Pow er Converters
N eil Marks
STFC ASTeC/ Cockcroft Institu te/ U. of
Liverpool,
Daresbu ry Laboratory,
Warrington WA4 4AD,
U.K.
n.m arks@d l.ac.u k
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CAS, Prague 2014
Contents
1. Requirements.
2. Basic elements of pow er supplies.
3. D .C. supplies:
i) simple rectification w ith diodes;
ii) phase controlled rectifiers;
iii) sw itch mode systems.
4. Cycling converters - w hat do w e need:
i) energy storage;
ii) w aveform criteria;
5. So how do w e do it:
i) slow cycling accelerators;
ii) medium and fast cycling – inductive storage;
iii) ‘modern’ medium cycling – capacitative storage;
6. The delay-line mode of resonance.
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1. Basic Requ irem ents
1. Typical requ irem ents for d .c. applications (storage rings,
cyclotrons, beam -lines, etc.):
• sm ooth d c (ripple < 1:105);
• am plitu d e stability betw een 1:104 and 1:105;
• am plitu d e ad ju stm ent over operating range (often 1:10 ).
2. Ad d itionally, for accelerating synchrotrons:
• energy storage (essential so as not to d issipate stored
energy at peak field w hen ‘resetting’ for next injection)
• am plitu d e control betw een m inim u m and m axim u m
cu rrent (field );
• w aveform control (if possible).
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2 - Basic com ponents.
Generic stru ctu re of a ‘Pow er Converter’:
regulation
(level setting
-usually a ‘servo
system’)
transformer
monitoring
rectifier
sw itch-gear
smoothing
feedback
control room
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LOAD
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Typ ical com p onents (cont.)
i) sw itch-gear:
• on/ off;
• protection against over-cu rrent/ over-voltage/ earth
leakage etc.
ii) transform er:
• changes voltage – ie m atches im ped ance level;
• provid es essential galvanic isolation load to su pply;
• three phase or (som etim es 6 or 12 phase);
iii) rectifier/ sw itch (p ow er electronics):
• u sed in both d .c. and a.c. su pplies;
• nu m ber of d ifferent types – see slid es 7, 8, 9, 10;
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Typ ical com p onents (cont.)
iv) regu lation:
• level setting;
• stabilisation w ith high gain servo system ;
• strongly linked w ith ‘rectifier’ [item iii) above];
v) sm oothing:
• u sing either a passive or active filter;
vi) m onitoring:
• for feed -back signal for servo-system ;
• for m onitoring in control room ;
• for fau lt d etection.
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Sw itches - d iod es
10 A; 300 V
75 V; 0.15 A
•
•
•
•
•
cond u cts in forw ard d irection only;
350 A; up to 2.5 kV
m od ern pow er d evices can cond u ct in ~ 1 ms;
has voltage d rop of (< 1 V) w hen cond u cting;
hence, d issipates pow er w hilst cond u cting;
ratings u p to m any 100s A (average), kVs peak reverse volts.
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Sw itches - thyristors
+
-
• Withstand s forw ard and reverse volts;
• then cond u cts in the forw ard d irection w hen the
gate is pu lsed ;
• cond u cts u ntil cu rrent d rops to zero and reverses ( to ‘clear’ carriers);
• after ‘recovery tim e’, again w ithstand s forw ard voltage;
• sw itches on in ~ 5 ms (d epend s on size) – as the forw ard voltage
d rops, it d issipates pow er as cu rrent rises;
• therefore d I/ d t lim ited d u ring early cond u ction;
• available ratings are 100s A average cu rrent, kVs forw ard and
reverse volts.
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Sw itches - thyristors
+
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-
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Sw itches – i.g.b.t. s
The insu lated gate bi-polar transistor (i.g.b.t.):
1.2 kV
module
• gate controls cond u ction, sw itching the d evice on and off;
• far faster than thyristor, can operate at 10s of kH z;
• d issipates significant pow er d u ring sw itching;
• is available at ~ 2 kV forw ard , 100s A average.
• w ill not w ithstand appreciable reverse volts (a series blocking d iod e
som etim es need ed );
• w ill not cond u ct reverse cu rrent (sometimes a parallel reverse ‘freew heeling’ d iod e is need ed ).
