This topic was not presented at the School but is included here to widen the scope of the syllabus. OCR Output

fast rise time and precise current f`1at—top regulation) are to be compared with some penalizing features (pulsed low

magnets, possibility of pulse-to-pulse current modulation in time and amplitude, possibility of multiple pulsing,

The advantages of pulsed power converters (high efficiency and reduced power consumption, compact

industrial applications (< 25 kA) [3,4].Complete fast cycling ultra-compact accelerators and beam extraction gantry systems for medical and

Special radiation-hard post target magnets (< 100 kA)

Particle collecting and matching solenoids behind e+ production targets (< 20 kA)

Super strong quadrupoles for colliding beam focusing (< 0.5 MA) [2]

for final focus schemes (< 1.2 MA)Lithium and plasma lenses either to focus the primary p beam in front of the target, or to collect p- or

Coaxial lenses to collect neutrino parents or p- behind a target (< 0.5 MA)

Current carrying targets for p- production (< 0.5 MA)

Thin monoturn septum magnets (< 50 kA)

Quadrupoles for gamma transition schemes (< 2 kA)

Magnets for local excitation of beam oscillations for ejection (< 2 kA)

Switching magnets for beam distribution or recombination (< 1 kA)

Switching magnets to select final beam destination (< 3 kA)

Transversal beam position scanning dipoles (< 200 A)

Correction and steering dipoles (< 100 A), [1]

Beam transport bending and quadrupole magnets (I < 3 kA)

levels are given to characterize the equipment):

accelerator operations, as shown by the following non-exhaustive list of possible applications (usual current

in very compact deflecting or focusing devices and to perform particular time dependent current patterns for

These power converters are used, instead of dc or programmed converters, to achieve high magnetic fields

current pulses with a duration of up to several ten ms and an amplitude of over 1.2 MA.

discharge of either lumped element Pulse Forming Networks (PFN) or of capacitor banks, designed to produce

Under the term 'pulsed power converters' one understands the family of apparatus based on the charge

1. INTRODUCTION

trends mentioned while specialized reports are referred to for more technical details.

parts and electronics. Present technical solutions are described and development

converters classified according to the specificity of their basic constituent power

The applications whcrc pulscd power converters are used are recalled and the

ABSTRA

CERN, Gcncva, Switzerland

F.V0clkcr

AN LNTKODUCIORY OVERVIEW!

BULSED@AC1'l`OR DISCHARGE POWER CONVERTER

— 315

special low-inductive pulse capacitors. OCR Output

duration with rise and fall time better than 0.1 tts at a repetition frequency of 1 Hz (Fig. 4) [6]. They require

Lumped element PFN‘s (Ro = 25 Ohm) have been designed to produce current pulses of up to 200 tts

2.2

controller on primary of stepping-up transformer

> 0.3 6—pulse or 12—pulse (if > 200 kW) thyristor

0.05 - 0.3 Higher frequency chopper

interruption or 'deQuing'

< 0.05 Resonant charging with controlled charge

(S)

Charging time | Implemented charging circuit solutions

Charging circuit classification

Table 1

power ratings.

designs foresee charging at constant active input power to alleviate mains loading, especially in the case of higher

mean value of the charging current is kept constant for linear charging of the energy storage element. More recent

a higher frequency chopper (Fig. 2) and linear charging (Fig. 3) [3]. In general for charging times > 0.3 s the

converter. As indicated in Table 1, the most common methods are resonant capacitor charging (Fig. l) [5], use of

The type of charging circuit depends both on the time available and on the power level of the pulsed

2.1 Charging circui

iv) electronics to fulfil the control, monitoring, timing and regulation functions.

current flat-top regilation, a pulse transmission line and a load impedance matching transformer:

iii) a discharge circuit, possibly including an energy conversion or recovery unit, an active filter for

ii) an energy storage PFN or capacitor bank, possibly with third harmonic current pulse shaping;

i) a mains fed energy supply and charging circuit;

subassemblies, namely:

To classify the pulsed power converters one can consider them as consisting of a number of basic functional

