELERATORS; MAGNETIC FIELDS; CURRENT DENSITY; DESIGN; EXPERIMENTAL DATA; FREQUENCY MEASUREMENT; TUNING; ACCELERATORS; DATA; INFORMATION; NUMERICAL DATA 430200* -- Particle Accelerators-- Beam Dynamics, Field Calculations, & Ion Optics

Citation Formats

Billen, J.H. Analysis of measured post-coupler fields in a ramped-gradient drift-tube linac. United States: N. p., 1988. Web.
Billen, J.H. Analysis of measured post-coupler fields in a ramped-gradient drift-tube linac. United States.
Billen, J.H. 1988. "Analysis of measured post-coupler fields in a ramped-gradient drift-tube linac". United States. doi:. https://www.osti.gov/servlets/purl/6807935.
@article{osti_6807935,
title = {Analysis of measured post-coupler fields in a ramped-gradient drift-tube linac},
author = {Billen, J.H.},
abstractNote = {Post couplers longitudinally stabilize the axial electric field in a drift-tube linac (DTL) against tuning errors. Displacing the post couplers toward one end of the structure results in a ramped field that increases toward the other end. Such a ramped-gradient DTL is also stable, but some of the cavity power is dissipated on the post couplers to maintain the ramp. This paper reports bead-perturbation measurements of the post-coupler magnetic fields near the tank well. A simple physical model reproduces the measured fields with an accuracy of about 2% and yields current-density distributions on the post couplers in terms of the current density on the cavity wall. The peak power density occurs on the sides of the post couplers where the TM/sub 010/ magnetic field adds to the post-coupler magnetic field. In the Los Alamos Ramped-Gradient DTL (RGDTL), the peak power densities on post couplers located in the steepest part of the ramp exceed by more than 10 times the maximum power density on the tank wall far from any post couplers. Methods to reduce such power losses to levels comparable to a flat-gradient DTL will be discussed. 2 refs. , 6 figs., 1 tab.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1988,
month = 1
}

Conference:
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  • This paper reports low-power tuning and stabilization measurements on the Los Alamos Ramped-Gradient Drift-Tube Linac (RGDTL). The RGDTL is a 425-MHz, 1.87-m-long structure containing 29 drift tubes, 14 post couplers, 2 tuners, and 2 drive loops. The design calls for an axial electric field gradient that increases from 2.0 MV/m to 4.4 MV/m over 1.5 m for accelerating H/sup /minus// from 2.07 to 6.67 MeV. Asymmetric post couplers adjacent to every other drift tube both stabilize and ramp the field. The two tuners provide 1.4 MHz of dynamic frequency adjustment around the frequency selected by a one-time trimming of twomore » tuning bars that are bolted inside the tank alongside the drift-tube stems. Field measurements obtained by the bead-perturbation method determine how to adjust the post couplers for the desired ramp. Comparison of two field distributions for different deliberate frequency perturbations quantifies the structure's tilt sensitivity and indicates whether to tune the post coupler frequencies lower or higher with respect to the TM/sub 010/ accelerating mode frequency. 3 refs., 6 figs., 1 tab.« less
  • Ramped accelerating gradients in drift-tube linacs are made possible by post couplers with extreme differential coupling between adjacent accelerating gaps. By bringing the energy of the beam up through ramping, better overall control of longitudinal emittance can be accomplished and a more compact accelerator achieved. Such a machine has been designed at Los Alamos and built through industrial participation with the Hanford Engineering Development Laboratory (HEDL), the Brobeck Corp., and Grumman Aerospace Corp. The design of the Ramped-Gradient Drift-Tube Linac (RGDTL) involved development of high-powered drive loops, specially instrumented drift tubes, and a new rotary-type dynamic tuner. The assembly ofmore » the machine from the industrial components has been completed, and it is now installed and in operation at the Accelerator Test Stand (ATS) in Los Alamos. 6 refs., 7 figs.« less
  • A short, high-power linac structure has been designed and is being built; it incorporates a ramped accelerating field for matching a radio-frequency quadrupole (RFQ) to a high-gradient drift-tube linac (DTL). The tank is made of aluminum and can operate at high duty factor. The drift tubes are copper and use new neodymium-iron-boron quadrupoles. The linac is tuned using conical-sweep post couplers and is driven by multiple rf loops. Rapid acceleration of the beam occurs from low to high gradients in a short, compact, and lightweight structure. Because this linac will also be used to test open drift tubes, test resultsmore » are included on vacuum measurements of high-grade epoxies and plastics that might be used in a drift-tube body.« less
  • Post-coupler characteristics have been measured on a model Alvarez tank for drift-tube-to-outer tank diameter ratios from 0.10 to 0.25. A maximum ratio beyond which field stabilization is not possible and a minimum ratio where stabilization is difficult have been determined. Field stabilization has been studied using two to twenty-one post couplers in a 22 cell tank. A sensitive dependence of the drift-tube stem currents near the tank end walls on the end cell geometry has been found.
  • New permanent magnets for quadrupole focusing make possible smaller drift tubes in drift-tube linacs (DTLs), resulting in higher shunt impedance. However, ordinary post couplers cannot stabilize a DTL whose drift-tube-to-wall spacing exceeds one-quarter wavelength (lambda/4) by more than a few per cent for the accelerating mode frequency. We have built and tested post couplers that operate in the 3lambda/4 mode. These 3lambda/4 post couplers, when substituted for lambda/4 couplers, had similar stabilization properties. In addition, the coupling between post and drift tube, and the post's resonant frequency can be independently adjusted. 5 refs., 5 figs.