ACCELERATION TEST OF DAW STRUCTURE
T. Tanaka, K. Tsukada, K. Satoh, K. Hayakawa, T. Ozaki,
N. Nakamura, K. Okamoto, 0. Takeda and M. Nishinaka
Atomic Energy Research Institute, Nihon University
Abstract
A test of a disk-and-wa sher (DAW) acceleratio n structure has
been performed using a pulsed electron beam which has incident energy about 2 MeV. The energy gain 4.5 MeV was obtained when thepeak rf power 1.8 MW was provided. From this result, the effective
shunt impedance of the DAW tube was found to be about 51 Mn/m,which is about a half of a calculated value for a DAW tube without
end-cavity or rf coupler.
1. A feature of tested DAW tube
The DAW tube consists of five acceleratio n cavities, whichcontains a rf coupler and three washers. Total length of the tube
is about 30cm. An outline of the DAW tube is illustrated in fig.l.
Each .washe·r is· supported by two radial stems which have an angleof 90° from each other. The stems are made of copper plated stainless steel pipes so that the cooling water may flow through thestems into the washers. The size of each part of the tube was fitted using a computer code "SUPERFISH" so that the effective shunt
impedance of the DAW cavity might be maximized under conditions
that the bore of the washers was fixed to 15 mm, and that the acceleration mode lay near 2856 MHz. The optimized values of theeffective shunt impedance and the unloaded quality factor werecalculated to be 95 Mn/m and 31200 respectivel y, where the effectsof the end-cavitie s or the coupler were not taken into account.
The frequency of the acceleratio n mode was tuned by using of tuners which were fitted to cylinders. The measured values of the unloaded quality factor and the frequency of the completed DAW tube
are about 20000 and 2857 MHz at room temperature , respectivel y.
2. Acceleratio n test
The rf power can be provided from a 5 MW pulse klystron invariable from 0 to 2.5 MW. The rf pulse width is about 7 ~s, andthe maximum repetition rate is 200pps. The DAW tube was aged over
200 hr at 10 pps, increasing input rf power up to 2.5 MW gradually.
The washers and the coupler were cooled by air instead of water
because of vacuum leak at the coupler. Therefore, the resonancefrequency of the tube was changed several hundred kHz during anexperiment, which was matched by varing input rf frequency.
An electron beam of pulse width 2 ~s and peak current lOOmA
was injected in the tube. The incident energy was about 2 MeV. The
electron beam accelerated in the DAW tube was momentum analysed inthe air using a 30° deflecting magnet. The beam current was monitored at the entrance and the exit of the tube. The beam penetrability was found to be about 70 % when the beam was enough accelerated, where the beam size at the exit of the tube was less than
10 mm wide.
The energy of the accelerated electron beam was obtained bymeasuring the field strength of the deflecting magnet when thecenter of the beam profile was seemed to be deflected by an angleof 30°. Fig.2 shows the relation between the electron energy andinput rf power. The beam profile monitored using a ZnS plate at
the down-stream of the deflecting magnet was several em wide because of the scattering of electrons by a Ti-foil window at theexit of the tube. Therefore, the electron energies obtained using
above method will have large errors because of the uncertainty of
the center of beam profile.
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The effective shunt impedance of the DAW tube hs been estimated from the energy gain as Sl.Mil/m, which is about a half ofthe calculated value. If the effects of the end-cavitie s and thecoupler is taken into account, the effective shunt impedance isexpected to be about 73 Mnjm. The measured value is still lessthan this one, which will be due to existence of stems. However,further discussion will need more detailed and accurate data. We
are now preparing for more accurate measurement of the electronenergy, and a 1 m long DAW tube is under constructio n at Mitsbishi
Electric Corporation . The effective shunt impedance of the new DAW
tube is expected more than that of tested one because of relatively small effects of the end-cavitie s or the coupler.

Fig. 1. An outline of the tested DAW tube. The electron beam is
injected from left hand side of this figure.

(MeV)

(500:
UJ 6zUJz 4-00:000~u 2UJ.....JUJIIII

0.5

1.0

1.5

2.0

RF POWER (MW)Fig. 2. The result of the measurement of relation between input rfpower and the electron energy.

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