HIGH POWER TEST OF A KANTHAL-COATED L-BAND LOSSY CAVITY
F. Miyahara#, Y. Arakida, Y. Higashi, K. Kakihara, S. Matsumoto,T. Higo, KEK, Tsukuba, JAPANK. Saito, H. Sakurabata, Hitachi Ltd., Hitachi, JAPANAbstract
We have been developing a Kanthal (Al-Cr-Fe)-coatedcollinear load as a possible candidates for the L-band
accelerating structure of SuperKEKB positron capturesystem. We understand that the Kanthal-coated cellshould be studied in high power to confirm the feasibility
at our design field of 10 MV/m level. To this end, wehave made a high power test cavity. The cavity consists of3 cells, one of which was composed of Kanthal-coateddisks to lower the intrinsic Q value from 20000 to theorder of 1000. The cavity reached 10 MV/m level after 30
hour operation but showed frequent vacuum activity
which prevent the stable long pulse operation of 1.52 s.
In this paper, the cavity production and the experimental
result in high power are reported.

INTRODUCTION
In order to achieve the 5 times higher capture efficiency
in SuperKEKB comparing to that of KEKB, the upgradeof the positron production and capture section is required.
The system consists of a Tungsten conversion target witha flux concentrator followed by accelerating structures
surrounded by solenoid magnets. For the upgrade ofSuperKEKB linac, two possible system layouts for thepositron capture section are considered. One of themconsists of two L-band accelerating structures with thecollinear loads followed by four 2-m long large aperture
S-band accelerating structures (LAS), while the other ismade up from six LAS’s. Especially for the former case,the output coupler part of the accelerating structure ispreferred to be replaced by the collinear load to compose
the system with compact magnets and to minimize the dip
in the solenoid field. The load consists of 5 Kanthalcoated cells. For both cases, the reduction of satellite
bunches is one of the important features of the capture
section, because it causes the beam loss in the damping
ring after injection [1].
If the L-band frequency(1298 MHz, which is 5/11 of S-band linac frequency) isadopted, the coprime frequency relation effectively
reduces the satellites. In the latter case, a preliminary
simulation study indicates that the accelerating field
higher than 14 MV/m in the first accelerating structure isrequired. From cost consideration, we adopted the latter(6 LAS’s) system for the first stage of the SuperKEKB.
However we keep the possibility to adopt the former caseafter beam commissioning. In both cases, the positron
beam loss and the production of satellite bunches arecaused between the 1st and the 2nd accelerating
structures. Therefore, it is also better to shorten the gap.
The present study of the collinear load gives us theimportant information for the application to LAS.
Considering the above ideas, we have been continuing thedevelopment of the collinear load even after the choice tochoose the LAS system for the initial commissioning.
The design of the load and the properties of Kanthal
were reported in previous works [2, 3]. The high power
evaluation of Kanthal-coated cell at S-band is reported [4].
However, there is no precise information of properties ofthe Kanthal-coated load especially in a high power RF
field at the frequency of L-band, except for the existenceof L-band linac based on Kanthal-coated cell [5], wedesigned a Kanthal-coated standing wave cavity andperformed the high power test. In this paper, we reportdesign of the load and the test results

LOSSY CAVITY
Although the collinear load is connected to thetravelling wave (TW) L-band accelerating structure [6],we adopted standing wave cavity for high RF power test
to save cost.

Design of the Cavity

Figure 1: The cross section of the lossy cavity.
A schematic of the cavity is shown in Fig. 1, where it
consists of the input wave guide connected to thematching cell, the load cell and the detuning cell which
supress /2 mode. Disks in the load cell are coated with
0.1 mm thick Kanthal layer.
The coating area, the radiuses of irises and thecylinders were optimized by using CST [7]. The load cell
was designed so that the cells has the same Q-value as the
1st cell of the TW collinear load cells, the Q=1300. The
field is also designed to be in the similar valance to thelast regular cell in TW case. The reflection coefficient
lower than -20 dB is realized.
Because the surface resistance of Kanthal layer isinfluenced by its microstructure, which depend on
spraying techniques [2], the electric conductivity  andthe permeability are not well known. We have chosenthe =2.58 [8] and the =22000 S/m to realize the same

Q-value reduction ratio. Then, the coating area isdetermined from r=71 to 87.5 mm.
The electric field strengths of 0 and  mode are shownin Fig. 2. In the case of -mode, the maximum field in theload cell is about 80% of the matching cell, which is close
to the TW collinear load case. In the Table 1 are listed themaximum fields on axis (Ez) and on surface (Esurfmax), inthe SW cavity at 2 MW RF input power and the TW
accelerating field and the surface field (Esurfmax).

waveguide to cavity coupling was almost 1 (criticallymatched). The field balance between the two cells is alsomeasured by using bead pull method. The field balance
Eload/Ematching was about 0.74, almost the same as designed.

Figure 4: High power test configuration.

HIGH POWER RF TEST
Figure 2: The electric field distribution on axis for
2 MW RF power. Red line and blue dashed line
represent  and 0 modes, respectively.
Table 1: Accelerating and Maximum Surface Field in
Each Cell
Ez / Esurfmax [MV/m]
TW last regular cell

10/20

TW 1st load cell

8/16

Matching cell

12/21 for Pin=2 MW

Load cell

10/19 for Pin=2 MW

Low Power Measurement

Experimental Setup
The high power test setup is shown in Fig. 4. Forward
and backward pulses at the klystron [3] exit and in frontof the cavity were monitored by directional couplers.
About 40% of the klystron power was transmitted to thecavity. The signal from the pickup antenna and thepressure measured at each ion pump and CCG are also
monitored. All readouts of the pressure were used for theinterlock system. The typical threshold of the interlock isset 2x10-4 Pa.
In the positron capture section, the accelerating
structure with the collinear load is confined in the 0.4 T
solenoid field. To make the present test system close tothis condition, the cavity was inserted in an existing largesolenoid magnet which gave a magnetic field of 0.09 T at
the current of 200 A, limited by the power supply.
Two photomultiplier tubes were set to view thewaveguide at just after the klystron and at the top of thecavity. These were used to catch the light emission due toany discharge or field emission. A faraday cup wasconnected to the detuning cell to measure the dark current.
A quadrupole mass-spectrometer was connected to thevacuum port attached to the detuning cell.

