NEW HIGH POWER VARIABLE-ATTENUATOR I PHASE-SHIFI'ER SYSTEMS
S. Ohsawa, I. Sato and A. Asami
National Laboratory for High Energy Physics
Oho, Tsukuba-shi, lbaraki-ken, 305, Japan
Abstract
For the prebuncher and buncher of the electron linac,
two kinds of waveguide systems were newly designed
which can be used not only as a variable attenuator but
also as a phase shifter. Either of the two systems has thedistinctive feature that it does not introduce an rf phaseshift as the attenuation is varied. This feature greatly
speeds up the process of converging on optimum phaseand power settings for both the prebuncher and buncher.
The principles of the new systems are described.
Introduction
In the electron linac electron bunches are produced by
the prebuncher and buncher. Since the characteristics ofthe beam are mainly determined by the bunches, it is very
important to optimize the rf powers and phases in thesedevices. For the optimizing process, a system consistingof an attenuator and a phase shifter is necessary for eachof these devices. However, the existing high-power
variable attenuators have a problem in that a phase shift is
introduced as the attenuation is varied. This fact makes itcomplicated to search for optimum parameters of theprebuncher and buncher. New systems were designed inorder to solve this problem.
Presently, the new systems are under production forrenewing the systems now in use in the Photon Factory
2.5-GeV electron linac. The system in use for the buncher
is pressurized with SF6, so that it requires rf windows
which part the gas from a vacuum. Bored holes withsmall cracks were found on the vacuum sides of thewindows this Spring. Fortunately, the holes still did notreach to the other side. In order to avoid this kind ofproblem, the new systems were designed so as to be used
under vacuum, as are those of the positron generator inKEK.

configuration illustrated schematically in Fig. 1. It is
composed of a 20-dB directional coupler as a power
divider, a 3-dB directional coupler with two shortplungers which move independently and a dummy loadfor termination. The power for the prebuncher isprovided from the main line for the buncher by the 20-dB
power divider. Then, the power is divided again by a 3dB directional coupler into two parts, each of which isreflected by a short plunger at a different position (Fig.
2); the two fields are then mixed by the same 3-dBcoupler.
Let us consider a determination of the output power
and the phase of the system. For simplicity, power losses
in the waveguides or by reflections owing to imperfectdevices are neglected in the discussions below. Let theinput rf field be 2Aexp(irot) at plane A as written in Fig.
1. The field is divided by the 3-dB directional coupler
into two fields, as mentioned above, each of which isgiven at the upper short plunger after reflection by...r2Aexp[i(OX+(M2+n)](1)
and at the lower short plunger after reflection by...r2Aexp[i( OX+fhl2+n+n12)](2)
respectively, where
81 = {lc + L1l) I [g.
£h = (lc- L1l) I [g,(:) = ( 81 + (h ) I 2 = lc /lg,(/) = (81- fh) I 2 = L1l/[g.

(3)andHere, 18 is the wavelength in the waveguide. The phasedifference 1t/2 in Eq. (2) comes from the 3-dB coupler
and 1t in Eqs. (1) and (2) indicates the phase shift as thefields are reflected at the short plungers. Each amplitude
is reduced to ViA since the input power is equally divided
into two. The fields reflected by the short plungers are
mixed by the same 3-dB coupler and become at plane A
in the input port as

Although the new systems are essentially very similar
they look quite different at first glance. It might therefore
be better to explain them separately.
Medium power system
In ·this section the system for medium power isdiscussed. The rf power required in the prebuncher isusually less than 10 kW. In this power range a dielectric
substance is usually used for varying the power or phase.
The attenuation can be altered by changing the position ofa loss dielectrics in the waveguide: The attenuation
becomes larger when the substance is moved toward therectangular waveguide center where the field strength of
the TEto mode becomes maximum. Another type of
attenuator has a rotary vane in a circular waveguide. In
any case, the rf phase is inevitably shifted as theattenuator is changed.
On the other hand, the new system has theB2Aexp(iro t)
AINPULOUTPUT

Fig. 1 The attenuator/phase-shifter system for medium
power. Bold arrows indicate directions of field flows.

-115-High-power system

Re[Aexp{i(o:t+8I+7r)} + Aexp{i(o:t+82+21r))]= 2Asint/) sin(mt+8)(4)

and at the same plane A in the output port as
Re[Aexp{i( o:t+8I+31CI2)} + Aexp{i( o:t+82+31CI2))}=2Acost/) sin(mt+8),(5)respectively.
It can be seen from Eq. (3) that t/J and 8 depend only
on til and lc, respectively. This means that the output
power or phase can be adjusted independently by meansof moving ·the plungers in the opposite or the same
direction by equal amounts, respectively. The output
power is changed without shifting the output phase in thefull range from zero to the input power according to t/);on the other hand, the output phase 8 is given by theaverage phase of the two fields before being added. Thus,
the new system has functions both of an attenuator and aphase shifter, as mentioned before.
The backward power returned to the input port ismainly absorbed by the dummy load connected at a portof 20-dB directional coupler. The part of the power turns
ba~k ~o the source through the 20-dB directional coupler.
This IS, however, less than - 40 dB of the main line
power, because the returning power to the source passes
through the 20-dB coupler two times. Hence, this becomes
negligibly small in practical use.
Although the plunger control becomes complex,
because it is necessary to shift two plungers independently
and simultaneously, the new system becomes simpler andmore compact since it requires no additional phaseshifter.

