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SLAC-PUB-8889

September 2001

physics/0108063

Room Temperature Accelerator Structures for Linear Colliders

Work supported by Department of Energy contract DE–AC03–76SF00515.

Presented at the IEEE Particle Accelerator Conference (PAC2001),

6/18/2001—6/22/2001, Chicago, IL, USA

R. H. Miller et al.

Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309

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ROOM TEMPERATURE ACCELERATOR STRUCTURES FOR LINEAR

COLLIDERS*

R.H. Miller, R.M. Jones, C. Adolphsen, G. Bowden, V. Dolgashev, N. Kroll Z. Li, R. Loewen, C.

Ng, C. Pearson, T. Raubenheimer R. Ruth, S. Tantawi, J.W. Wang, SLAC, Menlo Park, CA, USA

Abstract

Early tests of short low group velocity and standing

wave structures indicated the viability of operating X-

band linacs with accelerating gradients in excess of 100

MeV/m. Conventional scaling of traveling wave traveling

wave linacs with frequency scales the cell dimensions

with λ. Because Q scales as λ1/2, the length of the

structures scale not linearly but as λ3/2 in order to preserve

the attenuation through each structure. For NLC we

chose not to follow this scaling from the SLAC S-band

linac to its fourth harmonic at X-band. We wanted to

increase the length of the structures to reduce the number

of couplers and waveguide drives which can be a

significant part of the cost of a microwave linac.

Furthermore, scaling the iris size of the disk-loaded

structures gave unacceptably high short range dipole

wakefields. Consequently, we chose to go up a factor of

about 5 in average group velocity and length of the

structures, which increases the power fed to each structure

by the same factor and decreases the short range dipole

wakes by a similar factor. Unfortunately, these longer

(1.8 m) structures have not performed nearly as well in

high gradient tests as the short structures. We believe we

have at least a partial understanding of the reason and will

discuss it below. We are now studying two types of short

structures with large apertures with moderately good

efficiency including: 1) traveling wave structures with the

group velocity lowered by going to large phase advance

per period with bulges on the iris, 2) π mode standing

wave structures.

1 HIGH GRADIENT RF BREAKDOWN

The high gradient RF breakdown testing is reported in

detail by Adolphsen [1]. There are several interesting and

some surprising results that affect the choice of structure

design, which we will discuss here. The first is that as

suggested by Adolphsen the viable operating gradient

appears to vary almost linearly with the inverse of the of

the group velocity. A related observation is that the

structures process very rapidly with a relatively small

number of arcs up to a gradient that also varies slightly

less than linearly with the inverse of the group velocity.

Above that gradient the arcing rate increases dramatically

and progress to higher gradients is very, very slow.

The second and perhaps the most surprising result

occurred during the simultaneous testing of a 105cm long

structure and a 20cm long structure driven by the same

klystron through a 3 dB coupler so that the drive levels

and history would be identical. Both structures were

designed to be approximately constant gradient, but

precisely constant peak surface field on all disks. Both

had an initial group velocity of 5% of the velocity of light.

The short structure was identical to the first 20 cm of the

long structure. One might have expected the 105cm

structure to have roughly 5 times as many arcs as the

20cm structure. Instead, the two structures had equal

numbers of arcs at all power levels within the statistical

variation, except during the very early processing. This is

less surprising in view of the fact that the vast majority of

the arcs in the long structure occur in the first 20cm.

The third interesting fact emerging from the high

gradient testing is observed when a structure has been

processed up to some level with a short pulse and the

pulse length is increased significantly. The rate of

breakdowns increases dramatically with the arcs

distributed uniformly in time within the pulse, not

concentrated in the added portion of the pulse.

The fourth interesting result occurred in an experiment

studying high electric field gradients in rectangular

waveguide, Dolgashev [2]. The large dimension of the

waveguide had been reduced to lower the group velocity

to about 0.18c in order to raise the field strengths that

could be reached with available power. The striking

observation was that when the pulse length was less than

400ns and the peak surface gradient was 80 MV/m the

arcs never degraded the high gradient performance of the

waveguide. When the pulse length was more than 500 ns

the arcs frequently degraded the high gradient

performance. The degradation observed was a higher rate

of arcing, or the inability to reach 80MV/m and higher

xray levels on pulses where no arcing occurs.