RF Signals

Figure 3: Reflection as a function of frequency. Red line
represents simulation result and blue experimental datameasured in the atmosphere.

The S-parameter was measured using a network analyser.
Figure 3 shows the reflection coefficient of the cavity.
The resonance frequency of -mode is approximately
0.5 MHz lower than that of designed one. This error
partly due to the ~0.2 MHz drop by air with humidity and
0.3 MHz unkown, probably machining error. TheThe 2.2 s RF pulse signals transmitted to the cavity
and the pickup antenna from the cells are shown in Fig. 5.
This long pulse test was performed at the very low input
power due to the frequent vacuum jumps at higher power.
The reflection in the steady state was less than -20 dB,making most of the input power absorbed in the cavity.
The ratio of the pickup signals, load/matching, was 0.73,
which was consistent with the field balance measured by
the bead pull method.

Performances seen in High Power Operation
The high power test, with its goal of 4 to 6 MW at theklystron, was started at the pulse width of 500 ns,
minimum available from the LLRF system. The inputpower and the pulse width was increased slowly so thatthe base pressure did not exceed ~4x10-6 Pa and thevacuum trip rate was kept low. The input power reached
6 MW with 500 nsec after 30 hours operation.

Figure 5: Input/reflected RF power to/from the cavity(upper) and field monitor signal (lower) at pulse width
2.2 s.
A comparison of the vacuum activity between (a) thebeginning of the test and (b) the timing after 220-hourconditioning is shown in Fig. 7. The peak powers werealmost the same, but the pulse width at (b) was longer by
250 ns. It is seen that the amount of the out-gassing andthe frequency of the spike became small through the RF
conditioning. Note that the base pressure of (b) was
higher than (a), because the evacuation by IP’s in (a) waschanged to turbo molecular pumps (TMP) in (b). Afterthe evacuation by TMP’s was switched back to that byIP’s, we confirmed the base pressure around the cavity
was 8x10-7 Pa, lower than (a). Also to be noted is thatmany of the vacuum jumps in (b) were due to other parts
than the cavity, such as the klystron window.
The correlation between the vacuum spike and thevariation of the reflected pulse just after RF off wasobserved, though many of the spikes showed nocorrelation. The dominant residual gases such as H2O, COand N2, CO2, and H2 were observed. Note that the partialpressure pattern has not changed much. It is difficult toidentify the observed large out-gassing source, due to thehigh conductance through the L-band wave guide. Afterthe operation for several days, we checked the innersurface of the cavity by using an endoscope, but nodamage was observed.
We have measured the dark current by varying theinput power and the solenoid coil. We found that the darkcurrent was nearly proportional to the input power, which
is significantly different from the Fowler-Northeim fieldemission theory [9]. It is to be noted that it became
narrow maximum (16pA at 2 MW and at 25 Hz) at a
specific magnetic field of about 0.4kG, while at othermagnetic fields deviated by more than 10% it stayed at
the noise level of a few pA. The large PMT signal andbase pressure increase were also observed at thismagnetic field.
In summary, we observed the following characteristics,
 Cavity reached 10 MV/m level after 30 hoursoperation.
 Second peak of the reflected pulse varied when thepressure jumped but not always.

Figure 7: Comparison of vacuum activity between (a)left, early stage and (b) right, after 220 hour additional
conditioning. Figures from top to bottom are (up)pressure of the cavity, (middle) the klystron peak power
during 9 hours operation and (bottom) time interval
between pressure jump exceeding 2x10-6 Pa.
 Residual gas consisted of H2O, CO/N2, CO2, and H2.
 Dark current of a few tens of pA with big jitter wasobserved.

Discussions
The cavity reached the design gradient after 30 hour
operation, though in a shorter pulse. The RF conditioning
is still in progress and it is clear that the cavity processing
is certainly advancing. The main obstacle is the vacuum
burst actually happening sometimes in the cavity but its
mechanism is not understood. The dark current, pressure
increase and the light emission were observed at thespecific solenoid field. Simulations and the experimental
setup modification are needed to understand these. If
successfully performing these, we will confirm thefeasibility of the Kanthal-coated load.

REFERENCES[1] N. Iida, et al., “CSR in SuperKEKB superB DR”,IPAC2011, THYA01.
[2] F. Miyahara, et al., “Development of L-Band Positron
Capture Accelerating Structure with Kanthal-coatedCollinear Load for Super KEKB”,IPAC2012, TUPPR051.
[3] F. Miyahara, et al., 9th PASJ, THLR04, 2012 (in Japanese).
[4] M. Dohlus, et al., Internal Report, DESY M96-19, 1996.
[5] N. C. Pering, et al., IEEE Trans. Nucle. Sci. 16,1969, 316320.
[6] S. Matsumoto, et al. LINAC2010, MOP012.
[7] CST, http://www.cst.com/[8] X. D. He, “X-band Hybrid Dielectric-iris-loadedAccelerator Investigations”, Ph.D Dissertation, 2009 (inChinese).
[9] R. H. Fowler and L. Northeim, Proc. Roy. Soc. A119, 137,
1928.