ENE

E B KTERS FOR CABACITOR CLLARGE

role in the frame of future very high energy linear lepton colliders.

technology has accompanied the evolution of particle accelerators and will certainly play an even more important

thanks to the high adaptability of designs for an ever increasing number of applications, pulsed power converter

control the charge-discharge sequence, less conventional technology and mode of operation). Nevertheless,

cos ¢ power demand from the mains, higher ac current harmonics content, need of accurate timing pulses to

- 316 —

assembly for the AAC lithium lens pulser OCR OutputPig. 3 Converter with linear charging through thyristor controller on primary of transformer rectifier

Auto · TRANSFORHER

IHPEDAIIEE ADAPTING

IZ PULSE (NAREW6 SEIYION HU II UIEREV STORAGE DISUIARGE SWITCHES

AHA

AHA

septum magnets at ESRF (presently under construction)Fig. 2 Power convener with higher frequency chopper charging for the storage ring injection

AND LDADINVERTER AND RECTIFIERRESDNANT TRANSFORMER DISCHARGE CIRCUIT TRANSFORMERBUFFER [APACITCIRH -SHAFED STEP-UP ENERGY STORAGE AND MATCHINGDE SUPPLV AND

Fig. 1 Power convener with resonant charging within 8 ms for e-e+ convener solenoid of the LEP injector

swtutts

ul!write uncrrcu tomauumtn nwuttn un RHTIFIEH nlsttunsr

maui! Arm rurtuntiut suvnv Ann M-sunzn Hmtrn STEP - ur vsuusrnlmtn | 'rnvnnnu

ri ] II ‘| »?*

nttovtlv (Intuit

unuton nm

(NERGY SVUIAGE

· 317

required. OCR Output

thyratrons are still used where high voltage, high current and di/dt, fast rise time and pulse repetition rate are

thyratrons or, more rarely, by ignitron switches. Ignitrons have practically been replaced by thyristors while

The PFN or the energy storage capacitor bank is discharged into the magnet load by means of thyristors,

2.3 Discharge circui

different columns.

N.B. Tables 2 and 3 list different characteristics of pulsed power converters without systematic relation between lines of

E > 20 kJ Integrated LC (Fig.5 (2))U > 10 kV

l kJ < E < 20 kJ1 kV <U< l0 kV Special I Separate LC (Fig.5 (1))

E < 1 kJU < l kV (LV) Industrial < 20 kJ | Not present

Voltage level Stored energy I Type of capacitors I Third harmonic

Energy storage circuit classification

Table 2*)

lithium lens (see Fig.3) [8]. A tentative classification of the energy storage circuits is shown in Table 2.

safety, a power converter has been recently built with a capacitor bank of 200 kJ for the pulser of the p- collecting

Conceming the maximum stored energy, which is kept to about 20 kJ per cubicle for reasons of industrial

a third harmonic component of given amplitude to the basic sinusoidal discharge current (Fig.5).

the capacitor bank is subdivided into two parts, or an extra parallel LC circuit is added to it, in order to superpose

specified as industrial 50 Hz ac units with appropriate ratings to simplify their procurement [7]. In certain cases

aluminium armatures and either natural (mineral or castor oil) or synthetic oil impregnation. These capacitors are

common energy storage capacitor for pulsed applications is the mixed dielectric type (plastic film, paper) with

When approximately sinusoidal current pulses are required, simple capacitor banks are used, The most

of the IDIS power converterFig. 4 Lumped element, 28-cell, PFN energy storage for fast current pulses of 200 its

AND MAGNET TRANSMISSION LINE

r MATCHING IIESISTUR COAXIAL PULSE

Ra I[n•cn•1 Icn-1 IE1

Tn1I:::I (:::)rnz

Ln I Ln I Ln-1 I L1°I·'Iat IA

PULSE FORHING NETWORK (Z7 CELLS I SWITEHINE HDDULESPFII CHANGING SECTION

— 318

capacitance and of the circuit resistances [9,10]. OCR Output

due to residual ripple of charging voltage or current shape, and to non-compensated thermal drift of the

with its separate supply and isolated drive, acting in a fast servo·loop and absorbing any peak current variation

components and with a choke for the insertion of an active filter (Fig.7). This is a MOS-amplifier of class C,

better than 10*4 is required, the energy storage section is equipped with additional third harmonic pulse-shapingIn applications where a current plateau of Tp 2 100 tts duration and current stability and reproducibility DI/I

in parallel and discharged in sequence.