These four observations suggest that it may be

important to consider the energy deposited at the site of

an arc when an arc occurs for two reasons. First, it may

alter the microwave parameters of the structure by causing

a tiny, deposited-energy-dependent change in the resonant

frequency of the cell in which the arc occurs and thus

change the phase advance and the match of the structure.

Secondly, there probably is a deposited energy threshold

above which the high gradient performance of the

____________________________________________

*Work supported by the U.S. DOE, Contract DE-AC03-76SF00515.

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structure is degraded. Above this threshold an arc is

likely to cause surface damage which causes successive

arcs to occur at or in the vicinity of the original site. For

many years people have observed that RF processing is

not always monotonic; that sometimes an arc causes a

setback, lowering the RF power which the device being

processed will accept. It has also been realized that it was

advantageous to high power process with a short pulse

until the desired gradients are reached, and then slowly

increase the pulse length. The energy deposited in an arc

in a travelling wave structure should vary linearly with the

incident power, the pulse length, and the group velocity.

It may also depend on other parameters such as the

frequency, phase advance per cell and geometric factors

such as the gap across which the breakdown occurs. The

damage threshold almost surely depends on the physical

properties such as melting point of the material from

which the structure is made. Despite our ignorance of

these issues, it still may be useful to define a damage

parameter, D.

D = PTvg/c (1)

P is the input power, T is the pulse length, vg is the group

velocity and c is the velocity of light. To get an idea of

what may reasonable we can look at SLAC (ignoring any

frequency and geometry dependence). The highest power

operation of the standard SLAC S-band sections with a

rectangular pulse (unSLEDded) occurred in the injector

where several sections ran routinely with 25 MW 2.5 µs

long pulses. The initial group velocity of these constant

gradient sections was .02c. With these parameters the

damage parameter is 1.2 Joules. It is important to note

that only a minuscule fraction of this can be dissipated at

the site of the arc. In the high gradient testing the X-band

1.8 meter Damped Detuned Structures (DDS) processed

very easily and essentially monotonically up to about 35

MeV/m with a 44 MW, 240ns pulse. The initial group

velocity is .12c, giving a damage parameter of 1.3 J,

suggesting that this may be a useful parameter. The four

observations from the high gradient testing reported above

all suggest that for gradients above some damage

threshold most of the arcs result from damage caused by

previous arcs rather than from the initial existing defects

such as inclusions and microscopic points in our copper

structures. We have also observed both with the SLAC S-

band structures and with the NLC X-band structures that

it is possible to have arc damage from which it is

impossible to recover. That is that a reasonable amount of

high gradient processing cannot recover the gradient at

which the structure had been operating. The conservative

course of action may be to design to run below the arc

damage threshold for which our damage parameter D may

(or may not) be a useful indicator. If D were a precise

measure of the limit for monotonic processing then the

gradient at which monotonic processing ends would scale

linearly with the 1/vg. We observe a less than linear

scaling, but the number of samples is very small. Perhaps

vg/c in the damage parameter needs an experimentally

determined exponent which is less than unity.

2 TRAVELING WAVE STRUCTURES

The present design for the unloaded gradient for NLC is

72 MeV/m. To achieve this with a damage parameter of

the order of 1 Joule will require an initial group velocity

in our approximately constant gradient structures of

between .03c and .05c. We are presently testing

structures at each of these values. The tested structures

would not be satisfactory for NLC because the apertures

are too small causing excessive dipole wakefields. Z. Li

[3] has designed structures for each of these group

velocities using 150o phase advance per cell and thicker

disks to achieve these lower group velocities with the

same average iris diameter as in the vg = 0.12c structure.

The 0.05c structure is 90cm long, while the 0.03c

structure is 60 cm long. R.M. Jones [4] is studying

detuning these and damping them using either manifold

damping or local damping to reduce the long-range dipole

wakes by a factor greater than 100. The initial results

look quite promising. The wakefield for the 0.05c

structure with 10% Gaussian detuning and manifold

damping is shown in Fig. 1. It is difficult to go below an

initial group velocity of .03c without reducing the average

iris diameter, which we don't want to do because of the

short-range dipole wakes. We are uncomfortable about

100

Wake ?V?pC?mm?m?

2468

????

s ??????? ?

m ?

0.001

0.01

0.1

1

10

Figure 1: Wakefield for two 90cm long manifold-damped

10% detuned structures with .05c initial vg.

going to phase advances larger than 150o per period

because of the reduced bandwidth, and thicker disks hurt

the shunt impedance.

