Proceedings of the 25th Linear Accelerator Meeting in Japan (July 12-14, 2000, Himeji, Japan)[12P-18]

Design of W-band photoinjector
Xiongwei Zhu, Kazuhisa Nakajima
High Energy Accelerator Research Organization
1-1 Oho ,Tsukuba , Ibaraki ,305 , Japan
Abstract
we present a design study on W-Band photocathode RF gun which is capable of generating
and accelerating 300pC electron bunch. The design system is made up of 91.392GHz
photocathode RF gun and 91.392GHz travelling wave linac cells. Based on the numericalsimulation using SUPERFISH and PARMELA and the conventional RF linac scaling law, thedesign will produce 300pC at 1.74MeV with bunch length 0.72ps and normalized tranverse
emittance 0.5mm mrad. We study the beam dynamics in high frequency and high gradient; due tothe high gradient, the pondermotive effect plays an important role in beam dynamics; we foundthe pondermotive effect still exist with only the fundamental space harmonics (synchrotronmode )due to the coupling of the transverse and longitudinal motion.

1.Introduction
Recently, high gradient, high frequency
accelerators of mm-scale are of interest inadvanced accelerator research [1]. Researchon short wavelength, high gradient, RF
driven acceleration has concentrated on
90GHz which is in the range of W-Band(75GHz-110GHz),
itinvoles
theunderstanding of millimeter wave powersource, high power operation of millimeter
wave, and beam dynamics in high frequency
and high gradient, and technologies forfabrication and measurement of millimeter
accelerator.
The scaling of RF accelerator
withrespect to RF frequency f is as follow
Shunt impedance r~f1/2,
Accelerating gradient G~f,
Peak power required per meter~f1/2.
In this paper, we use the above scaling lawand codes (SUPERFISH, PARMELA) todesign the W-Band photoinjector, and study
the beam dynamics in high frequency andhigh gradient.

2.Design approaches and system
The design system is made up ofphotocathode
RFcavity,emittancecompensation drift section, and 60 2π/3
travelling wave cells. We use theconventional RF linac scaling law to scale
down the BNL S-Band photocathode RFcavity[2,3] and S-Band SLAC 2π/3 travellingwave cells. The RF cavity length is 0.46cm,
the drift length 2.3cm and the linac section
length 6.55cm as shown in Fig.1, the driftlength is decided in order to make a good

match between the photoinjector and linac cells.

... ...
cavity solenoid

linac cells

Fig.1 Setup of the design system
Table 1 and Table 2 list the structure
parameters of the RF cavity and the linac cells
which are calculated using SUPERFISH. The
shunt impedance and quality factor show agood agreement with the scaling law.
Table 1 The gun cavity design parameters________________________________
Inner radius of the cell(mm)1.3Radius of the iris(mm)0.39
Width of the iris(mm)0.689Length of the cavity
4.69
π mode frequency(GHz)91.392
Shunt impedance(MΩ/m)
269.67
Quality factor
2806_________________________________
Table 2 The linac cell design parameters________________________________
Inner radius of the cell(mm)1.303
Radius of the iris(mm)0.409
Width of the iris(mm)0.182
Length of the cavity
1.092
2π/3 mode frequency(GHz)91.392
Shunt impedance(MΩ/m)
284.5
Quality factor
2435________________________________−180−Because of the high frequency (smallstructure), it is difficult to achieve a highcharge of electron bunch. the maximum
bunch charge in the simulation is 300pC.
Above this value, the beam breaks up fastwith space charge effects. Fig.2 shows ther.m.s. emittance with bunch charge for thecondition B=5.8T, and the average RF cavity
gradient=1GV/m. In the W-band linac, thecharge of a single bunch charge isconstrainedbyabeam
break-up.
has
estimated
theZimmermann[6]longitudinal and transverse wakefield in a WBand ( 91GHz ) accelerating structure. The
transverse wakefield is almost completely
determined by the structure geometry ( irisradius ). For a 60pC charge, and a/λ≥0.18, thetransverse beam break up is negligible. Since
PARMELA does not include wake field or
beam loading in its calculation, we havestudied the 60pC bunch charge extensively .
In order to mininize the beam emittance,
we design the emittance compensation
solenoid using POISSON, whose peak field
is as high as 5.8T. We have simulated theemittance dependance on the solenoid field
shown in Fig.3 for the condition Q=60pC, theaverage gradient=1GV/m.
In our case, the peak electric field in thehalf and the full cell of the RF cavity is not
the same, the electric field amplitude haseffect on RF focusing and defocusing whichplay a role in the emittance evolution of thebeam, Fig.4 shows the beam r.m.s emittancedependance on the average field for thecondition bunch charge Q=60pC, B=5.8T.

