The unprecedentedly high charge
density and short bunch length in the CLIC damping ring could aggravate
the effects of coherent synchrotron radiation and Touschek scattering.
The Touschek lifetime can now be computed by a MADX development version
for the exact ring optics, using a formalism due to Piwinski. A novel
programme by K. Yokoya and T. Agoh at KEK allows – for the first time –
simulations of the microwave instability caused by coherent synchrotron
radiation in a lepton storage ring. Both effects are found to be
surprisingly benign for the CLIC parameters and the present CLIC
damping-ring lattice.
Maxim described the ATF damping
ring and presented emittance and damping time measurements.
Dispersion measurements were performed by changing the RF frequency.
The vertical dispersion was corrected from ~10 to 5mm using SAD
calculations. Beta functions were measured by gradient strength
changes. The damping times were measured by a streak camera. The
results with wiggler magnet operating were: τ
x=7ms, τ
y=22ms,
τ
z=7ms; without wiggler the values were: τ
y=27ms,
τ
z=11.4ms,. These values were compared to MAD calculations.
While the emittance and the damping time for the vertical plane agrees
well between model and measurement, the MAD predictions for the
horizontal emittance and horizontal and longitudinal damping times are
a factor 2 higher than the measurements.
Maxim also showed his damping ring design for the ILC where he could
obtain a factor 2 smaller horizontal emittance compared to previous
designs.
It was mentioned that ATF is usually run without the wigglers and the
reason for this was not obvious to us.
copy of his transparencies (pdf
format)
After recapitulating the Touschek effect,
Frank presented recent results about
Touschek scattering in the CLIC DR. The effect had been implemented in
MAD-X and simulations show that the Touschek lifetime in the CLIC DR
has a comfortably high value of 4.2 hours, even higher than in ILC.
Also for coherent synchrotron radiation (CSR), a new tracking code
including CSR has been developed, taking into account the shielding
effect by vacuum chamber geometry. It differs substantially from the
free space solution and for small wave numbers, the CSR impedence is
strongly suppressed. For the CLIC DR with 2cm vacuum chamber radius,
the tool predicts a bunch lengthening by only 5% and no momentum spread
increase, for 4mm the bunch length would grow by ~20%. So the CSR
effect for the present CLIC DR parameters is surprisingly benign.