A semi-automated system for the characterization of NLC accelerating structures Page: 1 of 4
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A SEMI-AUTOMATED SYSTEM FOR THE CHARACTERIZATION
OF NLC ACCELERATING STRUCTURES*
S.M. Hanna, G.B. Bowden, H.A. Hoag, R. Loewen, A.E. Vlieks, J.W. Wang
Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309, USA
A system for characterizing the phase shift per cell of a
long X-band accelerator structure is described. The fields
within the structure are perturbed by a small cylindrical
metal bead pulled along the axis. A computer controls the
bead position and processes the data from a network analy-
zer connected to the accelerator section. Measurements
made on prototype accelerator sections are described, and
they are shown to be in good agreement with theory
Linear accelerators require precise phase relationship
to be maintained between the driving rf field and the
bunched beam throughout the interaction length .
Structures for the Next Linear Collider (NLC) will
probably be machined and assembled to such tight
mechanical tolerances that no provision for cell tuning is
needed. However, prototype structures being developed for
the NLC Test Accelerator (NLCTA) will need to be tuned,
and will have provision for tuning built into each cell. The
X-band structures are long, and employ a large number of
cells (204). Tuning has traditionally been checked by the
so-called nodal-shift method , in which a shorting
plunger rests on the irises and is moved from cell to cell
along the horizontal structure. Concerns over high-field
breakdown and dark current generation in the operating
accelerator prohibit the use of this method, which could
damage the finely machined copper surfaces.
In Section II of this paper, we describe a semi-
automated measurement system being built for testing and
tuning NLCTA accelerator sections. The results of travel-
ing wave (TW) perturbation measurements on two proto-
type sections are reported in Section III. Finally, in Section
IV, the measured data are compared to predictions calcula-
ted from our analytical model.
II. ACCELERATING STRUCTURE
The system being developed utilizes a bead field-
perturbation technique to evaluate the tuning of X-band
* Work supported by Department of Energy contract DE-
accelerators. A small metallic cylindrical bead is attached
to a thin nylon string running along the axis of the
vertically-mounted accelerator section, as shown in Fig. 1.
The position of the bead is determined by a stepper motor-
driven lead screw and carriage attached to the external
return loop of the nylon string. This method allows TW
perturbation measurements to be made without contacting
the clean inner surfaces of the section. The bead can be
replaced by a light metal cylinder if it is necessary to make
M2-", Lead Screw
Figure 1. Schematic diagram for the semi-automated test setup.
A rotary encoder is shaft-coupled to the stepper motor.
A computer interfaces with both, reading back position data
from the encoder and stepping the motor as required by the
measurement program. The computer also collects and
processes the complex reflection coefficient data from a
microwave network analyzer connected to the accelerator
RIl~ _g SGIEP
Presented at the 16th IEEE Particle Accelerator Conference (PAC 95) and
International Conference on High-Energy Accelerators, Dallas, Texas, May 1--5, 199
C46TRBUTJON OF THIS DOCUMENT IS UNLIMITED
C1cPJFq5 -00'162 -336
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Hanna, S.M.; Bowden, G.B.; Hoag, H.A.; Loewen, R.; Vlieks, A.E. & Wang, J.W. A semi-automated system for the characterization of NLC accelerating structures, report, June 1, 1995; Menlo Park, California. (digital.library.unt.edu/ark:/67531/metadc624900/m1/1/: accessed November 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.