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3. DC Su p p lies
A single phase fu ll-w ave rectifier:
+

Classical ‘fu ll-w ave’ circu it:
-
• u ncontrolled – no am plitu d e variation or control;
• large ripple – large capacitor sm oothing necessary;
• only su itable for sm all load s.
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DC – a 3 phase d iod e rectifier
Rectifier
Fast switch
Lf
Lf
Three phase
transformer
3 phase I/p
Cf
Cf
1Vdc
period
Three p hase, six pu lse system :
Vsw
Lf
Lf
• no am plitu d e control;
• m u ch low er rip ple (~ 12% of 6th harm onic – 300 H z)
bu t low -p ass filters still need ed .
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Thyristor phase control
Replace d iod es w ith thyristors - am plitu d e of the ou tpu t
voltage is controlled by retard ing the cond u ction phase:
V out
Zero volts ou tpu t
V out
Fu ll cond u ction – like d iod e
N egative volts ou tpu t-‘inversion’.
V out
H alf cond u ction
But current must alw ays be in the forw ard direction.
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V out
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Fu ll 12 pu lse phase
controlled circu it.
Lf
Iload
Ii
Vi
Cf
Ipi
Lf
LOAD
3 phase i/p
11kV or 400V
Iii
Lf
Cf
Vload
Vii
Lf
• like all thyristor rectifiers, is ‘line com m u tated ’;
• prod u ces 600 H z ripple (~ 6%)
• sm oothing filters still need ed .
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The thyristor rectifier.
The ‘stand ard ’ circu it u ntil recently:
• gave good precision (better than 1:103);
• inversion protects circu it and load d u ring fau lts;
• has bad pow er factor w ith large phase angles (V and I
ou t of p hase in ac su pply) ;
• injected harm onic contam ination into load and 50 H z
a.c. d istribu tion system at large phase angles.
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Mod ern d .c. ‘sw itch-m od e’ system .
The i.g.b.t. allow s a new , revolu tionary system to be u sed :
the ‘sw itch-mode’ pow er su pply (see you r m obile phone
charger!):
50Hz Mains
Network
Rectifier
Inverter (kHz)
H.F.
Transformer
H.F.
Rectifier
Passive Filter
D.C. Output
DCCT
Load
D.C Bus
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Mod e of operation
Stages of p ow er conversion:
• incom ing a.c. is rectified w ith d iod es to give ‘raw ’ d .c.;
• the d .c. is ‘chopped ’ at high frequ ency (> 10 kH z) by an
inverter/ chopper u sing i.g.b.t.s;
•
a.c. is transform ed to requ ired level;
1
• transform er size is ∝ (determined by 𝜕/𝜕𝑡 in transform er core) so

is m u ch sm aller and cheaper at high frequ ency ;
• transform ed a.c. is rectified – d iod es;
• filtered (filter is m u ch sm aller at 10 kH z);
• regu lation is by feed -back to the inverter (m u ch faster, therefore
greater stability and faster protection);
• response and protection is very fast.
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Inverter – or ‘Chopper’
The inverter is the heart of the sw itch -m od e su pply:
+
B
A
-
+
-
-
+
The i.g.b.t. s
provid e fu ll
sw itching
flexibility –
sw itching on
or off
accord ing to
external
control
protocols.
Point A: d irect voltage sou rce; cu rrent can be bid irectional (eg,
ind u ctive load , capacitative sou rce).
Point B: voltage squ are w ave, bid irectional cu rrent.
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4. Cycling convertersw hat d o w e need to d o?
We need to raise the m agnet cu rrent d u ring acceleration - w ill
‘ord inary’ A.C. d o?
Bu t the requ ired m agnetic field (therefore the requ ired m agnet cu rrent)
is unidirectional – acceleration low to high energy: - so ‘normal’ a.c. is
inappropriate:
extraction
1
•
•
•
•
only ¼cycle u sed ;
excess rm s cu rrent;
high a.c. losses;
high grad ient at
injection.
injection
0
0
7
-1
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N atu re of the Magnet Load
R
LM
IM
C
Magnet cu rrent:
Magnet voltage:
Series ind u ctance:
Series resistance:
Distribu ted capacitance to earth
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IM ;
VM
LM ;
R;
C.
