Proceedings of the 2nd Annual Meeting of Particle Accelerator Society of Japanand the 30th Linear Accelerator Meeting in Japan (July 20 - 22, 2005, Tosu Japan)

STAINLESS STEEL RF DRY LOAD WITH SUPPRESSINGSURFACE ELECTRIC FIELD
A. Lounine, T. Higo 1 and N. Kudo, KEK, Tsukuba, JapanK. Watanabe, The Graduate University for Advanced Studies, Hayama, Japan

Abstract
An X-band RF dry load made of magnetic stainlesssteel, SS430, was designed to work at a low surface
electric field. The optimization process of the lossy
section was performed using an S-matrix formalism andthe maximum surface electric field is below 55MV/m at
an input power of 100MW. A new elliptical polarizer wasproposed making a smooth transition from the linearlypolarized point at the input of the load to the circularly
polarized point at the entrance to the lossy region. This
design suppresses the local enhancement of theelectromagnetic field, resulting in a very broad bandwidthof 1 GHz below –20dB.
Based on this design, a test load was fabricated. It wasclarified that the surface resistance of the magnetic
stainless steel SS430 was decreased by a factor of ~1.2
due to the high temperature brazing process. This load
has been high-power tested at XTF of KEK at 50MW.

2 DESIGN
2.1 Schematic shape of the present load
A schematic shape of the present load is shown in Fig.
1. It is composed of the upstream mode converter
followed by a series of lossy load cells. The mode
converter changes firstly the TE11 mode in the rectangularwave guide to the TE11 mode in the circular wave guide.
Then it converts the linearly polarized TE11 mode to thecircularly polarized TE11 mode when passing through anelliptical cross section with the axis 45 degrees rotatedwith respect to the original linear polarization axis. Thelossy section is made of magnetic stainless steel, SS430,
which has a large surface resistance at 11.4GHz, 19 times
higher than copper [6]. Each cells are self-matched blockwith a pair of chokes, in which RF loss is enhanced in aresonant-like excitation of TM-like mode.

1 INTRODUCTION
RF dry loads are preferred because no worry about thewater leakage into vacuum side. At the X-band highpower use, many of the existing dry loads took smooth
taper design. This case a finite length is needed formatching performance.
A stainless-steel high power X-band RF load was
proposed at SLAC in 1995 [1] which utilized resonantmode in a self-matched lossy load cells. This design
opened the potential to make loads compact. Many loadsof this design have been in operation for months at SLACat the linear collider test facility [2,3]. KEK imported
some loads and also fabricated one. These loads wereused in high power at up to several tens of MW and witha pulse width ranging from 100ns to 1.6 microsec. In thisusage, we sometimes encountered breakdowns in the load[4]. The similar breakdown behaviour was observed atSLAC at the upstream part of the lossy region [5].
Since the electric field of the original SLAC design at
the upstream is larger than those of downstream, wesuspect that the high electric field there is the cause of thebreakdowns. The edge of the groove is fairly sharp andthis is also one of the causes. In the present paper, weimproved the design by reducing the surface electric field
not only the lossy material region stated above but also
the transformer part upstream of the lossy region.
1Fig. 1

Schematic of the load.

2.2 Lossy cell shape
The basic structure of each lossy load cell is shown inFig. 2. The circular pipe diameter is fixed at 1 inch thesame as the original SLAC
design. The cell is composed ofa pair of chokes. The junction
from the pipe into the groove isrounded by r2mm to reduce thesurface electric field. The lengthof the cell is fixed at 40mm,
though further optimisation of
surface field is estimated when
considering the reflected wave
phase tuning as described later.

E-mail: toshiyasu.higo@kek.jp-317-Fig. 2

Load cell shape.

Number of cells was fixed at eleven, taking the same

Proceedings of the 2nd Annual Meeting of Particle Accelerator Society of Japanand the 30th Linear Accelerator Meeting in Japan (July 20 - 22, 2005, Tosu Japan)smooth line the reflection from the polarizer and the lastline the reflection from the whole load assembly.

number as the original SLAC design. Fig. 3 shows thetransmission properties of the cells. In order to make theload wideband, the frequency tunings of the cells are
distributed.

Max Surface E-field, [kV/cm]

1000

0

S12, [dB]

-1-2

# 1-2
# 3-4
#5#6
#7
#8
#9# 10# 11-3-4-5
10.0

SLAC Design
KEK_V3

900
800
700
600
500
400
300
200
100
00

12

3

4

5

6

7

8

9

10

11

12

Cell's Number
10.5

11.0

11.5

12.0

12.5

13.0

Freq., [GHz]Fig. 3Fig. 5 Electric field along the lossy section of the present
load in green bars. For comparison, the values of theSLAC load are also plotted in red bars.

Transmission performances of each cell.

2.3 Optimization of cell parameters
In the present design, we tried to reduce the surface
electric field rather than making the heat dissipation
uniform along the lossy section. To this end, anoptimisation process was performed as described below.
Firstly, we define a geometry factor of each cell,ki = Pi / PN ,where i is the cell number, P i – power dissipated in the ith cell and N the number of cells in the load. Then, we
can simply write a set of equations
where Aj is the transmission through cell i and ksum is theNºªAi »«N1(1  Aj ) Ai « N
i j »Aj »«i 1»«  Ai¼¬i jk gj
k sum,

2.4

Change of magnetic property at high temperature

Because of the possible change of the crystal
characteristic structure of the magnetic stainless steel
SS430 at a high temperature, the surface resistance may
decrease during the high-temperature brazing processes
during assembly. Fig. 6 shows the one-pass attenuation
studied with a test sample equivalent to the cell #1. Fourcurves are those measured as of machined, after firstcopper brazing at 1100C, after gold brazing at 1040C andafter second processing at 1100C. As shown in the figure,it was found that the first heat treatment mainly
determines the decrease of resistance. The change is notvery big but we designed the load based on this final
characteristics.

