R.W de Leeuw, J.I.M. Botman, C.J. Timmermans,
W.J.G.M. Kleeven, H.L. Hagedoorn,
Two standing wave accelerating structures have been built for the operation
of two AVF racetrack microtrons (RTM). For the first RTM a 3 cell 1.3 GHz
on axis coupled standing wave structure has been designed to accelerate a
50 A peak current beam in 9 steps from the injection energy of 6 MeV to a
final energy of 25 MeV. The beam will be used as drive beam for the free
electron laser TEUFEL. The second structure accelerates a 7.5 mA beam in 13
steps from the injection energy of 10 MeV, to a maximum energy of 75 MeV.
This 9 cell on-axis coupled structure operates at 3 GHz and was designed
with a relatively large aperture radius (8 mm) in order to avoid limitations
on the RTM's acceptance. Design, fabrication and testing of the structures
have been done in house. For the design of the structures the combination of
the codes Superfish and Mafia has been used. Low and high power tests
proved that the structures live up to the demands. With the experiences
gained a design for the accelerating structure of the H
The Racetrack Microtron Eindhoven (RTME) has been designed to accelerate a
pulsed 7.5 mA electron beam from the injection energy of 10 MeV to the final energy
of 75 MeV [1].
The acceleration is achieved in 13 subsequent passages by a 5 MeV,
3 GHz standing wave on-axis coupled cavity, see sec.
The linac of the accelerator based neutron spallation source ESS project will
accelerate a 100 mA H
Fig. 1 depicts the schematic lay-out of the on-axis coupled
RTME cavity.
Table 1 lists some of the measured and related parameters of this
cavity [3].
Design and construction of standing wave accelerating structures
at TUE
Eindhoven University of Technology (TUE), Cyclotron Laboratory,
P.O. Box 513, 5600MB Eindhoven, The Netherlands.Abstract:
linac of the
ESS project has been made. The design of the cells as well as a novel type
of single cell bridge coupler will be presented.
Introduction
-beam over 660 m length from 70 to 1334 MeV. This paper
describes the cell and brigde coupler design in sec.
The RTME cavity
Figure 1: Schematic layout of the RTME cavity.
The 9 accelerating and 8 coupling cells are formed by stacking 18 accurately
fabricated square bricks of OFHC-Cu. These square bricks are used for
a repetitive mounting on the lathe and for the alignment of the total
cavity in a ridge. Here the pieces are kept together with a force of 
N by a multi-spring based clamping mechanism.
For the tuning the parts are mainly stacked in sets of 2 and 4 terminated with plates forming respectively 3 and 5 coupled resonators with 3 and 5 mode frequencies. From the three mode frequencies the accelerating and coupling cell resonant frequencies and the coupling coefficient are obtained. From the five mode frequencies also the direct coupling coefficient for the accelerating cells is obtained.
To tune the end parts they are stacked with their two tuned nearest neighbour
parts. This structure is covered with a plate. The 
-mode resonant
frequency of this structure is adapted to the tuning frequency by adapting the
frequency of the end part.
Table 1: Measured and related parameters of the RTME cavity,
K.
The dimensions of the waveguide-cavity coupling iris are determined by repetitive VSWR measurements in the waveguide.
Since in a perfectly tuned structure there will only flow major RF currents on
the outher surface of the accelerating cells, it was decided to only join the
two halves of the accelerating cells by brazing, whereas the two halves of the
coupling cells are joined by O-rings.
As brazing material Ag 
Pd 
Cu 
with a melting temperature of
780 
C has been used.
After constitution of the different parts in the ridge no vacuum leaks could be
detected.
After the completion of the structure the electric field profile of the
-mode has been determined by means of the pertubating ball method, see
fig. 2.
The standard deviation in the measured field amplitudes corresponds with 1%
of the average amplitudes in the cells, indicating that the structure is
properly tuned. It is not possible to quantify the magnitude of the electric fields
in the coupling cells.
Figure 2: The measured electric field profile in the RTME cavity.