3 STANDING WAVE STRUCTURES

There is some argument for designing the NLC

structure to operate at unloaded gradients as high as 100

MeV/m, to accommodate an energy upgrade above 1 TeV

in the same tunnel. We think this forces us to consider

standing wave structures. Standing wave structures have

several advantages over travelling for high gradient

operation. The first is that for a given gradient the input

power required scales roughly as length, and stranding

wave structures can be made arbitrarily short without

sacrificing efficiency. Secondly, because a standing wave

structure is a high Q resonant cavity the reflection

coefficient goes very, very close to unity almost instantly

when loaded by an arc. In this way a standing wave

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cavity is more self-protecting than a travelling wave

structure. We are studying 20 and 30cm π-mode

structures with 15 and 23 cells, respectively. We think

this range is a reasonable compromise between tolerances

and the costs associated with a large number of short

structures. Figure 2 is computer cutaway of our first 15-

cell π mode cavity. Two of these are currently in high

power testing. We propose to braze 6 to 10 of these short

structures together to form a single precision aligned

assembly. Each multicell cavity will have an odd number

of cells and be driven through the center cell. Center

driving relaxes the tolerances by a factor of 4.

Figure 2: Computer cutaway of 15-cell π-mode cavity.

The standing wave cavities in each half of the assembly

will be driven by a single oversize rectangular waveguide

with a directional coupler splitting off the appropriate

power for each cavity. When this is done right all the

power reflected from the cavities goes into dummy loads

in the approximation that all the cavities have the same

coupling and the same resonant frequency. Thus, in this

approximation the assembly looks like a matched load to

the RF source. Of course when a cavity arcs it upsets the

match, but since each cavity is getting a small fraction of

the total power the reflected power goes like the small

fraction squared.

3.1 Dipole Wakefield Reduction

The same general methods, namely detuning and

damping, are available for reducing dipole wakefields in

standing wave cavities as were used in the traveling wave

structures. However, because the cavities are so short the

implementation will be different. Tapering the iris

diameters with compensating changes in the cell

diameters still appears to be the best way to tune the first

dipole band while keeping fundamental π mode frequency

constant. By decreasing the thickness of the disks as the

iris diameters decrease we find we can reduce the spread

in cell to cell coupling of the fundamental. We can get an

8% detuning of the first dipole band with the fundamental

mode cell to cell coupling varying from 5.2% at the large

iris cells down to 2.2% at small iris end. We intend to

vary the iris size and therefore the dipole frequencies

monotonically from one end to the other of a 6 or 9 cavity

assembly having a total of about 135 cells. Fig. 3 shows

the amplitude and phase from an equivalent circuit for

each cell of a 15 cell cavity in which the coupling varies

from 3.5% to 2.2% as it might in the last cavity at the high

dipole frequency end of a 6 cavity assembly. The phase

shifts, which are caused by the power flow, can be

compensated for by adjusting the length of the cells to

keep a velocity of light beam on the crest in each cell.

Fig. 4 shows the dispersion curve for each cell (as if it

were in a periodic structure). The cell fundamental mode

resonant frequencies have been tuned to achieve a flat

field, which requires that the π mode frequencies for all

the interior cells coincide at about 11.4 GHz. The cells at

each end, because they are full cells rather than a half-

cells, must be tuned so the π/2 modes are at 11.4 GHz.

Because the cavities are so short, we tried what might

be called end damping. We put a low Q (10) dipole mode

cavity in the drift tube between each pair of 23 cell

cavities and at each end of the full assembly. We have a

preliminary simulation of this and the wakefield is

presented in Fig. 4. It appears to be promising, but this is

certainly neither a full nor optimized design. We have a

concept for the lossy dipole cavities but they have not

been designed.

Figure 3: Phase and amplitude of the fundamental mode

in the 15 cells of a dipole detuned π mode cavity.

Figure 4: Dispersion curves for the fundamental mode of

the 15 cells of a π-mode cavity with dipole mode detuning

Figure 5: Wakefield for six 23 cell π-mode cavities in a

monotonically detuned assembly with a low Q dipole

cavity every 23 cells.

4 REFERENCES

[1] C. Adolphsen, Paper ROAA003 this conference

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[2] V. Dolgashev, Paper FPAH057 this conference

[3] Z. Li, Paper FPAH061 this conference

[4] R.M. Jones, Papers FPAH058 & MPPH068 this conf.