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M. W. Bruker† , J. Grames, J. Guo, J. Musson, S. A. Overstreet, G. T. Park,
T. Plawski, M. Poelker, R. Rimmer, H. Wang, C. Wilson, S. Zhang
Thomas Jefferson National Accelerator Facility, Newport News, VA, USA
M. Pablo, B. Roberts, D. Speirs, Electrodynamic, Albuquerque, NM, USA
Abstract

INTRODUCTION
In circulating accelerators, the minimum bunch spacing
needed to accommodate the rise/fall time of a kicker canlimit the design options for the bunch train. Originally developed for the Circulating Cooler Ring, a hypothetical ringwith 11 revolutions driven by an energy-recovery linac thatwas intended to be part of the Jefferson Lab Electron-Ion
Collider [1], the harmonic kicker cavity offers a new option
for applications where every 𝑛th bunch must be deflected [2].
One such application is the injection into the Rapid Cycling
Synchrotron at the Electron-Ion Collider [3], where one outof four consecutive bunches is injected at a time; here, thebunch spacing is 1.6 ns, and each set of four bunches is followed by a gap of 12 µs, encouraging the use of a pulsed
device but out of reach for a stripline due to the short bunch
spacing.
A harmonic kicker is a transversely deflecting cavity thatcan be excited at multiple harmonic frequencies at the sametime, allowing one to Fourier-synthesize any deflection waveform containing only these frequencies; the high 𝑄 of themodes compared to a stripline makes the drive power manageable, while pulsed drive is still possible if the gap between∗ Work supported by the U.S. Department of Energy, Office of Science,

Office of Nuclear Physics under contract DE-AC05-06OR23177. MultiHarmonic driver development has been supported by DOE’s NP SBIR
program (DE-SC0020566).
† bruker@jlab.org

ers

n
Tu

Beam

Input coupler

Figure 1: CAD model of a 5-mode harmonic kicker cavity.
Five stub tuners are needed to tune all modes. The RF signal
is coupled in through a single port; another port serves as
the field probe (not shown here).
sets of bunches is sufficiently long. Figure 1 shows a modelof the prototype reduced to the most important parts.
An example waveform providing a kick to 1 out of 11bunches is shown in Fig. 2. The usefulness of this type ofwaveform is not characterized by its absolute flatness outsideof the main peak but by the locations of its zero crossingsor minor peaks, depending on design. While conceptually
flexible, the mode structure of such cavities must be chosen
according to the bunch timing of the intended application.
1
0.5
0Waveform
Bunches

−0.5
−1

0

2

4

8
6
Time (ns)1012Figure 2: Kick action only on bunches at a bunch frequency equal to the fundamental of the kick waveform,
𝑓HK = 86.6 MHz; in this example, all 11 buckets are filled
at a bunch frequency of 11 𝑓HK = 952.6 MHz.
Compared to placing multiple cavities with different resonant frequencies in series, combining the modes in one
cavity has the upside of providing all the deflection in the

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05 Longitudinal Diagnostics and Synchronization

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A harmonically resonant kicker cavity designed for beam
exchange in a circulator cooler was built and successfully
tested at the Upgraded Injector Test Facility (UITF) at Jefferson Lab. This type of cavity is being considered for the
injection scheme of the Rapid Cycling Synchrotron at theElectron-Ion Collider, where the spacing of neighboringbunches demands very short kicks. Operating with fivetransversely deflecting modes simultaneously that resonate
at 86.6 MHz and consecutive odd harmonics thereof, the prototype cavity selectively deflects 1 of 11 electron bunches
while leaving the others unperturbed. An RF driver wasdeveloped to synthesize phase- and amplitude-controlled
harmonic signals and combine them to drive the cavity while
also separating the modes from a field-probe antenna for RF
feedback and dynamic tuning. Beam deflection was measured by sweeping the cavity phase; the deflection waveform
agrees with expectations, having sub-nanosecond rise andfall times. No emittance increase is observed. Harmonicallyresonant cavities like the one described provide a new capability for injection and extraction at circulators and rings.