the capacitors by means of an auxiliary circuit or the presence of multiple discharge branches, which are charged

voltage after energy recuperation). Double pulsing within the same operating cycle requires either fast recharge of

dissipated between pulses (if the subsequent peak current value is expected to be smaller than that produced by the

When pulse—to·pulse peak magnet-current modulation is required, the residual energy in the capacitors is

the degree of voltage reversal on the capacitors.

load (3) or through the charging choke (4)

the energy recuperation method, i.e. through an auxiliary inductance (1,2), through the magnet

the type of magnet and the degree of magnet current reversal

A number of different discharge schemes are in use, as shown in Fig.6, depending on:

electrical circuit characteristics and waveformsFig. 5 Superposition of third harmonic component on main discharge pulse:

?uL,’ \_`\ $4` "7ur, \X LJ"II\~\I"`', \

_ . .r " ’` 1 \\<n u, L, \ \ ,»`_

-/x _ \ .··c··l·—-. }"'¤`LUlu

`\ IIic 2\I \

. —. y · 2 ‘‘ aK, ,°.. 1 l/`*’K\\ +‘* `\.\ +‘*

_ "j/_ L.x/ it;\

;L·§ all tI A ./ jk z

r,,, tu,

cu tt?Tly,

. T lt_+r¤. -, t, _ , ‘¤~ 1; ·I·4¤,·, I .t c T in ul' 1* ""‘*t r »T»““‘

ut.;ul.; ul

- 319 —

the discharge circuits is given in Table 3. OCR Output

auto-transformer followed by an 18:1 toroidal pulse transformer with dc current bias. A possible classification of

have been used for pulsed septum magnet applications. The lithium lens pulser (1.2 MA, 4kV) [8] has a 1.5:1

with the dc component of the excitation current. Transformers with turns-ratio of 6:1, 8:1, 10:1, 12:1, 20:1, 24:1

or via an auxiliary tertiary winding, or an air gap of the order of 5 mm are foreseen in the magnetic circuit to cope

interleaved windings, located on the central limb, for minimum stray inductance. Either a dc current bias, directly

duration which is proportional to N1t(LC)m. The transformer has normally a three-limb construction with

to the magnet. The transformer turns-ratio determines both the discharge current amplitude (I/N) and its pulse

primary of an impedance matching transfomier with a turns-ratio N11, whose high current secondary is connected

The discharge circuit can either feed the magnet directly through a pulse transmission line or feed the

Fig. 7 Active filter insertion in circuits equipped with third harmonic pulse-shaping element

FLAT - TDP

FOR CURRENT

AUIVI FILTERUIAREIPIE UREIIIT

ADA

*1 $ fz

(lR(Ul\'

AIID DISUIAIEE

[NERGY STREAM

THIRD IIIIIIDIIIK

Fig. 6 Example of discharge circuit layouts used in pulsed power converters

manu

I: I

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-320

septum magnets with complex operating requirements OCR OutputFig. 8 Advanced pulsed power converter layout for beam injection/ejection monotum

CURRENT FLAT · TDP AND SEPTUH MAGNETAND FAST RECIIARGE CIRCUIT

ACTIVE FILTER FOR IIATCIIING TRANSFORHERREST ENERGY IZIISSIPATINGIASIC (IIARGINE CIRCUIT

AHA

N Z I

<¤ $ t ¢ r

DISCNARGE CIRCUIT

ENERGY STORAGE AND

TNIRD IIARHORIK

inserted between the load and the pulse uansmission line.

