Download DLS/CCLRC Cockcroft Institute 2005/6, © N.Marks, 2006

A.C. Magnets (II)
Neil Marks,
CCLRC,
Daresbury Laboratory,
Warrington WA4 4AD.
[email protected]
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Philosophy
1.
Present practical details of how a.c. lattice magnets differ from d.c.
magnets.
2.
Present details of the typical qualities of steel used in lattice magnets.
3.
Present an overview of the design and operation of power supply
systems, both d.c. (for storage rings) and cycling (for cycling
accelerators).
4.
Give a qualitative overview of injection and extraction techniques as used
in circular machines.
5.
Present the standard designs for kicker and septum magnets and their
associated power supplies.
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Contents
Core Syllabus
Variations in design and construction for a.c.
magnets;
Effects of eddy currents;
‘Low frequency’ a.c. magnets
Coil transposition-eddy loss-hysteresis loss;
Properties and choice of steel;
‘Extension’
Inductance in an a.c. magnet;
Power supply systems – d.c. and a.c.;
Injection and extraction schemes;
‘Fast’ magnets;
Kicker magnets-lumped and distributed power
supplies;
Septum magnets-active and passive septa;
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
High Frequency – Kicker Magnets
Kicker Magnets:
•used for rapid deflection of beam for injection or extraction;
•usually located inside the vacuum chamber;
•rise/fall times << 1µs.
•yoke assembled from high frequency ferrite;
Ferrite Core
•single turn coil;
•pulse current  104A;
•pulse voltages of many kV.
beam
Typical geometry:
Conductors
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Magnet & Power Supply
Because of the demanding performance required from these
systems, the magnet and power supply must be strongly
integrated and designed as a single unit.
Two alternative approaches to powering these magnets:
Distributed circuit: magnet and power supply made up of delay line circuits.
Lumped circuits: magnet is designed as a pure inductance; power supply can
be use delay line or a capacitor to feed the high pulse current.
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Kickers - Distributed System
Standard (CERN) delay line magnet and power supply:
L, C
L, C
Z0
dc
 Power Supply  Thyratron Magnet
Resistor
The power supply and interconnecting cables are matched to the surge
impedance of the delay line magnet:
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Mode of Operation
•the first delay line is charged to
by the d.c. supply;
V
•the thyratron triggers, a voltages wave:
magnet;
V/2 propagates into
•this gives a current wave of
propagating into the magnet;
V/( 2 Z )
•the circuit is terminated by pure resistor
to prevent reflection.
Neil Marks; DLS/CCLRC
Z,
Cockcroft Institute 2005/6, © N.Marks, 2006
Physical assembly
Magnet:
Usually capacitance is introduced along the length of
the magnet, which is split into many segments:
ie it is a pseudodistributed line
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Physical assembly.
Power supply:
Can be:
•a true ‘line’ (ie a long length of high voltage coaxial
cable);
•or a multi-segment lumped line.
These are referred to as ‘pulse forming networks’
(p.f.n.s) and are used extensively in ‘modulators’ for:
• linacs;
• radar installations.
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Parameters
The value of impedance Z (and therefore the added
distributed capacitance) is determined by the required rise
time of current:
total magnet inductance
capacitance added
surge impedance Z0
transit time (t) in magnet =
so
Z0
for a current pulse (I), V
=
=
=
 (LC);
=
=
=
L;
C;
(L/C);
L/t;
2ZI;
2IL/t.
The voltage (V/2) is the same as that required for a linear rise
across a pure inductance of the same value – the distributed
capacitance has not slowed the pulse down!
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Suitability:
Strengths:
•
•
•
•
•
the most widely used system for high I and V applications;
highly suitable if power supply is remote from the magnet;
this system is capable of very high quality pulses;
other circuits can approach this in performance but not improve on it;
the volts do not reverse across the thyratron at the end of the pulse.
Problems:
•
•
•
•
the pulse voltage is only 1/2 of the line voltage;
the volts are on the magnet throughout the pulse;
the magnet is a complex piece of electrical & mechanical engineering;
the terminating resistor must have a very low inductance - problem!
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Distributed power supply– lumped magnet
Z 0
R= Z 0
dc
L
I = (V/Z) (1 – exp (-Z t /L)
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Example of such a kicker system
SNS facility (Brookhaven)– extraction kickers:
• 14 kicker pulse power supplies & magnets;
• operated at a 60 Hz
kicker magnet inductance
repetition rate;
• kicks beam in 250 nS;
magnet current
• 750nS pulse flat top.
Neil Marks; DLS/CCLRC
0.76 -0.8 uH
2 - 2.5 kA
blumlein PFN Voltage
35 kV
pulse current rise time
200nS
current pulse width
750 nS
pulse repetition
60 Hz
Cockcroft Institute 2005/6, © N.Marks, 2006
Extraction systems layout
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Kicker p.f.n simulation model
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Simulated current waveform
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
EEV Thyratron CX1925
EEV
HV = 80kV
Peak current 15 kA
repetition 2 kHz
Life time ~3 year
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Kickers – Lumped Systems.
•The magnet is (mainly) inductive - no added distributed
capacitance;
•the magnet must be very close to the supply (minimises
inductance).