Kick voltage (arb. unit)

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same location, thereby avoiding bunch displacement due tothe extra drift between cavities; however, note that only odd
harmonics can readily be produced in a single cavity of thistype.
Drawing from a diverse collaboration for fabrication [4],tuning [5], and drive-circuit design, we produced working
prototypes of the cavity and the drive circuit and performed
a beam test to verify the basic aspects of the cavity performance. The following sections describe the methods andresults.

HARMONIC ARBITRARYWAVEFORM GENERATOR
The cavity is controlled by a specially designed drive
circuit called the Harmonic Arbitrary Waveform Generator (HAWG). A synthesized RF source phase-locked to themaster oscillator of the accelerator is used to generate an
86.6 MHz harmonic comb. The desired harmonic signals are
divided from the harmonic comb onto individual channels
using a harmonic combiner and divider [6]. Once isolated,each harmonic signal is phase- and amplitude-controlledusing 𝐼/𝑄 modulator ICs, amplified, and recombined to thecavity’s drive antenna using a second harmonic combiner.
While the cavity is designed for an input power of multiple
kW, the present version of the HAWG provides 100 W per
channel. With this power, the cavity produces its nominal
deflection of ∼ 4 mrad at 6.5 MeV/𝑐.
Signals from the coupling antenna of the cavity are directed to a third harmonic divider where the harmonic signals are separated to individual 𝐼/𝑄 demodulator ICs, allowing for both RF feedback and stub-tuner control for eachmode.

This is a preprint — the final version is published with IOP

BEAM GENERATION ANDSYNCHRONIZATIONThe beam test of the cavity was performed at Jefferson
Lab’s Upgraded Injector Test Facility (UITF). Designed tobe accessible and reconfigurable with relatively little logistical effort, this accelerator delivers short (<1 ps) electron bunches at up to 8 MeV with an average current of
100 nA (limited only by radiation shielding). The beam isgenerated by a laser-driven DC photogun at 200 keV, ballistically bunched, and accelerated to the final energy by
a compact, superconducting booster [7]. All RF elements
run at 𝑓0 = 1497 MHz; the signal pulsing the laser determines the bunch timing. In principle, the machine supports
bunches at any subharmonic of 𝑓0 ; also, configurable lowfrequency (50–200 Hz) macropulses gating the drive laser
can be set up regardless of bunch timing.
The frequencies useful for this experiment are determined by the cavity design. The cavity fundamental 𝑓HK ≈86.6 MHz is related to the machine base frequency 𝑓0 by
𝑓HK
7
=
.
𝑓0
121

The highest frequency that is a subharmonic of both 𝑓HK and
𝑓0 is 𝑓0 /121 ≈ 12.4 MHz, making this frequency ideal for
placing all bunches on the same cavity phase, whereas its
11th harmonic, 𝑓0 /11 ≈ 136.1 MHz, could be used to probe
all 11 phases of interest at the same time. The simplest way
to measure the deflecting action of the kicker cavity is tosend bunches at 𝑓0 /121; this way, all bunches are deflected
the same, avoiding the complication of temporally selective
detection downstream. The angle of deflection can thenbe measured as a function of relative phase by shifting thecavity waveform in time.
The frequencies needed for all RF systems (normally including the laser) come from the main RF synchronization
chassis of the machine, which contains multiple independent phase-locked loops (PLLs) operating from a common
10 MHz reference. Because the 𝑓0 /121 signal needed forthis experiment was not readily available from the laser PLL,
we instead used a commercial frequency generator (CG635
by Stanford Research) synchronized to the same 10 MHz
reference. Due to the nature of the frequency ratio, there is
no way to produce the exact frequency with this generator,
but with 16 digits of frequency resolution, the phase drift
due to frequency mismatch is < 0.1◦ per day.
Another CG635 set to the same frequency supplied thereference signal for the PLLs in the HAWG that generate all
frequencies for the harmonic cavity, allowing for independent phase control of the HAWG with a single knob.
Apart from phase-synchronous RF drive of all components, an additional complication was the necessity forpulsed operation of the HAWG. While the device is designed
for CW operation in principle, the duty cycle was limited
to 10 % for this experiment as a precaution. In order to prevent bunches from being delivered with the cavity off, weoperated the laser in macropulse mode with a pulse width of
400 µs and a delay of 100 µs with respect to the 50–200 Hz
trigger signal, and gated the HAWG with a stretched copyof this trigger signal so that it would turn on sooner thanthe beam and turn off later, giving the cavity time to ring in.
The timing is shown schematically in Fig. 3.
RF on
Beam onBunches
at 12.4 MHz
−100

