PROPOSAL ABOUT HIGH-CURRENT POLARIZED ELECTRON SOURCE
WITH LONG LIFETIMEON THE BASE OFSECONDARY-EMISSION MAGNETRON INJECTION GUN
S.A. Cherenshchykov, Kharkov Institute of Physics and Technology, Kharkov, Ukraine,e-mail cherench@kipt.kharkov.ua
Abstract
Polarized electrons from the secondary emission
were obtained from different materials by several authors.
The secondary emission magnetron injection gun is a highcurrent electron source with the cold cathode. The gun isnovel universal electron source for charged particle
accelerators. This gun is based on an unconventional
principle. The principle is the self-sustained secondary
emission multiplication in crossed fields. It can be
expected that the beam from the secondary emission
magnetron injection gun may be polarized. The
possibility of creation polarized electron source on thesimilar principle is discussed. The main expectedadvantage is unlimited lifetime times at ordinary vacuumin high current (up to any hundred amperes) pulse mode.
Expected level of polarization is up to 50% or more over.

1 INTRODUCTION
The secondary emission magnetron injection gun
is a high current (the current up to 200 A and more) anddurable (the lifetime is more than 100,000 hours) electron
source. The gun is novel universal electron source forcharged particle accelerators. This gun is based on an
unconventional principle. The principle consists in the
self-sustained secondary emission multiplication incrossed fields.
Many years ago secondary emission polarized
electrons were obtained from different materials by seriesof authors. It can be expected that the beam from thesecondary emission magnetron injection gun may bepolarized. The possibility of creation of polarized electron
source on the similar principle is discussed. The main
expected advantage is unlimited lifetime at ordinary
vacuum in high current (up to any hundred amperes)
pulse mode.

2 THE PROBLEM SETTING
In 1930, Fuse and Hellman [1] put forward an
idea to obtain polarized electrons via the secondary
electron emission from magnetic materials. The earlyexperiments have already shown that the polarization up
to 32% at low currents from the europium oxide (EuO)
cathode can be obtained by means of this method.
Polarized electrons from the secondary emission wereobtained also from metals (with polarization level up to50% for Fe).

However, now only polarized sources based
upon the photoemission method using the semiconductor(e.g., GaAs) photocathodes have been successfully usedin nuclear and particle physics experiments. Typically,
the lifetime of such cathodes exceeds 100 hours at thebeam I=0.2 mA current with keeping an extremely highvacuum regime (~10-13Torr) [2,3] and decreases as 1/I
with the current increase. Other possible methods ofcreating polarized electron beams were not developed
intensively during the two decades in view of difficultiesof their realization and relatively low extracted beam
currents.
The contradiction between the wishes to get thesatisfactory current and lifetime values simultaneously
takes place also for usual (unpolarized) electron sources
with thermionic cathode [4]. However, this contradiction
is much more dramatic in case of polarized electron
sources.
The secondary emission magnetron injection gun(SEMIG) [4,5,6] is proposed as a high current (the current
density is about 100 A / cm2 with the emitting surfacebeing large) and durable electron source with the cold(the temperature varies from 0 K to the melting point)
cathode.
At present, guns of such type have pulse
duration up to 3000 µs at the low current and repetition
rate of 50 Hz [4]. The current beyond 200 A is achieved[6] for short (about 1 µs) pulse duration values. The beam
current density exceeds 100 A/cm2 [4]. The gun is thenovel universal electron source for charged particle
accelerators. It was tested in linac [5]. Recently thecurrent up to 4 kA was achieved.
The operation lifetime of similar cathodes [7] inmagnetron amplifiers is estimated as 100,000 (hundred ofthousand) hours.

3 GUN DESCRIPTIONS
It can be expected that the beam from thesecondary emission injection magnetron gun with amagnetic cathode should be polarized. The secondaryemission magnetron injection gun with a magneticcathode (SEMIG-MC) design is rather simple. See Fig.1

Solenoid

Insulator

CathodeAnodeFigure 1: Schematic representation of the SEMIGThe SEMIG-MC will consist of a hollow tube
anode and a coaxial finger-like magnetic secondaryemission cathode inside the anode. A solenoid provides
the magnetic field directed along the cathode axis. The
correspondence between the magnetic field and applied
voltage pulse values is chosen such that electrons cancome back to the cathode. The electron multiplication
starts at the trailing edge of the negative high-voltage
pulse applied to the cathode. See Fig.2.