R
dc
L
I = (V/R) (1 – exp (- R t /L)
i.e. the same waveform as distributed power supply, lumped magnet systems..
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Improvement on above
C
R
dc
L
The extra capacitor C improves the pulse substantially.
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Resulting Waveform
Pulse Waveform
Example calculated for the following parameters:
1.2
mag inductance
rise time
resistor
trim capacitor
L = 1 mH;
t = 0.2 ms;
R = 10 W;
C = 4,000 pF.
1
0.8
0.6
0.4
The impedance in the lumped
circuit is twice that needed in the
distributed! The voltage to
produce a given peak current is the
same in both cases.
Performance:
0.2
0
0.00E+00
2.00E-07
4.00E-07
6.00E-07
Time m s
at
t = 0.1 ms, current amplitude = 0.777 of peak;
at
t = 0.2 ms, current amplitude = 1.01 of peak.
The maximum ‘overswing’ is 2.5%.
This system is much simpler and cheaper than the distributed system.
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Septum Magnets – ‘classic’ design.
Often (not always) located inside the vacuum and used to deflect
part of the beam for injection or extraction:
The thin 'septum' coil on the front
face gives:
•high field within the gap,
•low field externally;
Yoke.
Beam
Problems:
•The thickness of the septum must be
minimised to limit beam loss;
•the front septum has very high
current density and major heating
problems
Neil Marks; DLS/CCLRC
Single turn coil
Cockcroft Institute 2005/6, © N.Marks, 2006
Multiple septa
These engineering problems can be partially overcome by using
multiple septa magnets (the septa can get thicker as the beams
diverge).
eg – KEK (3 GeV beam):
Operation:
Beam:
Energy:
Field strength:
Effective length:
Field flatness:
Neil Marks; DLS/CCLRC
DC
H+
3.0 GeV
0.41067 T (SEPEX-1)
0.75023 T (SEPEX-2)
0.87418 T (SEPEX-3)
1.00530 T (SEPEX-4)
0.9 m
+/- 0.1 %
Cockcroft Institute 2005/6, © N.Marks, 2006
‘Opposite bend’ septa magnets
KEK also use ‘opposite bend’ septum magnets at 50
GeV:
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
Septum Magnet – eddy current design.
•uses a pulsed current through a
backleg coil (usually a poor design
feature) to generate the field;
•the front eddy current shield must be,
at the septum, a number of skin depths
thick; elsewhere at least ten skin
depths;
•high eddy currents are induced in the
front screen; but this is at earth
potential and bonded to the base plate
– heat is conducted out to the base
plate;
•field outside the septum are usually ~
1% of field in the gap.
Neil Marks; DLS/CCLRC
Eddy current
shield
-
+
Single or multi turn
coil
Cockcroft Institute 2005/6, © N.Marks, 2006
Comparison of the two types.
Classical:
Eddy current:
Excitation
d.c or low frequency pulse;
pulse at > 10 kHz;
Coil
single turn including
front septum;
single or multi-turn on
backleg, room for
large cross section;
Cooling
Yoke
complex-water spirals
in thermal contact with
septum;
conventional steel
Neil Marks; DLS/CCLRC
heat generated in
shield is conducted to
base plate;
high frequency
material (ferrite or
radio metal).
Cockcroft Institute 2005/6, © N.Marks, 2006
Example
Skin depth in material:
is given by:
resistivity r;
permeability m;
at frequency w
d = (2 r/wµµ0 )
Example: SRS injection eddy current septum.
Screen thickness (at beam height):
"
" (elsewhere)
Excitation
Skin depth in copper at 20 kHz
Neil Marks; DLS/CCLRC
1
mm;
10
mm;
25
µs,
half sinewave;
0.45 mm
Cockcroft Institute 2005/6, © N.Marks, 2006
SPS fast extraction (450 GeV)
The proposed extraction
septum system will consist of
six 3.2 m long magnets,
operating at a field of about
1.1 T at 450 GeV.
Peak field at 450 GeV/c:
1.078 T;
Magnetic length
6 x 3.2 m;
Kick at 450 GeV/c:
13.8mrad;
Pulse duration:
250 ms;
Septum thickness:`5 (Cu) + 1 (Fe) mm;
Peak current at 450GeV/c 17.16 kA
Peak voltage at 450 GeV/c: 3.40 kV;
Type:
eddy.
Note: twin vacuum systems!
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
‘Out of Vacuum’ designs.
Benefits in locating the magnet outside the vacuum.
But a (metallic) vessel has to be inserted inside the magnet -the
use of an eddy current design (probably) impossible.
eg the upgrade to the APS septum (2002):
‘The designs of the six septum magnets required for the APS facility have
evolved since operation began in 1996. Improvements .. have provided
better injection/extraction performance and extended the machine
reliability...’
‘Currently a new synchrotron extraction direct-drive septum with the
core out of vacuum is being built to replace the existing, in-vacuum eddycurrent-shielded magnet.’
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006
‘New’ APS septum magnet.
Synchrotron extraction septum conductor assembly partially installed in the laminated
core.
Neil Marks; DLS/CCLRC
Cockcroft Institute 2005/6, © N.Marks, 2006