VM
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‘Reactive’ Pow er and Energy
voltage:
VM
= R IM + L (d IM /dt);
‘pow er’:
V M IM
= R (IM )2 + L IM (d IM /dt);
stored energy:
EM
= ½ LM (IM )2;
d EM /dt
= L (IM ) (d IM /dt);
V M IM
= R (IM )2 + d EM /dt;
so
resistive
power loss;
reactive’ power –
alternates between +ve
and –ve as field rises and
falls;
The challenge of the cyclic pow er converter is to provid e and
control the positive and negative flow of energy - energy
storage is requ ired .
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Waveform criteria
– ed d y cu rrents .
Generated by alternating m agnetic field cu tting a
cond u cting su rface:
ed d y cu rrent in vac. vessel & m agnet;  B/ t;
ed d y cu rrents prod u ce:
• negative d ipole field - red u ces m ain field m agnitu d e;
• sextu pole field – affects chrom aticity/ resonances;
ed d y effects proportional (1/ B)(d B/ d t) – critical at
injection.
B/t
B
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Waveform criteria
– d iscontinu ou s operation
Circu lating beam in a storage ring slow ly d ecays
w ith tim e – very inconvenient for experim ental
u sers.
Solu tion – ‘top up mode’ – d iscontinu ou s operation by
the booster synchrotron – beam is only accelerated and
injected once every n booster cycles, to m aintain
constant cu rrent in the m ain ring.
1.5
1.5
0
0
0
-1.5
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1.5
0
0
10
1.5
10
-1.5
time
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0
0
-1.5
10
0
-1.5
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10
Fast and slow cycling accelerators.
‘Slow cycling’:
• repetition rate 0.1 to 1 H z (typically 0.3 H z);
• large proton accelerators;
‘Fast cycling’:
• repetition rate 10 to 50 H z;
• com bined fu nction electron accelerators (1950s and
60s) and high cu rrent m ed iu m energy proton
accelerators;
‘Medium cycling’:
• repetition rate 1 to 5 H z;
• separated fu nction electron accelerators;
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Exam ple 1 –
the CERN SPS
A slow cycling synchrotron.
Original d ipole pow er su pply param eters (744 m agnets):
•
•
•
•
•
•
•
p eak proton energy
cycle tim e (fixed target)
p eak cu rrent
p eak d I/ d t
m agnet resistance
m agnet ind u ctance
m agnet stored energy
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450
8.94
5.75
1.9
3.25
6.6
109
GeV;
secs;
kA;
kA/ s;
;
H;
MJ;
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SPS Cu rrent w aveform
7000
current (A)
6000
5000
4000
3000
2000
1000
0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
time (s)
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SPS Voltage w aveform s
40.0
Total volts
voltage (kV)
30.0
20.0
10.0
0.0
-10.0
0.0
-20.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Inductive volts
-30.0
time (s)
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SPS Magnet Pow er
200.0
power (MVA)
150.0
100.0
50.0
0.0
-50.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
-100.0
time (s)
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Exam ple 2 – N IN A (ex D.L.)
A fast cycling synchrotron
Origional m agnet pow er su pply param eters;
•
•
•
•
•
•
•
p eak electron energy
cycle tim e
cycle frequ ency
p eak cu rrent
m agnet resistance
m agnet ind u ctance
m agnet stored energy
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5.0
20
50
1362
900
654
606
GeV;
m secs;
Hz
A;
m ;
mH;
kJ;
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N IN A Cu rrent w aveform
Current (A)
1500
1000
500
0
0.0
5.0
10.0
15.0
20.0
time (ms)
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N IN A Voltage w aveform
200
Inductive voltage
150
Voltage (kV)
100
Resistive voltage
50
0
-50 0.0
5.0
10.0
15.0
20.0
-100
-150
-200
time (ms)
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N IN A Pow er w aveform
150
Power (MVA)
100
50
0
-50
0.0
5.0
10.0
15.0
20.0
-100
-150
time (ms)
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Cycling converter requ irem ents
Su m m ing u p - a pow er converter system
need s to provid e:
•
•
•
•
•
•
a u nid irectional alternating w aveform ;
accu rate control of w aveform am p litu d e;
accu rate control of w aveform tim ing;
storage of m agnetic energy d u ring low field ;
if possible, w aveform control;
if need ed (and if possible) discontinuous
operation for ‘top u p m od e’.