0

100

200 300
Time (µs)

400

500

600

Figure 3: Macropulse timing for clean beam delivery while
the cavity is on. Bunch spacing exaggerated for visual clarity.
The macropulse beam mode increases the bunch charge
for the same average current, improving the signal-tobackground ratio of the BPMs, which is too low for them tobe sensitive in CW mode because of the average-current
limitation of the UITF. On the other hand, because the

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macropulses are generated electro-optically with a finite contrast ratio of ≈ 250, very low macropulse duty cycles such as
those needed for viewscreen-safe operation result in a highrelative level of CW background during the nominally-offtime window, obscuring the desired image on viewscreens
after the kicker. This issue is not significant for other diagnostics because they can be used at higher beam duty cycles;
also, the BPM data acquisition is gated to the macropulse
signal and therefore insensitive to CW background.

BEAM-LINE SETUP
For this experiment, we chose a beam momentum of
6.5 MeV/𝑐 for deflection angles ≤ 4 mrad at full HAWG
output power. The deflection was detected by a pair of BPMs
located downstream of the cavity. With the cavity off, thebeam orbit was set up to be as flat as possible throughout thissection and geometrically centered in the cavity. A pair ofquadrupole magnets combined with a wire scanner locatedat the downstream end of the beam line was used to measure
the transverse phase space; because the bunch frequency
was chosen to place all bunches at the same kicker phase,the deflection from the kicker could be steered out with apair of correctors, restoring the orbit through the emittancemeasurement section. Figure 4 shows a schematic layout,
while Fig. 5 shows a photo of the cavity installed in the UITF
beam line.

RESULTS
Deflection Waveform
Due to time constraints, the beam test was performed without automatic stub-tuner control and RF feedback. However,after iterative matrix-based tuning of all modes and withtemperature-regulated water flowing through the inner conductor of the cavity, we found that the frequency stabilityof all modes was well within the bandwidth. The limited
RF duty cycle helped keep detuning due to RF heating to aminimum.
As a first test, we drove the cavity at half power per channel one mode at a time, the other amplifiers off, and scanned

the HAWG reference phase to measure the deflection waveform at each frequency. The angle of deflection for eachphase follows from the position signals of two subsequent
BPMs located downstream of the cavity. Figure 6 shows
an example of this measurement performed at the ninth harmonic, demonstrating the single-mode deflection is a cleansine wave.
Having tested each mode individually, we powered allmodes at the same time, measured the superposition waveform, and adjusted the phase and amplitude setpoints of themodes to give equal phases and amplitudes in the deflection
waveform as determined by a fit of𝑓 (𝜙) = 𝑓0 +5∑︁𝐴 𝑘 cos [(2𝑘 − 1)𝜙 − 𝜙 𝑘 ]

(1)

𝑘=1

with 𝜙 = 7𝜙ref . With all 𝐴 𝑘 equal and all 𝜙 𝑘 = 0, this would
give the desired waveform shown in Fig. 2.
Table 1: Fit Parameters for Waveform in Fig. 7𝑘𝐴 𝑘 (mrad)