Let us next consider the system for high rf power. The
medium-power system mentioned above has a simple
configuration. However, this system is not suitable for use
under high power, because in the high-power system it
might usually be impossible to use a very weakly coupledpower divider as the 20-dB coupler in the medium-powersystem. Without a weak coupler, high power might tum
back to the source and destroy it. Even if a circulator isadopted, the transient part of the pulsed power might beturned back to the source.
Accordingly, another new attenuator/phase-shifter
system was designed for high power, so that the reflected
power is perfectly absorbed by a dummy load, at least in
principle, and does not tum back to the source. The new
system is illustrated schematically in Fig. 2. It consists offour 3-dB directional couplers combined in a body, twoof which have short plungers, and two dummy loads. In
this case, the short plungers at a 3-dB coupler are joined
and move together, but each of the joined plungers moveindependently. The system consisted of a 3-dB coupler
and joined short plungers is the well-known phase shifter.
Let us now see how works the new system. Let theinput rf field be 2Aexp(imt) at the plane A in Fig. 2 assame as the previous case. The input power is divided at
the first 3-dB coupler into two parts, each of which is
given at the plane B as...J2Aexp[i(o:t+o)](6)
and at the plane B as...J2Aexp[i(o:t+0+1r12)]'(7)respectively. The phase ocorresponds to the electricallength between planes A and B. It has been assumed forsimplicity that planes B and B or C and C' are symmetricwith respect to the central plane of the system. After
1

1INPUT
2Aexp(iro t)

~l/2.

I

Lc/2·Bl

BLc/2),

it.l/2I

cc~Fig. 2 The attenuator/phase-shifter system for high power. Bold arrows indicate directions of field flows.

-116-passing through the corresponding phase shifter, eachfield becomes at the exit plane B of the right phase shifteras( 8)-v-2Aexp[i( ox+ ()I'+H)]
and at the exit plane B' of the left phase shifter as(9)-v-2Aexp[i( ox+8l+3n/2)},
respectively, where
81' = (lc + 11/) I /g + 0 + n/2(10)
82' = (lc- 11/) I /g + o+ n/2.
andThe phase 81' corresponds to the electrical length fromplane A to exit-plane B of the phase shifter. The fields
given above by (8) and (9) are mixed by the next 3-dB
directional coupler. Then, the final fields are given byequations at the plane C as

different, but are determined in every beam, because it
becomes easier to switch over one setting to another. Thisfeature also holds for the medium-power system.
Now let us compare the characteristics of the newhigh-power system with those of the high-power variableattenuator now in use which has a straight waveguideinstead of the phase shifter shown on the right-hand sidein Fig. 2. In this case, only the phase of one field ischanged; then, the output field is given by

Re[Aexp{i(wt+fh+n/2)} + Aexp{i(mt+£h+3n12)}](14)=2Asin<P cos( M+8),
where(15)92 = 82' + 0' =(le-d) I /g +O+o'+ n/2,
8 = (81 +82) I 2 ={(le-d) I [g +o +O'+o"+n/2]12,

Re[Aexp{i( mt+8I+n)} + Aexp{i( mt+£h+2n)}](11)= 2Asin<P sin( mt+ 8)
andand at the plane C' in the output port as

Re[Aexp{i( mt+8I+3n/2)} + Aexp{i( mt+£h+3n/2)})(12)= 2Acos<P sin(mt+8),
where

and

81 = 81' + o' = (lc+J.l) I /g + o + o',£h = 82' + o' = (lc-J.l) I /g + o + o',
8 = ( 81 + 82) I 2 = lc I /g + o + o',<P = (81 - (h) I 2 = 11/ I [g.

(13)

The phase o' corresponds to the electrical length between
planes Band C, orB' and C'.
Equations (11) and (12) are exactly the same as (4) and(5), respectively. Although phases 91,92 and (} defined by
E9. (13) are slig~tly different from those of Eq. (3), thedifference o+o' IS not important since it is the constantpha~e. Therefore, tJ;te two new systems are equivalent.
Owmg to the equations obtained above it is concluded
that this system has the same functions 'explained in theprevious section. The difference is that as the twoplungers shift in the same direction the attenuation
changes, and as they move in the opposite direction theoutput phase shifts. Of course it is possible to change boththe o':l~ut power and the phase simultaneously by meansof sh1ftmg the two plungers independently (by moving
the s~ort plun~ers to the different amounts). This
operation mode IS useful when the rf optimum settings are./2

1 ....., .."'::::::·········..·········.....

(}] = o",<P = (81 -82) 1 2 = - {(lc-111) 1 /g +o +o'-o"+1t12]12.

The phases 8 and <P both depend on (lc-111)12, as indicatedin Eq. (15). It is therefore impossible to keep the output
phase constant as the attenuation is varied. Figure 3aclearly shows this feature in the case that the phase of afield is shifted by nl2. The added fields before and after
the shift are shown by arrows A and B, respectively. Not
only the resultant amplitude is changed from 2 to -{2, but
the phase also shifts by as much as nl4. For a comparison,
the corresponding figure for the new system is illustrated
in Fig. 3b. In this case, the resultant field is changed
from A to B without any phase shift.
Summary
Two waveguide systems which can be used as avariable attenuator and a phase shifter are newlydesigned. Either of the two systems has the distinctive
feature that it does not introduce an rf phase shift as theattenuation is varied. Hence, it is expected that the processof converging on optimum phase and power settings forthe prebuncher and buncher will be greatly speeded up bythe new systems.

1 ........,,:.:·:~·························· ...:BA012Fig. 3a Addition of two fields in the attenuator now inuse. The resultant field changes from A to B.

-117-Fig. 3b Addition of two fields in the new system.
The resultant field changes from A to B.