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5. Cycling convertersso how d o w e d o it?
It d ep end s on w hether w e are d esigning for:
Slow ;
Medium; or
Fast;
cycling accelerators.
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‘Slow Cycling’ Mechanical Storage
Thryistor w aveform
control – rectifying and
inverting (see slide 13.
d .c. m otor
to m ake
u p losses
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high
inertia flyw heel to
store
energy
a.c
alternator/
rectifier/
magnet
synchronou s
inverter
m otor
Examples: all large proton
accelerators built in 1950/60s.
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‘N im rod Pow er Su pply’
of the 7 GeV w eak-focu sing
synchrotron, N IMROD – note
– tw o u nits, back to back.
The alternator/
synchronou s m otor.
fly-w heel
d .c. m otor
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‘Slow cycling’ direct connection
to supply netw ork
N ational su pply netw orks have large stored (ind u ctive)
energy; w ith the correct interface, this can be u tilised to
provid e and receive back the reactive pow er of a large
accelerator.
Com pliance w ith su pply au thority regu lations m u st
m inim ise:
• voltage ripple at feed er;
• phase d istu rbances;
• frequ ency flu ctu ations over the netw ork.
A ‘rigid ’ high voltage line in is necessary.
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Exam ple – SPS Dipole su pply
14 converter m od u les (each 2
sets of 12 p u lse phase
controlled thyristor
rectifiers) su pply the ring
d ipoles in series; w aveform
control!
Each m od u le is connected to
its ow n 18 kV feed er, w hich
are d irectly fed from the 400
kV French netw ork.
Satu rable reactor/ capacitor
parallel circu its lim it voltage
flu ctu ations.
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Medium & fast cycling inductive storage.
Fast and m ed iu m cycling accelerators (m ainly
electron synchrotrons) d eveloped in 1960/ 70s
u sed ind u ctive energy storage:
ind u ctive storage w as rou ghly half the cap ital
cost per stored kJ of capacitative storage.
The ‘stand ard circu it’ w as d eveloped at
Princeton-Pen accelerator – the ‘White Circu it’.
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White Circu it – single cell.
Energy
storage
choke
LCh
a.c.
C2
C1
su pply
accelerator
magnets
LM
DC
Supply
Examples: Boosters for ESRF, SRS; (medium to fast cycling
‘small’ synchrotrons).
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White circu it (cont.)
Single cell circu it:
•
•
•
•
•
m agnets are all in series (LM );
circu it oscillation frequ ency ;
C 1 resonates m agnet in parallel: C 1 = 2/ LM ;
C 2 resonates energy storage choke:C 2 = 2/ LCh ;
energy storage choke has a prim ary w ind ing
closely cou pled to the m ain w ind ing;
• only sm all ac p resent in d .c. sou rce;
• no d .c. present in a.c sou rce;
• N O WAVEFORM CON TROL.
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White Circu it m agnet w aveform
Magnet cu rrent is biased sin w ave – am plitu d e of IAC and
1.5controlled .
IDC ind epend ently
Usu ally fu lly
biased ,
so IDC ~ IAC
IAC
0
0
IDC
0
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-1.5
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Multi-cell White Circuit
(N IN A, D ESY, CEA & others)
L M
For high voltage circu its,
the m agnets are
segm ented into a nu m ber
of separate grou ps.
C
L Ch
L M
L Ch
C
dc
earth point
ac
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Mu lti-cell White circu it (cont.)
Benefits for an ‘n’ section circu it
• m agnets are still in series for cu rrent continu ity;
• voltage across each section is only 1/ n of total;
• m axim u m voltage to earth is only 1/ 2n of total;
• choke has to be split into n sections;
• d .c. is at centre of one split section (earth point);
• a.c. is connected throu gh a paralleled prim ary;
• the paralleled prim ary must be close cou pled to
second ary to balance voltages in the circu it;
• still N O w aveform control.
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Mod ern Capacitative Storage
For Medium cycling accelerators:
Technical and econom ic d evelopm ents in electrolytic capacitors
m anu factu re now resu lt in capacitiative storage being low er cost than
ind u ctive energy storage (p rovid ing voltage reversal is not need ed ).
