High Gradient Testing of O-Axis Coupled C-band Cu and CuAg Accelerating Structures Mitchell SchneiderValery Dolgashev John W. Lewellen Sami G. Tantawi and Emilio A. Nanni

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High Gradient Testing of Oﬀ-Axis Coupled C-band Cu and CuAg Accelerating

Structures

Mitchell Schneider,∗Valery Dolgashev, John W. Lewellen, Sami G. Tantawi, and Emilio A. Nanni

SLAC National Accelerator Laboratory, Stanford University, Menlo Park, CA, U.S.A

Muhammed Zuboraj,†Ryan Fleming, Dmitry Gorelov, Mark Middendorf, and Evgenya I. Simakov

Los Alamos National Laboratory, Los Alamos, NM, U.S.A.

(Both M. Schneider and M. Zuboraj Contributed Equally)

We report the high gradient testing results of two single cell oﬀ-axis coupled standing wave accel-

erating structures. Two brazed standing wave oﬀ-axis coupled structures with the same geometry

were tested: one made of pure copper (Cu), and one made of a copper-silver (CuAg) alloy with

a silver concentration of 0.08%. A peak surface electric ﬁeld of 450 MV/m was achieved in the

CuAg structure for a klystron input power of 14.5 MW and a 1 µs pulse length, which was 25%

higher than the peak surface electric ﬁeld achieved in the Cu structure. The superb high gradi-

ent performance was achieved because of the two major optimizations in the cavity’s geometry: 1)

the shunt impedance of the cavity was maximized for a peak surface electric ﬁeld to accelerating

gradient ratio of ∼2 for a fully relativistic particle, 2) the peak magnetic ﬁeld enhancement due

to the input coupler was minimized to limit pulse heating. These tests allow us to conclude that

C-band accelerating structures can operate at peak ﬁelds similar to those at higher frequencies while

providing a larger beam iris for improved beam transport.

Reducing the overall physical footprint of a particle

accelerator has become important for many accelerator

applications in industry, medicine, national security, and

basic sciences [1–4]. In response, there has been a con-

certed eﬀort to develop high gradient accelerating struc-

tures which allow achieving the required beam energy

within a shorter length. These compact accelerators can

be used for many applications such as high brightness

light sources [1, 2], high energy facilities [3–5], medi-

cal radiotherapy [6, 7] and industrial LINACs [8]. The

high gradient accelerators must often meet a user’s re-

quirement of transporting a high charge or a high cur-

rent particle beam which restricts the size of the mini-

mum aperture of the accelerating structures and makes it

preferable to operate at lower microwave frequencies. In

many previous works, high gradient operation of X-band

(11.424-11.992 GHz) accelerating structures was success-

fully demonstrated and studied [9]. Here we explore de-

velopment of high gradient accelerators at C-band (5.712

GHz) where the beam aperture can be twice as large as

at X-band, reducing the level of higher order modes that

can be excited by the accelerating particle beam.

The C-band cavities tested in this project were opti-

mized as standing wave accelerating structures to be used

with distributed coupling topologies [10]. However, this

cavity’s geometry can be utilized for accelerating multi-

ple species of particles by adjusting the phase advance

between subsequent cells [4, 6, 10, 11]. For example,

fully relativistic electrons or protons would have a ∼100o

phase advance/cell in this structure, and β= 0.5 protons

would accelerate at a 180ophase advance/cell.

∗mitchs@slac.stanford.edu

†zuboraj@lanl.gov

The important ﬁgures of merit for performance of any

high gradient structure are the achievable peak surface

ﬁelds, accelerating gradient and subsequent breakdown

rate (BDR) at the given ﬁeld [9, 12–15]. A breakdown

is a vacuum arc discharge inside of the structure which

generates an excursion of gases, particulates, and ions

from the surface. Radiofrequency (RF) breakdowns are

related to multiple phenomena including pulse heating

and ﬁeld emission/dark current [13, 16]. The BDR is

deﬁned as the probability of a breakdown event per a

RF pulse normalized to the length of the accelerating

structure for a given RF pulse length.

C-band accelerating structures have been previously

studied by multiple institutions and projects, such as

SwissFEL [17], SINAP [18], SPARC LAB [19], the FEL

Spring-8 [20], and the Korean National Fusion Research

Institute (KNFRI) [21]. All previous projects used

traveling-wave C-band structures with exception of KN-

FRI [21] that used standing-wave structures. The peak

surface ﬁelds in those C-band structures were in the range

of 80-150 MV/m while requiring input power from the

klystron on the order of 10s MW. The breakdown rates

in these multi-cell accelerating structures varied between

1×10−5and 1×10−6(1/pulse/meter) for the pulse length

in the range of 0.5-1 µs.