The high power tests have been done with a 2 MW EEV M5125 magnetron that was connected to the cavity via a 4-port circulator. By means of an EH-tuner located after the second port of the circulator the amount of power sent to the cavity at the third port could be regulated [3]. At most as much as 1.6 MW of power was sent to the cavity, implying an energy gain of 6.1 MeV for the electrons. This means operation at a maximum field surface strength of 1.17 Kilpatrick field limit. At this field strength hardly any voltage breakdowns occured and no sign of multipacting was observed.
The fabrication of the TEUFEL cavity was done similarly as the RTME cavity. Table 2 lists some characteristics of the TEUFEL cavity [4].
Table 2: Accelerating cavity parameters
Due to the high peak currents in the cavity the structure will operate
under high beam loading conditions.
Therefore the coupling ratio is relatively high, 
.
The precise beam current to be accelerated in the microtron is not known yet.
The generator and reflected power in dependence of the macro pulse current
is depectid in fig. 3.
This was calculated with formula[5]
where r is the normalized reflected power (normalized w.r.t. the wall losses),
p is the normalized beam power,
,
, 
is the RF frequency and 
is the accelerating phase.
Figure 3: Required RF generator power
and reflected power as a function of the average macropulse current
for the TEUFEL cavity
The proposed lay-out of the linac of the European Spallation Source (ESS)
project [6] has two front ends, each with a H
source (70 mA, 50 kV, 10% d.c.), low energy beam transport, an RFQ, a beam
chopper and a second RFQ. Funneling is at 5 MeV.
A drift tube linac (DTL) operating at 350 MHz accelerates the beam up to 70 MeV.
In the reference design a 700 MHz normal conducting side coupled cavity linac
(CCL) further accelerates the beam to 1.334 GeV to feed the rings [7].
In the CCL a single 2 MW klystron will feed 2 tanks connected via a bridge
coupler.
The tank length is determined by limiting the peak power per tank to 0.75 MW.
It than varies from 1.27 (16 cells at 70 MeV) to 1.95 m (10 cells at 1.334 GeV),
short enough to allow constant cell length in one tank
(the phase slip per tank is 4 deg.).
The intertank gaps have a length of 5/2 
and 3/2 .
Over the shorter gap the two tanks are connected via a bridge coupler.
In the design of the CCL first the shunt impedance and the transit time of the individual cavities is maximised. Various parameters determining the shape of a cell are of importance. The shunt impedance increases with decreasing bore hole radius, web thickness between cells and nose cone thickness.
Fig. 4 depicts the values for the shunt impedance as optimised with Superfish. It is reasonable to lower this values by 20 % to account for the losses due to the coupling slots between accelerating and coupling cells and manufactoring imperfections.
Figure 4: Calculated shunt impedance as a function of the velocity B .
The lower curve represents the values for the shunt impedance lowered by
20 %.
In previous designs of long side coupled CCL's the outher diameter of the cells has been kept constant in order to minimise fabrication costs. With modern machining techniques, as programmable lathes, this is no longer necessary. The extra costs due to the variating outher diameter will not imply a significant cost increase. The diameter will be kept constant within a single tank.
For the calculation of the cell geometries we have the availability of the accurate 2D code Superfish and the less accurate 3D code Mafia. For the calculation of the coupling coefficient between the accelerating and coupling cells we need accurate 3D results. Therefore for this calculation the combination of the codes Superfish and Mafia has been used as described in ref. [8]. By varying the offset of the symmetry axis of the coupling cells to the symmetry axis of the accelerating cells the coupling coefficient can be varied between 2 and 8 %.
Due to the varying length of the bridge couplers a number of higher order
modes in these bridge couplers are within the passband of the accelerating
tanks [9].
At lower energies the TE 
mode crosses the passband. This mode can easily be
expelled from the passband by placing two round disks with a diameter of about
half the coupler diameter at the end of the coupler at the locations where
the electric field is maximum.
At higher energies the TE 
mode crosses the passband. This mode is expellled
from the passband by placing two rings in the coupler where the amplitude of
the TE mode is maximum.
The rings are large enough to expell the perturbing mode from the passband, but
small enough to assure that the resonator still operates as a single cell
resonator.
Mafia calculations on the combination of two accelerating cells that are connected
via coupling cells to the bridge coupler show that the method works.
One has to assure however that the shifted mode is well outside the passband
to avoid mixing with one of the chain modes.