Abstracts Brochure - CERN

THPCH — Poster Session 29-Jun-06 16:00 - 18:00
for long-pulse LIAs such as DARHT-II is the ion-hose. We also performed experiments with the 6.7-MeV long-pulse
configuration of DARHT-II in which we deliberately induced ion-hose by raising the background pressure far above
its normal value. The results of these experiments were used to show that ion-hose will not be a problem for to the
final DARHT-II configuration.
Coupling Impedances of Small Discontinuities for Non-ultrarelativistic Beams
The beam coupling impedances of small discontinuities
of an accelerator vacuum cham- S.S. Kurennoy (LANL)
ber have been calculated (e.g., * and references
therein) for ultrarelativistic beams using Bethe’s diffraction theory. Here we extend the results for an arbitrary
beam velocity. The vacuum chamber is assumed to have an arbitrary, but fixed, cross section. The longitudinal and
transverse coupling impedances are derived in terms of series over cross-section eigenfunctions, while the discontinuity
shape enters via its polarizabilities. Simple explicit formulas for the circular and rectangular cross sections are
presented. The impedance dependence on the beam velocity exhibits some unusual features. For example, the reactive
impedance, which dominates in the ultrarelativistic limit, can vanish at a certain beam velocity, or its magnitude
can exceed the ultrarelativistic value many times.
*S. S. Kurennoy et al. Phys. Rev. ·10 52 (1995) 4354.
Wakefields in the LCLS Undulator Transitions
We have studied longitudinal wakefields of
very short bunches in non-cylindrically sym- K.L.F. Bane (SLAC) I. Zagorodnov (DESY)
metric (3D) vacuum chamber transitions using
analytical models and the computer program ECHO. The wake (for pairs of well-separated, non-smooth transitions)
invariably is resistive, with its shape proportional to the bunch distribution. For the example of an elliptical
collimator in a round beam pipe we have demonstrated that—as in the cylindrically symmetric (2D) case—the wake
can be obtained from the static primary field of the beam alone. We have obtained the wakes of the LCLS rectangularto-round
transitions using indirect (numerical) field integration combined with a primary beam field calculation. For
the LCLS 1 nC bunch charge configuration we find that the total variation in wake-induced energy change is small
(0.03% in the core of the beam, 0.15% in the horns of the distribution) compared to that due to the resistive wall wakes
of the undulator beam pipe (0.6%).
Reflectivity Measurements for Copper and Aluminum Films in the Far Infrared and the
Resistive Wall Impedance in the LCLS Undulator
In the Linac Coherent Light Source (LCLS),
a SASE X-ray FEL, a short, intense electron
bunch passes through a 100 meter-long undulator
beam pipe. Longitudinal wakefields
K.L.F. Bane, G.V. Stupakov (SLAC) J. Tu (City College of The City
University of New York)
are excited that can have a deleterious effect on FEL performance. Of the sources of wakefields, of prime importance
is that due to the surface resistance of the beam pipe wall (resistive wall wake). For wavelengths within the bunch
spectrum (∼10–100 microns) the standard resistive wall wake formulas do not apply, and one needs to also consider
effects such as the conductivity dependence on frequency (the so-called "ac conductivity" effect), the anomalous skin
407
THPCH071
THPCH072
THPCH073