2
1.5
10.5
00

100

200

300

Q(pC)

Fig.2 The r.m.s emittance dependance on thebunch chargeemittance(mm mrad)

Table 3 photoinjector parameters________________________________
solenoid peak field[T]5.8
gradient[GV/m]
1charge[pC]
300
bunch length[ps]0.72emittance[mm mrad]
0.5
energy[MeV]
1.74
energy spread
1.7%________________________________

0.2
0.15
0.1
0.05
04

567Bz(T)Fig.3 The r.m.s emittance dependance on thepeak magnetic fieldemittance(mm mrad)

We use PARMELA to study the beam
dynamics of our system. The RF cavity
works in π mode, while the Linac cells workin 2π/3 mode. Table 3 gives the typicalparameters of the photoinjector.

emittance(mm mrad)

3.Simulation result

10.8
0.6
0.4
0.2
00.75

11.25

1.5E(GV/m)

Fig.4 The r.m.s emittance dependance on thegradient
Up to now, there are few works on the beam
dynamics in high frequency and high gradientcase. Due to high electric field, the RF focusing
and defocusing will become more important.
We use the 60 linac cells operated at thegradient 1GeV/m to study the beam dynamics
in the W-band, the cells operated at thefundamental 2π/3 mode. PARMELA cann’ttake into account the high space harmonics
mode, where the field coefficients arecalculated using SUPERFISH. Therefore nohigh space harmonics exist. However, wecan observe the beam envelope osillation, theperiodical time of thisoscillation
approximately scales like the square of theelectric field amplitude, and is almost
independent of the bunch charge. Hence, wecan conclude that there exists the transverse
pondermotive effect of the fundamental mode.
According to the rf pondermotive focusing
theory developed by Hartman andRosenzweig[4,5], there will be no such effect if
only fundamental travelling mode exists.
However, if the coupling between thelongitudinal and transverse motion isconsidered, there will exist the pondermotive
effect for the case of fundamental travellingmode.

Fig.5 The beam envelope motion

4.Conclusion
The conventional RF linac scaling law works
well in the W-band structure. The pondermotive
effect becomes obvious due to the high gradient.
We conclude that there will exists thepondermotive effect in the case of fundamental
travelling mode due to the coupling between thelongitudinal and transverse motion.

Acknowledgement
This work is supported by the High Energy
Accelerator Research Organization (KEK) andthe Japan Society for the Promotion ofScience(JSPS).

Reference:[1]R.H.Siemann, Advanced Electron
Linacs ( 1997 ).
[2]D.Palmer, the next generation
photoinjector, Ph.D Dissertation,
Stanford University .
[3]W.Gai, Nuclear Instruments andMethods, A410(1998)431.
[4]J.Rosenzweig, L.Serafini, Transverse
particle motion in radio frequency
linear accelerators, Physical Review
E,Vol.49,No.2(1994)1599.
[5]S.C.Hartman, J.Rosenzweig,
pondermotive focusing inasisymmetric rf linacs, PhysicalReview E,Vol.47, No.3(1993)2031.
[6]F.Zimmermann, D.H.Whittum,
C.K.Ng, Wake fields in a mm-wave
Linac, SLAC-PUB-7899(1998).