High gradient testing of accelerating structures oper-

ating at X-band is routinely conducted at SLAC Na-

tional Accelerator Laboratory demonstrating peak sur-

face electric ﬁelds greater than 350 MV/m for 1×10−4

to 1×10−2BDR(1/pulse/meter) for RF pulse length of

85-300 ns [12–14]. This is similar with testing of X-band

structures at CERN/KEK [22] and S-band work form

CERN [23] with both showing a maximum achievable

surface electric ﬁeld of 225 MV/m for a BDR of less

than 1×10−7(1/pulse/meter) for the 250 ns pulse length

arXiv:2210.17022v1 [physics.acc-ph] 31 Oct 2022

at X-band and surface ﬁeld of 220 MV/m for a BDR of

less than 1×10−6(1/pulse/meter) for the 1.2 µs pulse

length at S-band. Those same X-band (11.424 GHz)

experiments showed that the breakdown rate correlates

with peak pulse surface heating. It is hypothesized that

pulsed heating results in cyclic fatigue, defect mobility

and physical changes to the surface that enhance surface

ﬁelds and consequently lead to breakdowns [24]. The ac-

celerator cavity made for this project was designed with

the goal of maximizing shunt impedance, minimizing the

peak surface magnetic ﬁeld at the input waveguide cou-

pler, and reducing the pulse heating. The structure itself

is a C-band version of an S-band (2.856 GHz) accelerat-

ing cavity that was designed by X. Lu et al. [6] to serve as

an energy modulator for β=0.5 protons utilized in pro-

ton radiation therapy. The geometry was scaled down

in size to increase the resonant frequency to 5.712 GHz.

The two geometric diﬀerences are an increased length of

the cell to ease machining requirements and the RF feed

shape which was tapered due to the relatively smaller

dimensions of the WR-187 waveguide compared to the

WR-284 waveguide. Fig. 1a shows a mechanical modal

for the cavity. The two structures were fabricated for the

experiment described. One was made of pure Cu and the

other one was made of a CuAg alloy with 0.08% concen-

tration of Ag to further investigate breakdown rates in

CuAg structures compared to Cu structures [14]. The

hypothesis, as proposed in new computational work [24],

was that adding a small concentration of silver is supe-

rior in copper alloy to increase resistance to the thermal

and mechanical stresses during an RF pulse which could

result in higher achievable ﬁelds for the CuAg structure

before the onset of breakdown. Fig. 1b shows the reﬂec-

tion coeﬃcient S11 that were measured during the RF

cold tests of the two fabricated cavities.

Fig. 1c shows the results of Ansys electronics simula-

tion toolkit HFSS simulations for the magnitude of the

electric ﬁeld plotted at the central plane of the structure.

Fig. 1d shows the results of HFSS simulations for the

magnitude of the magnetic ﬁeld on the surfaces of the

cavities and illustrates that the maximum magnetic ﬁeld

is located on the upper walls of the accelerating structure

near the input coupler, far away from the beam iris. This

is the location where the maximum of peak pulse surface

heating will occur during high gradient testing. The ac-

celerating parameters of the cavity computed with HFSS

can be seen in Table I. Compared to the S-band cavity

(2.856 GHz), the scaled C-band cavity has an increased

shunt, impedance Rs which is proportional to the square

root of the cavity’s frequency, √f. Table II shows the Q

factors and resonant frequencies extracted from the RF

cold test results of the two fabricated cavities with the

copper-silver cavity having a slightly higher Q-factor.

The high gradient testing of the two structures was per-

formed at the C-band Engineering Research Facility in

New Mexico (CERF-NM) at Los Alamos National Lab-

oratory (LANL)[25]. The schematic and a photograph

of CERF-NM are shown in Fig. 2. The facility is built

FIG. 1: (a) The mechanical CAD Modal of the 5.712

GHz copper cavity, (b) the reﬂection coeﬃcients

measured during cold-testing of two fabricated cavities,

one made of copper and the other one of copper-silver.

The results of HFSS computations for (c) the electric

ﬁeld plotted at the central cross-section the cavity and

(d) the magnetic ﬁeld on the surface of the cavity. The

dashed lines show the symmetry axis ﬁeld are for 4MW

of power dissipated in the cavity.

TABLE I: Parameters of the C-band cavity as

computed by HFSS for a copper structure. For a

v=0.5c protons and v=c electrons acceleration.

Parameter Cu Cu

v=0.5c Proton v=c Electron

Length 1.58 cm

a/λ 0.0525

Frequency 5.712GHz

σ58 MS/m

Q09762

Qext 10165

61.51 115.8

MΩ/m MΩ/m

62 MeV/m 81 MeV/m

×pP[MW ]×pP[MW ]

Ep/Ea2.42 1.84

Hp∗Z0/Ea1.40 1.07

TABLE II: Cavity parameters measured in the RF cold

test of the Cu and the CuAg cavities.

Cold Test Results Cu CuAg

Frequency 5.71205 GHz 5.71133 GHz

Q09621 9720

Qext 9742 9805

2∗τ269 ns 272 ns

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HighGradientTestingofO-AxisCoupledC-bandCuandCuAgAcceleratingStructuresMitchellSchneider,ValeryDolgashev,JohnW.Lewellen,SamiG.Tantawi,andEmilioA.NanniSLACNationalAcceleratorLaboratory,StanfordUniversity,MenloPark,CA,U.S.AMuhammedZuboraj,yRyanFleming,DmitryGorelov,MarkMiddendorf,andEvgenyaI.Simakov...

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High Gradient Testing of O-Axis Coupled C-band Cu and CuAg Accelerating Structures Mitchell SchneiderValery Dolgashev John W. Lewellen Sami G. Tantawi and Emilio A. Nanni

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