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

    A.C. Magnets (II)Neil Marks,

    CCLRC,Daresbury Laboratory,

    Warrington WA4 4AD.

    n.marks@dl.ac.uk

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    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 cyclingaccelerators).

    4. Give a qualitative overview of injection and extraction techniques as usedin circular machines.

    5. Present the standard designs for kicker and septum magnets and theirassociated power supplies.

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

    ContentsContents

    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;

    Inductance in an a.c. magnet;

    Fast magnets;

    Kicker magnets-lumped and distributed power

    supplies;

    Septum magnets-active and passive septa;

    Extension

    Power supply systemsd.c. and a.c.;

    Injection and extraction schemes;

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    High FrequencyKicker Magnets

    Kicker Magnets:

    used for rapid deflection of beam for injection or extraction;

    usually located inside the vacuum chamber;

    rise/fall times

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    Neil Marks; DLS/CCLRCCockcroft 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.

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Kickers - Distributed System

    Standard (CERN) delay line magnet and power supply:

    dc

    L, C L, C

    Z 0

    Power Supply Thyratron Magnet ResistorThe power supply and interconnecting cables are matched to the surge

    impedance of the delay line magnet:

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    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.

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    Neil Marks; DLS/CCLRCCockcroft 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 pseudo-distributed line

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    Neil Marks; DLS/CCLRCCockcroft 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.

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    Neil Marks; DLS/CCLRCCockcroft 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 = 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 valuethe distributed

    capacitance has not slowed the pulse down!

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    Neil Marks; DLS/CCLRCCockcroft 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!

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Distributed power supplylumped magnet

    Ldc

    R = Z

    Z0

    0

    I = (V/Z) (1exp (-Z t /L)

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    Neil Marks; DLS/CCLRCCockcroft 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

    repetition rate;

    kicks beam in 250 nS;

    750nS pulse flat top.

    kicker magnet inductance 0.76 -0.8 uH

    magnet current 2 - 2.5 kA

    blumlein PFN Voltage 35 kV

    pulse current rise time 200nS

    current pulse width 750 nS

    pulse repetition 60 Hz

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Extraction systems layout

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Kicker p.f.n simulation model

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Simulated current waveform

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    EEV Thyratron CX1925

    EEV

    HV = 80kV

    Peak current 15 kArepetition 2 kHz

    Life time ~3 year

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    KickersLumped 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) (1exp (- R t /L)

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

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Improvement on above

    Ldc

    R

    C

    The extra capacitor C improves the pulse substantially.

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Resulting Waveform

    Example calculated for the following parameters:

    0

    0.2

    0.4

    0.6

    0.8

    1

    1.2

    0.00E+00 2.00E-07 4.00E-07 6.00E-07

    Time ms

    mag inductance L = 1 mH;rise time t = 0.2 ms;resistor R = 10 W;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 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.

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Septum Magnetsclassic 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 heatingproblems

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    Neil Marks; DLS/CCLRCCockcroft 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 beamsdiverge).

    egKEK (3 GeV beam):

    Operation: DC

    Beam: H+Energy: 3.0 GeV

    Field 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 m

    Field flatness: +/- 0.1 %

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    Neil Marks; DLS/CCLRCCockcroft Institute 2005/6, N.Marks, 2006

    Opposite bend septa magnets

    KEK also use opposite bend septum magnets at 50

    GeV:

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