S12, [dB]sum of all ki. Secondly the linear functional form isassumed for ki and calculate Aj. Then, calculate the actual
field of each cell by using a 3D electromagnetic fieldsolver HFSS and follow a multi-parametric optimization
algorithm [7] to obtain a parameter set which gives therequired Ai while keeping the matched condition. Then,
we modify the ki trying to make the electric field uniform.
The iteration is repeated until the resultant electric field
becomes uniform to proceed.
The optimized result is shown in Figs. 4 and 5.

-0.60
-0.62
-0.64
-0.66
-0.68
-0.70
-0.72
-0.74
-0.76
-0.78
-0.80
-0.82
-0.84
-0.86
-0.88
-0.90
-0.92
-0.94
-0.96
-0.98
-1.00
11.40

C1a_no_heat_refl_0deg
0C1a_after_heat_N1(+1100 C)_refl_0deg
C1a_after_heat_N2(+10400C)_refl_0deg
C1a_after_heat_N3(+11000C)_refl_0deg

11.41

11.42

11.43

11.44

11.45

Freq., [Ghz]Fig. 60

Material change due to a series of hightemperature processes.

-10

S11, [dB]-20

2.4 Polarizer

-30

-40

-50
Load3_S12_Trans
Elliptic_Polarizer_Refl
Load3_Refl

-60

-70
11.0

11.1

11.2

11.3

11.4

11.5

11.6

11.7

11.8

11.9

12.0

Freq., [Ghz]Fig. 4 The characteristics of the optimized load. The top
curve is one pass insertion loss of lossy cells, the lower

As shown in Fig. 1, a smooth geometrical transition isformed from the rectangular cross section to the circular
one by connecting both periphery shapes with straightlines. The linearly polarized TE11 mode to the circularly
polarized TE11 mode is performed with inserting an
elliptical shape in between with 45 degrees rotated. Dueto its smoothness, the matching frequency dependence iswider as shown in Fig. 7.
-318-

Proceedings of the 2nd Annual Meeting of Particle Accelerator Society of Japanand the 30th Linear Accelerator Meeting in Japan (July 20 - 22, 2005, Tosu Japan)

bandwidth below –20dB is 1GHz.

0-10S11, [dB]-20-30-40
Elliptical polarizer-50-6011.2

Pin-polarizer

11.3

11.4

11.5

11.6

11.7

11.8

Freq., [GHz]Fig. 7 Comparison of two types of TE11 mode circularpolarizer, pin-type (SLAC) and smooth elliptical type(present one).
Fig. 93 FABRICATIONAn RF load of the present design has been fabricated atKEK. The process of load fabrication consists of threesteps as listed below. The lossy load cells are processedin a high-temperature treatment at least three times.
Polarizer part
1. Polariser parts are made of oxygen-free copper. The
inside shaping is made by wire electric dischargemachine. These parts are cleaned by chemicalpolishing.
2. These parts are brazed with gold-copper alloy at atemperature at 1040C.
Lossy stainless steel part
3. The lossy magnetic stainless-steel parts are made byturning lathe. The surfaces were cleaned by chemical
polishing.
4. These parts are brazed to form a stack of lossy cells.
This brazing is done with copper metal at 1100C.
5. Water cooling jackets are brazed on lossy cells.
Final assembly
6. All sub-assembled parts are brazed at around 1040C.
7. Vacuum pumping port flange is TiG-welded.

Fig. 8 Parts for the present load. Left: elliptical polarizer
and right: lossy load cell.
The final matching performance of the load is shownin Fig. 9. A smooth line is for polarizer itself, while theother two lines are those for the whole load, one bycalculation and one by experimental values. The
matching at 11.4GHz is better than 40dB and and theFinal matching performance of the present load.

4 DISCUSSION
The degradation of the lossy property of magnetic
stainless steel was clearly observed and it is not too large
so that we can use the material. A design of –40dB
matching with 1GHz bandwidth below –20dB wasestablished. The load has been fabricated and recently
tested in high power. It was processed easily beyond
50MW with a pulse width of 400ns [4], proving that it isa good candidate for X-band RF high-power load.

REFERENCES[1] S. G. Tantawi and A. E. Vlieks, “Compact X-band
High Power Load Using Magnetic Stainless Steel”,
Proc. 1995 Particle Accelerator Conf., p. 2132.
[2] W.R. Fowkes, E.N. Jongewaard, R.J. Loewen, S.G.
Tantawi and A.E. Vlieks, “An All-Metal High Power
Circularly Polarized X-Band RF Load”, Proc. 1997
Particle Accelerator Conf., Vancouver, Canada, 1997.
[3] D. Schultz, C. Adolphsen, D. Burke at al., “Status of a
Linac RF Unit Demonstration for the NLC/GLC Xband Linear Collider”, Proc. 9-th European Particle
Accelerator Conference, EPAC’04, Lucerne, 5-9July, 2004.
[4] K. Watanabe et al., “X-band High Power Component
Study for High Gradient RF Linac”, 20P080, in thisconference.
[5] M. Ross, Private communication.
[6] H. Sakai et al., “Measurement of the surface
resistance of iron, stainless steel and Kanthal at
3~16GHz range”, JLC-NOTE No.40, 1993, inJapanese.
[7] R.HOOKE, T.A.JEEVS, “Direct search solution ofnumerical and statistical problems”, Journ. Ass.
Comput. Mech., 8, 1961.

-319-