of a MOS active filter. 'I`he septum magnet requires a current of up to 40 kA and a 12:1 matching transformer isand pulse·to-pulse current amplitude modulation. A current regulated high stability flat top is obtained by meanstimes 2 1.2 s. A residual energy dissipation and a fast recharge circuit are added to perform multiple pulsinginto two sections for third-harmonic, current-pulse shaping and is charged linearly for pulse repetitionPS to illustrate the different characteristics of this type of equipment [11]. The energy storage capacitor is divided

Figure 8 shows the power converter of the most important monotum pulsed septum magnets at the CERN

dissipationRest energy

devices).branchesother solid-state reactorThyristors (or I Via the charging I Multiple capacitor

formertoroidal trans- insertion chokechokeThyratrons I Through separate I Fast recharge I Three limb or I MOS filter with

Ignitrons I Through magnet I No multi-pulsing I No transformer I No active filter

transformer

filter(auto-)discharge switchI energy recoveryI pulsingMatching I ActiveType of I Type of I Multi

Discharge circuit classification

- 321

loop, and for displaying the current waveform and peak value. OCR Output

performance dcct's (e.g.. HOLEC, NL or DANPHYSIK, DK). The signal is used for the flat-top regulation

The magnet current is measured by means of pulse transformers (e. g.. PEARSON, US) or high

3.3 Monitoring

adequate pulse indication and time interval measuring facilities are part of the normal auxiliary equipment.

Because of the importance of the timing pulses for successful operation of any pulsed power converter,

corresponds to the beam presence and to peak discharge current.

MEASURE (MEA) which triggers Sample/Hold acquisition of the discharge current and which normally

given time interval

START (ST) which triggers the discharge so that maximum current is reached in the magnet after a

all actions foreseen to assure a safe discharge

voltage stabilized to the required level. This pulse sets the reference to zero and starts

WARNING (W) which blocks the charging circuit once the capacitors have been charged and their

operation and initiates the charge of the energy storage capacitors

FOREWARNING (FW) which announces that the power converter is asked to operate during the next beam

The four standard timing pulses are:

the highest possible operational reliability and transparency.

accelerator timing system. Whenever possible, all four pulses are delivered extemally to the power converter for

time intervals which are initiated by four timing pulses, at least two of which must come from the extemal general

To meet these constraints the operating cycle of a pulsed power converter has been subdivided into specific

the switches must have recovered their voltage blocking capability before recharging the capacitors.

circuiting of the charging section during the capacitor discharge. Similarly, the discharge must be completed and

the required level, that the voltage has been stabilized and that all precautions have been taken to avoid short

beam is present. This means that before each discharge one must ensure that the capacitors have been charged to

with the accelerator operation in order to achieve maximum stable field in the corresponding magnets when the

The timing function is an essential aspect in pulsed power converters because they must be synchronized

3.2 Timing

information on detailed operational situations (e.g. intemal or extemal interlocks and faults).

and acquisition have either 12-bit (STH) or 14-bit (STD) resolution. A number of indicator bits provides

actuation bits: OFF - STANDBY - ON · RESET, and the corresponding acquisition bits. The current reference

words for actuation, reference setting, current and status acquisition. The control protocol foresees four exclusive

digital (STD) or have analogue current reference and acquisition (STH-hybrid). Control is based on four 8·bit

The computer control interface in the CERN-PS is a single transceiver board [10], which can be either fully

3-1 Qmrupls

TIME! RLN G AND REGULATION

— 322 —

therefore be energetically pursued. OCR Output

lepton colliders, will make extensive use of pulsed power converters. Development work in this field should

applied [12]. More sophisticated operations on present accelerators, and the next generation of high energy linear

dominating so far and pulse compression techniques with the help of modem magnetic materials are already being

particular applications. Modem semiconductor switches are entering applications where thyratrons were

The combination of a lumped element PFN with an active ripple filter may become an interesting solution for

solutions and build mains and user friendly pulsed converters.

the use of advanced power devices (e.g. IGBT's and GTO's) and microprocessors to implement smarter technical

briefly described and references made to more detailed specific technical reports. Trends of development concern

several examples of recent realisations. The composition of the converters and the technical solutions have been