Figure 2: Excitation process in SEMIG.
Upper curve is voltage; lower curve is beam current.
After initiation, the electron emission continues
at the top of the next part of the pulse. The secondary
emission is caused by an additional energy of electrons
coming back from the turbulent electron flow in a crossfield gap due to increasing electron space charge. Near
the cathode edge, the electron space charge gives an axial
component of the electric field. The axial component
leads to the electron injection and producing electronbeam. The electron beam polarization arises due topolarization of electrons of the cathode material.

4 DISCUSSION
Let's consider the factors that influence on thelevel of polarization. After an emission the electrons
move in the crossed fields. Polarization of the electrons
directs along a magnetic field. The spatial position ofpolarization direction is unchanged, but only the directionof its motion fluctuates. The motion in magnetic fieldwith direction of polarization along magnetic field does
not influence at all (in a non-relativistic approximation)
on spin direction. Therefore gun creates a beam withlongitudinal polarization. The electron interaction in thespace charge is only electrostatic and does not influenceon spin. Such interaction even in case of scattering ofelectron gas on by a twelve order (1012) greater thandensity in easy metals, as shown experimentally [3] does
not influence on absolute direction of a spin.
The negligible influence to spin direction canoccur for escaping electrons from a magnetic field. The
magnetic-mechanical spin ratio is twice greater then theorbit alone. In this case, the divergence of spins
approximately twice will exceed an angular divergence ofa beam. The angular beam divergence, as was shown
experimentally [5] in the accelerator, did not exceed 10-2
radian due to a choice of thin cathode. Even twice morechange of spin direction gives the contribution to a
relative modification of polarization in 4·10-4, i.e.
magnitude smaller then errors of a polarization
measurement.
Consider further factors capable to increase adegree of polarization. As it is noted above stream ofprimary electrons was polarized too due to self-support
character of an emission. Inelastic reflected electrons areunpolarized in case of unpolarized primary beam. In ourcase it is save polarization of primary electrons. The partof inelastic reflected electrons in relation to amount ofsecondary electrons weakly depends from energy ofprimary electrons, its angle of incidence and material of
the emitter. In the roughest approximation the amount of
inelastic reflected electrons will be about a one third ofprimary electrons and nearly one fifth of all secondaryelectrons. As a first approximation the relative value ofpolarization will increase on the same magnitude also, i.e.
on 20 %. The effectiveness of electron interaction withuniformly directed spins is higher due to effects ofexchange interaction. This contribution to magnificationof polarization will be difficult for evaluating
theoretically or numerically from want of the satisfactory
theory of a secondary emission. It gives the additional
contribution to magnification of a degree of polarization
for primary polarized beam (i.e. self-support secondaryemission). The other factor of increasing polarization will
be selection of low energy electrons, due to preferential
incoming of electrons with a small energy to an electron
cloud. The electrons with the greater energy are returnedto the cathode with the greater probability. The
experiments on a nickel show, that after selection of low
energy secondary electrons, the polarization is increased
from 8 % up to 17 %, i.e. approximately twice. Nickel is
not most effective material for polarization of secondary
electrons. For iron (Fe) the polarization for unpolarizedprimary beam without selection of low energy secondary
electrons achieves 50 %. Here it is necessary notice, thatthe oxidation of a nickel gives a drop of polarization from
17 % up to 12 %. It is possible assume, that the inverse
replacement oxide on metal will give appropriatemagnification of polarization. Having multiplied relativemagnifications of polarization obtained with above
factors we shall receive value close to 100 %. It will be
the optimistic prognosis of polarization level for theoffered source. Realization only the pessimistic prognosis(50% for Fe) for which the total influence of these factors
will appear insignificant will give a source competitive on
quality of polarization. The realization only of thepessimistic prognosis with of lifetime two orders greater
will be anew pitch in development of sources of polarized
electrons.

ACKNOWLEGMENT
Author thanks Prof. C.K Sinclair for attention tothe proposal. Thank Dr. V. Gorbenko and Dr. L.Levchuk
for fruitful discussion and support. Thank Dr. N.
Gaponenko and Dr. V. Zhiglo for assistance indevelopment of the SEMIG with current up to 4 kA.

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