Sem i-cond u ctor technology now allow s the u se of fu lly sw itchable
i.g.b.t. ‘choppers’ (see slid e 18) to control the transfer of energy to and
from the m agnet giving w aveform control.
Med iu m sized synchrotrons (cycling at 1 to 5 H z) now u se this
d evelopm ent for cheaper and d ynam ically controllable system s.
Waveform Control & D iscontinuous Operation!
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Example: S.L.S. Booster dipole circuit.
DC
CHARGING
STORAGE
CAPACITOR
TWO
QUADRANT
CHOPPER
FILTER
MAGNET
acknowledgment :Irminger, Horvat, Jenni, Boksberger, SLS
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SLS Booster param eters
Com bined fu nction
d ipoles
48 BD
45 BF
Resistance
600
m
Ind u ctance
80
mH
Max cu rrent
950
A
Stored energy
28
kJ
Cycling frequ ency
3
Hz
acknowledgment :Irminger, Horvat, Jenni, Boksberger, SLS
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35 0
30 0
25 0
20 0
15 0
10 0
50
1
P O W E R [k W ]
750
500
250
0
-250
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35 0
30 0
25 0
20 0
15 0
10 0
1000
50
1
C U R R E N T [A ] / V O L T A G E [V ]
SLS Booster Waveform s
1000
750
500
250
0
-250
-500
SLS Booster Waveform s
The storage capacitor only d ischarges a fraction of its
stored energy d u ring each acceleration cycle:
600
2 Q in p u t v o lta g e
500
[V ]
400
300
200
d c /d c in p u t c u r r e n t
100
0
[A ]
0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
0 .9
1
T IM E [s ]
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Assessm ent of sw itch-m od e circu it
Com parison w ith the White Circu it:
• the s.m . circu it d oes not need a costly energy storage
choke w ith increased pow er losses;
• w ithin lim its of rated cu rrent and voltage, the s.m .c.
provid es flexibility of ou tpu t w aveform ;
• after sw itch on, the s.m .c. requ ires less than one second
to stabilise (valu able in d iscontinu ou s ‘top u p’ m od e).
H ow ever:
• the cu rrent and voltages possible in sw itched circu its
are restricted by com ponent ratings.
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Diam ond 3 GeV Booster
param eters for SLS typ e circu it
Parameter
Number of turns per dipole:
Peak current:
Total RMS current (for fully biased sine-wave):
Conductor cross section:
Total ohmic loss:
Inductance all dipoles in series:
Peak stored energy all dipoles:
Cycling frequency:
Peak reactive alternating volts across circuit:
low turns
16
1271
778
195
188
0.091
73.3
5
1.81
high turns
20
1016
622
156
188
0.142
73.3
5
2.26
A
A
2
mm
kW
H
kJ
Hz
kV
N ote: operating frequ ency higher than the SLS; the 16 or 20
tu rn options w ere consid ered to ad ju st to the cu rrent &
voltage ratings available for capacitors and sem i-cond u ctors.
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6. Delay-line m od e of resonance
Most often seen in cycling circu its (high field d istu rbances
prod u ce d istu rbance at next injection); bu t can be present
in any system .
Stray capacitance to earth m akes the ind u ctive m agnet
string a d elay line. Travelling and stand ing w aves (cu rrent
and voltage) on the series m agnet string: different current
in dipoles at different positions!
BAD !
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Stand ing w aves in m agnets chain.
im
vm
Fundamental
voltage
current
1.5
current
2nd 0
harmonic
0
voltage
-1.5
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Delay-line m od e equ ations
LM is total m agnet ind u ctance;
C is total stray capacitance;
C
Then:
su rge im ped ance:
Z
R
LM
=
v m / im =
(LM / C);
transm ission tim e:

=
(LM C);
fu nd am ental frequ ency:
1
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1/ { 2 (LM C) }
CAS, Prague 2014
Excitation of d .l.m .r.
The m od e w ill only be excited if rapid voltage-toearth excu rsions are ind u ced locally at high
energy in the m agnet chain (‘beam -bu m p s’); the
next injection is then com prom ised :
propagation
V
• keep stray capacitance as low as possible;
• avoid local d istu rbances in m agnet ring;
• solu tions (d am ping loops) are possible.
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The End!
May the Pow er be w ith you!
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CAS, Prague 2014