Download - AC Magnets

7/27/2019 AC Magnets

1/29

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]


2/29


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.


3/29


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;


4/29

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


5/29


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.


6/29


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:


7/29


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.


8/29


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


9/29


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.


10/29


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!


11/29


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!


12/29


Distributed power supplylumped magnet

Ldc

R = Z

Z0

0

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


13/29


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


14/29


Extraction systems layout


15/29


Kicker p.f.n simulation model


16/29


Simulated current waveform


17/29


EEV Thyratron CX1925

EEV

HV = 80kV

Peak current 15 kArepetition 2 kHz

Life time ~3 year


18/29


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..


19/29


Improvement on above

Ldc

R

C

The extra capacitor C improves the pulse substantially.


20/29


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.


21/29


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


22/29


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 %


23/29


Opposite bend septa magnets

KEK also use opposite bend septum magnets at 50

GeV:


24/29

Neil Marks; DLS/CCLRC

Cockcroft Institute 2005/6, N.Marks, 2006

Septum Magneteddy current design.

uses a pulsed current through a

backleg coil (usually a poor designfeature) 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.

- +

Single or multi turn

Eddy currentshield


25/29



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 on

front septum; backleg, room for

large 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).


26/29



Example

Skin depth in material: resistivity r;

permeability m;

at frequency w

is given by: d = (2 r/w0 )

Example: SRS injection eddy current septum.

Screen thickness (at beam height): 1 mm;

" " (elsewhere) 10 mm;

Excitation 25 s,

half sinewave;

Skin depth in copper at 20 kHz 0.45 mm


27/29



SPS fast extraction (450 GeV)

The proposed extraction

septum system will consist ofsix 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!


28/29



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 haveevolved 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 eddy-

current-shielded magnet.


29/29



New APS septum magnet.

Synchrotron extraction septum conductor assembly partially installed in the laminated

core.