The wide domain of pulsed power converters for particle accelerators has been introduced by means of

4. CONCLUSION

operating point of the active filter are also avoided during equipment warm-up conditions.

any required time and amplitude pattem of magnet pulse sequences. By these means too large variations of the

between charging voltage and pulse current, with thermal drift due to ohmic losses in the power circuit and with

characteristics and by modelling the load, several corrections are applied to the loops to cope with non-linearities

feeds the active filter via a suitably phase-corrected isolated drive circuit. On the base of the power circuit

bandwidth (100 kHz). In this case the magnet current is directly compared to the reference and the error signal

The flat·top regulation by an active filter, when present, constitutes a separate servo-loop with larger

roughly constant power even outside the charging time (i.e. during the W—FW time interval).

mains during the charging period. For very large power ratings, solutions have been worked out to absorb

charging power circuit. It is possible to profit from this by imposing constant active power demand from the

A faster and more accurate voltage control is obtained by means of a higher frequency chopper in the

extinction of the power thyristors) or by keeping the source voltage constant and 'deQuing' the resonant circuit.

voltage source and interrupting the resonant charge as soon as the desired voltage has been achieved (by

The voltage regulation of resonant charging type power converters is done either by controlling the low

during stabilisation before the W pulse.

voltage is reached. Higher-gain voltage control takes over at transition to the final capacitor voltage level and

subsequent full current charge, the current reference is reduced for better transients when approximately 90% of

and current control servo-loops. Current control acts during the charging time. After initial soft start and

Pulsed power converters with linear charge have, in general, either cascaded or parallel-switched voltage

3-4 Kcaulation

— 323 —

Proceedings of EPE 89 Conference p. 1127 (1989)[12] A.L. Keet and M. Grocnenboom, High voltage solid state pulser for high-repetition—rate gas lasers,

the pulsed septum magnets in the PS straight sections 16 and 58, CERN/PS/88-26 (PO) (1988)[11] C. Ducastel, J.P. Royer and F. Voelker, The novel 4 kV, 40 kA capacitor discharge power converters for

[10] E. Asséo et al, Interface de controle des alimentations, CERN/PS/BRJ77-54 (1977)

PS-Booster, CERN/PS/BR/77-38 (1977)J.P. Royer and F. Voelker, The capacitor discharge current supplies for the injection septum magnets of the[9 1

Accumulator and Collector (AAC) at CERN, CERN/PS/89-61(PO) (1989)[8 1 F. Voelker, The 200 kJ pulser and power converter for the 36 mm lithium lens of the Antiproton

under energy storage/discharge conditions, CERN/EF/BT/81-2 (1981)[7] R. Grueb and B. Langeset, Accelerated life tests of DPO impregnated polypropylene capacitors operated

Proceedings of the Fifth Conference on Magnet Technology, p. 268. Frascati, 1975[6] P. Bossard and F. Voelker, A pulsed ferrite magnet system with rise and fall time below 100 ns,

focusing solenoid of the e- e+ converter in the CERN LEP pre-injector, CERN/PS/88-1 (PO) (1988)[5] J.P. Royer and F. Voelker, The 6 kA, 10 kV, 100 Hz power supply and pulser for the collecting and

Novosibirsk, USSR[4] A.D. Chemyakin, G.I. Sylvestrov et al. Small size gantry system for proton therapy, Preprint 90-32, INP,

Novosibirsk, USSR, (1990)[31 A.D. Chcmyakin, G.I. Sylvestrov etal, Project of small dimensional 200 MeV proton synchrotron, INP,

final focusing at linear colliders, INP, Novosibirsk, USSR, (1989)[2] G.I. Sylvestrov, A.N. Skrinsky and T.A. Vsevolozhskaya, Super strong quadrupoles of liquid metal for

g(1 )[1] Jlglgoyer and F. Voelker, Modular pulsed power supplies for the PS-Booster, CERN/PS/BR/80-2

- 324