2.3 The Storage Ring: Inside the Shield Walls
Having gone to the great expense of producing a relativistic high-energy charged
particle beam, we would like to use the particles more than once. This is done by
bending the particles in an approximate circle, hence the term storage ring. These are
essentially large high vacuum vessels, with magnets to bend and focus particles and
with one or more rf cavities to replace the energy lost to synchrotron radiation.
Machine physicists usually build magnets that have primarily one multipole term,
dipole, quadrupole, or sextupole, the fields for which are summarized in Appendix
C. We will see that each serves a different purpose.
2.3.1 Dipole Magnets
The most basic magnet component in a storage ring, the one that bends particles in a
circular orbit, is a dipole magnet often referred to as a bend magnet. An ideal bend
magnet for a horizontal orbit employs a constant vertical magnetic field with no
horizontal component. Most storage ring dipoles are electromagnets, employing
steel pole pieces and cooled copper coils. For example, a typical bend magnet
employs a ~1 kA current to achieve a 1.58 Tesla field, and weighs more than
2 tons (Fig. 2.5).
A horizontally moving relativistic electron (β ¼ v/c ~ 1) in such a field follows a
circular trajectory with a bend radius ρ in practical units:
ρ m
½ ¼
3:3E GeV
½
B Tesla
½
ð2:5Þ
For the ALS, with a ring energy of 1.9 GeV, this yields a bend radius of 4 m.
Since the physical circumference of the ring is ~200 m (a radius of 32 m), there is
clearly a lot of extra real estate between bend magnets. We will now see what else is
involved in that space.
Fig. 2.5 Left: schematic of the windings, yoke, and pole pattern of a dipole (“bend”) magnet.
Middle: the resulting magnetic field. Right: practical dipole magnet at SESAME
16
2 The Storage Ring Complex
Having gone to the great expense of producing a relativistic high-energy charged
particle beam, we would like to use the particles more than once. This is done by
bending the particles in an approximate circle, hence the term storage ring. These are
essentially large high vacuum vessels, with magnets to bend and focus particles and
with one or more rf cavities to replace the energy lost to synchrotron radiation.
Machine physicists usually build magnets that have primarily one multipole term,
dipole, quadrupole, or sextupole, the fields for which are summarized in Appendix
C. We will see that each serves a different purpose.
2.3.1 Dipole Magnets
The most basic magnet component in a storage ring, the one that bends particles in a
circular orbit, is a dipole magnet often referred to as a bend magnet. An ideal bend
magnet for a horizontal orbit employs a constant vertical magnetic field with no
horizontal component. Most storage ring dipoles are electromagnets, employing
steel pole pieces and cooled copper coils. For example, a typical bend magnet
employs a ~1 kA current to achieve a 1.58 Tesla field, and weighs more than
2 tons (Fig. 2.5).
A horizontally moving relativistic electron (β ¼ v/c ~ 1) in such a field follows a
circular trajectory with a bend radius ρ in practical units:
ρ m
½ ¼
3:3E GeV
½
B Tesla
½
ð2:5Þ
For the ALS, with a ring energy of 1.9 GeV, this yields a bend radius of 4 m.
Since the physical circumference of the ring is ~200 m (a radius of 32 m), there is
clearly a lot of extra real estate between bend magnets. We will now see what else is
involved in that space.
Fig. 2.5 Left: schematic of the windings, yoke, and pole pattern of a dipole (“bend”) magnet.
Middle: the resulting magnetic field. Right: practical dipole magnet at SESAME
16
2 The Storage Ring Complex