Specification of a new electron cooler for the low energy ion ...

400
between the ions and the electrons. From the
above relation one sees that a number of machine
and cooler parameters can influence the cooling
time. In our experiments the main points of
investigation were
* The ion beam lifetime as a function of the
electron beam intensity.
* The influence of the machine lattice parameters
on the cooling time.
* The cooling time versus the electron beam
intensity and the cooler length.
* Testing of stacking modes.
* Stack equilibrium emittances and emittance
growth.
2.1. Ion beam lifetime
The lifetime of the lead ion beam in the presence
of an electron beam was measured for a number of
charge states around 53+. An anomalously fast
recombination rate for Pb 53+ was observed in
initial measurements and forced us to switch to a
charge state of 54+. This unusually high rate
points to mechanisms other than radiative or
dielectronic capture and has been observed at
other heavy ion storage rings.
2.2. Influence of the lattice parameters on the
cooling time
Special importance was attached to the dependence
of the cooling time on the optical parameters
of the storage ring. For a given emittance
and momentum spread, the lattice parameters b
and D determine the size and angular distribution
of the ion beam. Cooling times were measured for
a wide range of lattice parameters at the cooler
with protons at 50 MeV and Pb ions at 4.2 MeV/u.
Contrary to what one would expect from theory,
the best cooling was obtained for intermediate b
values. It should however be mentioned that for
these machine settings a non-zero value of the
dispersion function was set in the cooler section. It
is clear that the influence of D is superimposed on
the bh dependence and may even be the dominant
effect.
ARTICLE IN PRESS
G. Tranquille / Nuclear Instruments and Methods in Physics Research A 532 (2004) 399–402
Guided by these observations a series of
measurements were made where only the dispersion
function was varied and the cooling times as a
function of the difference in beam alignment was
recorded. The results showed that with a finite
dispersion in the cooler the cooling times could be
halved and, depending on the sign of the dispersion
function, cooling was faster for positive or
negative offsets in beam position.
2.3. Cooling time versus electron current and cooler
length
In the 1997 experiments the cooler length was
doubled from 1.5 to 3 m. This enabled us to
complement the cooling time measurements as a
function of I e with the variable Z. The results show
that the linear increase in cooling rate, 1/t, asa
function of Ie and Z shows uponly for low electron
currents (o120 mA). Possible explanations include
the effect of the electron beam space charge
increasing the mean transverse velocity of the
beam, misalignment tolerances, which are more
critical for a longer cooler, and electron beam
instabilities observed at high currents due to the
gun design.
2.4. Stacking tests and equilibrium emittances
Cooling and stacking tests were performed with
the lead ion injector Linac cycling at a repetition
rate of 2.5 Hz. The ion beam was injected using a
combined horizontal and longitudinal multi-turn
injection scheme enabling upto 1.5 10 8 ions to
be injected per Linac pulse. After injection into the
ring the beam is cooled in all three planes and
‘dragged’ to the stack momentum leaving space for
the following pulse 400 ms later. This process is
repeated until the losses during the interval
between consecutive pulses balance the number
of particles added per injection. On average
6 10 8
ions were accumulated in about 10
injections with peaks reaching 7 10 8 ions.
The stack emittance was measured as a function
of stack intensity and the cooling was switched off
to record the emittance growth. The results show
that for the required LHC intensity, the emittance,