𝜙𝑘

1
2
3
4
5

0.47
0.46
0.46
0.43
0.38

1°
2°
4°
4°
−16°

The waveform so obtained is shown in Fig. 7, its parameters listed in Table 1. While the waveform visually resembles
the target, the numbers show a discrepancy in the highest
mode. The dominant limitation preventing an accurate andreproducible synthesis of the target waveform is the lack ofboth tuning feedback and RF feedback in the present setup,causing the phases to wander. In practice, this is exacerbatedby each phase being slightly interdependent with the corresponding amplitude due to imperfections of the RF elementsin the HAWG, especially in the highest mode.
The adjustability of the modes turned out to be further
limited by multipacting, which could be observed as a vacuum response and intermittent jumps in the cavity field for

(a)

(b)
HK
BPMs & correctors forlaunch adjustment

Quads
Kick measurement
and compensation

Wire
scanner

Emittance measurement

Dump

Figure 4: Conceptual sketch of the beam line for the kicker test. Part (a) shows the essential components of the setup formeasuring only the deflection, while part (b) includes a kick correction with two steerers to restore the orbit for consistentquad scans. The dashed line represents the beam orbit with the kicker turned on; it is exaggerated for clarity in the bottom
figure.

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12th Int. Beam Instrum. Conf.
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0

Data
Fit

−2
0

Kick angle (mrad)

Figure 5: Harmonic kicker installed in the UITF beam linewith the HAWG next to it.
Data
Fit: 𝑦 0 + 𝐴 sin(𝜙 − 𝜙0 )1Figure 7: Deflection waveform with all five modes poweredand optimized.
the deflection with steering coils. The results are listed inTable 2, showing no measurable impact on transverse phasespace regardless of kicker phase; the point at 0° is at peak
deflection, whereas that at 15.4° is at maximum negative
slope.
Table 2: Downstream Twiss Parameters in the Deflecting
Plane of the Kicker

0.5

0
0

50

100 150 200 250 300 350
7 × 9 × 𝜙ref (degrees)

Figure 6: Deflection waveform with only the 9th harmonic(779.4 MHz) being driven at nominally 50 % output power.
The phase 𝜙ref being scanned is that of the 12.4 MHz reference and is scaled up accordingly for clarity. The beam angle
is not explicitly corrected to give zero with the cavity off.
𝑦 0 = 0.372(3) mrad, 𝜙0 = 75.1(7) ◦ , 𝐴 = 0.333(4) mrad.

This is a preprint — the final version is published with IOP

100 150 200 250 300 350
7 × 𝜙ref (degrees)50

several different combinations of RF parameters. While ourattempts to process out the multipacting between beam shifts
were unsuccessful, it is not expected to be present at higher
levels of drive power as needed for the beam momenta ofpractical applications.

Transverse Phase Space
As is the case for all deflecting cavities, the time-dependent deflecting field of the harmonic kicker will transformthe spread of arrival times—i.e., the bunch length—into a
spread of transverse angles, deteriorating the 2-d emittanceif the bunch length is significant compared to the temporalslope of the field around the central bunch phase. Multipole
components of the field will further affect the transverse
phase space if the transverse beam size is significant. Asa sanity check to verify that neither effect is visible withthe default bunch length of the UITF and an optics setupthat ensures 𝜎𝑥,𝑦 ≪ 1 mm, we performed quad scans at
the end of the line at different cavity phases, correcting for

7𝜙ref

𝛼

𝛽 (m)

𝜖 (nm)

off
0°
15.4°

-9.9
-9.6
-9.5

21.4
19.9
20.0

26.3
26.7
24.4

CONCLUSION
We successfully operated the harmonic kicker with all fivemodes and performed a beam-based measurement and optimization of the deflection waveform, finding no unexpectedissues other than multipacting. We expect the technical limitations of the drive circuit to be fully resolved by ongoingdevelopment efforts, making harmonic kicker cavities a viable option for future accelerator designs.

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Kick angle (mrad)2

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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 3 1[4] S. Overstreet et al., “Manufacturing the Harmonic Kicker
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