Mechanism

1
Mechanism
The effect of surface is VERY sensitive to the
slope angle ? It is insensitive to W and the
strength of the B-field
Nucl.Instrum.Meth.A571588( 2007)
The electrons hit the surface several times, and
then the total number of electrons re-entering to
the beam chamber is reduced.
B-filed
Triangular surface in dipole magnet, Orbits of
electrons show the mechanism of confinement of
secondary electrons
The capture probability of secondary electrons in
magnetic field from a tilted surface with slope
angle ? for various values of the SEY coefficient
at zero energy ?0.
2
Effect of the surface shape
R_tip should be smaller comparing with W in order
to reduce the effect of round tips
Rtip
?max1.2
Transverse distribution of electron cloud in a
dipole magnet (a) and wiggler magnet (b).
Effective SEYs of a triangular surface with
different tips shown in Figure 6 (Rtip0.2mm,
W2mm, ?800) in a magnetic field of 0.2 Tesla
3
Effectiveness of grooves (POSINST)
(M. Venturini, LBNL)
The model of triangular groove with beam will be
included CLOUDLAND soon
B0.19T N21010 ? 1.75

• E-cloud density suppressed by a factor 200 for
  groove angles ?gt75o

Max of cloud density vs. groove-tip radius for
two groove height hg
4
Discussion
How to get the best geometry (small tips)?
Do we need coating with TiN, ect. ( We use Al.)
The secondaries is less important with a small
?max1, therefore, the suppression effect may be
not significant at low SEY. A larger ?max is
desired in order to clearly see the effect of
grooved surface
The impedance enhancement is small. It is less
than 15 in a dipole magnet