A.C. Magnets (II)

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

A.C. Magnets (II)Neil Marks,

CCLRC,Daresbury Laboratory,Warrington WA4 4AD.

[email protected]


Philosophy1. 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.


Contents

ContentsCore Syllabus

Variations in design and construction for a.c. magnets;Effects of eddy currents;‘Low frequency’ a.c. magnetsCoil transposition-eddy loss-hysteresis loss;Properties and choice of steel;

Inductance in an a.c. magnet;

‘Fast’ magnets;Kicker magnets-lumped and distributed power supplies;Septum magnets-active and passive septa;

‘Extension’

Power supply systems – d.c. and a.c.;

Injection and extraction schemes;


High Frequency – Kicker MagnetsKicker 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;•single turn coil;•pulse current 104A;•pulse voltages of many kV.

beam

Conductors

Ferrite Core

Typical geometry:


Magnet & Power SupplyBecause 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.


Kickers - Distributed SystemStandard (CERN) delay line magnet and power supply:

dc

L, C L, C

Z0

Power Supply Thyratron Magnet Resistor

The power supply and interconnecting cables are matched to the surge impedance of the delay line magnet:


Mode of Operation•the first delay line is charged to V by the d.c. supply;

•the thyratron triggers, a voltages wave: V/2 propagates into magnet;

•this gives a current wave of V/( 2 Z )propagating into the magnet;

•the circuit is terminated by pure resistor Z, to prevent reflection.


Physical assemblyMagnet:Usually capacitance is introduced along the length of the magnet, which is split into many segments:

ie it is a pseudo-distributed line


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.


ParametersThe value of impedance Z (and therefore the added distributed capacitance) is determined by the required rise time of current:total magnet inductance = L; capacitance added = C;surge impedance Z0 = (L/C); transit time (t) in magnet = (LC);so Z0 = L/t; for a current pulse (I), V = 2 Z I ; = 2 I L / 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!


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!


Distributed power supply– lumped magnet

Ldc

R = Z

Z 0

0

I = (V/Z) (1 – exp (-Z t /L)


Example of such a kicker systemSNS facility (Brookhaven)– extraction kickers:• 14 kicker pulse power supplies & magnets;• operated at a 60 Hz

repetition rate;• kicks beam in 250 nS;• 750nS pulse flat top.


Extraction systems layout


Kicker p.f.n simulation model


Simulated current waveform


EEV Thyratron CX1925

EEVHV = 80kVPeak current 15 kArepetition 2 kHzLife time ~3 year


Kickers – Lumped Systems.•The magnet is (mainly) inductive - no added distributed capacitance;•the magnet must be very close to the supply (minimises inductance).

Ldc

R

I = (V/R) (1 – exp (- R t /L)

i.e. the same waveform as distributed power supply, lumped magnet systems..


Improvement on above

Ldc

R

C

The extra capacitor C improves the pulse substantially.


Resulting WaveformExample calculated for the following parameters:

mag inductance L = 1 H;rise time t = 0.2 s;resistor R = 10 ; trim capacitor C = 4,000 pF.

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: at t = 0.1 s, current amplitude = 0.777 of peak;

at t = 0.2 s, current amplitude = 1.01 of peak.The maximum ‘overswing’ is 2.5%.

This system is much simpler and cheaper than the distributed system.


Septum Magnets – ‘classic’ design.

Often (not always) located inside the vacuum and used to deflect part of the beam for injection or extraction:

Yoke.

Single turn coil

Beam

The thin 'septum' coil on the front face gives:•high field within the gap,•low field externally;

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


Multiple septaThese 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: DCBeam: H+Energy: 3.0 GeVField strength: 0.41067 T (SEPEX-1)

0.75023 T (SEPEX-2)0.87418 T (SEPEX-3)1.00530 T (SEPEX-4)

Effective length: 0.9 mField flatness: +/- 0.1 %


‘Opposite bend’ septa magnetsKEK also use ‘opposite bend’ septum magnets at 50 GeV:


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.


Comparison of the two types.Classical: Eddy current:

Excitation d.c or low frequency pulse; pulse at > 10 kHz;

Coil single turn including single or multi-turn onfront septum; backleg, room forlarge cross section;

Cooling complex-water spirals heat generated in in thermal contact with shield is conducted to septum; base plate;

Yoke conventional steel high frequency material (ferrite or radio metal).


ExampleSkin depth in material: resistivity ;

permeability ;at frequency

is given by: d = (2 /µµ0 )

Example: SRS injection eddy current septum.

Screen thickness (at beam height): 1 mm; " " (elsewhere) 10 mm;

Excitation 25 µs, half sinewave;

Skin depth in copper at 20 kHz 0.45 mm


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 s;Septum thickness:`5 (Cu) + 1 (Fe) mm;Peak current at 450GeV/c 17.16 kAPeak voltage at 450 GeV/c:3.40 kV;Type: eddy.

Note: twin vacuum systems!


‘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 providedbetter 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 eddy-current-shielded magnet.’


‘New’ APS septum magnet.

Synchrotron extraction septum conductor assembly partially installed in the laminated core.

A.C. Magnets (II)

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