VIII.
MICROWAVE TUBE RESEARCH
A.
H. A. Haus
C. R. Russell
A. G. Barrett
C. Fried
L. D. Smullin
Prof. L. J. Chu
NOISE IN ELECTRON BEAMS
The shielding of the apparatus described in the previous Quarterly Progress Report
has been completed,
and very satisfactory semiautomatic operation has been obtained.
The focusing magnet has been rewound and calibrated.
With the improved focusing con-
ditions the interception current was reduced to below 0. 1 percent for all but very weak
Typical interception current curves are shown in Fig. VIII-1.
magnetic fields.
The Brillouin field corresponding to the curves of Fig. VIII-1 is approximately 370
gauss.
A number of noise measurements have been made both for dc and for pulsed
Figures VIII-2 to VIII-6 show some of these measurements.
operation.
that the beam diameter d is
0.20 inch as calculated,
VIII-2 to VIII-5 are 262,
ponding to Figs.
307,
362,
the Brillouin fields corres-
and 370 gauss respectively.)
curves including the fine structure were well reproducible.
a
L
U.b
z
w
cr
0.4
(If we assume
Instability, however,
The
was
r 0.2
z
O
0
-
w
ANODE
-02
o
-0.4
z
-0.6
MAGNET
SHIELD
a.5
SI
0.68
Fig. VIII-1
Interception current vs cavity position for convergent-flow
= 16. 6 4a;
pulsed electron beam V = 1500 volts; I
pressure
0 7 X 10
7
mm.
-35-
INTERCEPTION CURRENT< 1/10% , H 344 GAUSS
INTERCEPTION CURRENT< 1.5% , H 258 GAUSS
SHOT- NOISE LEVEL
'
0
-5
-10
iv
-15
-, ,,
9n
I\
tf
~_
4
1l
I
/
1 ,"
\/"
10
15
20
DISTANCE (CM)
25
30 32.5
DISTANCE (CM)
MAGNET
I I L
C.)
0.68
0.62
a"
INTERCEPTION CURRENT < 1/10%,
H- 430 GAUSS
o
INTERCEPTION CURRENT <
/
1/O %, H - 598GAUSS
SHOT- NOISE LEVEL
0
-5
20
0
DISTANCE (CM)
5
10
15
20
25
30 32.5
DISTANCE (CM)
Fig. VIII-2
Noise power vs cavity position for convergent-flow dc electron beam. Icoll = 1. 7 ma;
-7
col
V = 700 volts; pressure
4. 5 x 10
mm; beam diameter = 0. 035 inch.
IJTERCEPTION CURRENT < IV,
*-H 258 GAUSS
5
SHOT- NOISELEVEL
0
5
-10
-15
-20
10
5
15
20
25
30
32.5
DISTANCE (CM)
DISTANCE(CM)
M
MAGNET
jODSHEO
GAVITY
I
INTERCEPTION CURRENT< 1/20% . H- 430 GAUSS
0
SHOT-NOISE LEVaEL
-5
-10
-15
-20
0
5
10
15
20
25
30 32.5
DISTANCE (CM)
DISTANCE (CM)
Fig. VIII-3
Noise power vs cavity position for convergent-flow dc electron beam. Icoll = 2. 8 ma;
-7
ch
mm; beam diameter w 0. 035 inch.
5 x 10
V 0 = 1000 volts; pressure
INTERCEPTION CURRENT< 2% , H * 258 GAUSS
INTERCEPTION CURRENT<
I%, H
344 GAUSS
5
SHOT-NOISE LEVEL
S0
ii
uJ -5
z.01
/
/ I
P-in
V
/ /
-10
-5
v
W
Ln -15
2- 20
-20
5
10
15
20
DISTANCE (CM)
INTERCEPTION CURRENT
25
30 32.5
O
< 1/20 % , H430 GAUSS
IS
20
DISTANCE (CM)
25
30 325
INTERCEPTION CURRENT < 1/20 % ,H 602 GAUSS
SHOT -NOISE LEVEL
SSHOT- NOISE LEVEL
*0
2
-5
R
10
.0
5
0
5
-5
-10
-20
O
5
10
20
15
DISTANCE (CM)
25
30 32.5
O
5
10
15
20
DISTANCE (CM)
25
Fig. VIII-4
Noise power vs cavity position for convergent-flow dc electron beam.
-7
V o = 1500 volts; pressure
=
8 x 10
Icoll = 4. 8 ma;
mm; beam diameter~ 0.035 inch.
30 32.5
H 301 GAUSS
25
III
H 344 GAUSS
I
25
r
I
/
7!:..
I I
/
/
/T
/
LE/L
SHOT-NOISE
SHOT- NOISE LEVEL
SHOT- NOISE LEVEL
W
-5
cz 10
z -15
-0
-15
-20
-20
S 5
AwME
S 7 ?L
10
15
20
DISTANCE(CM)
25
30
32.5
0
5
10
20
15
DISTANCE (CM)
25
30 32.5
C7VITY
II
8 40.62
H- 430 GAUSS
H -602 GAUSS
//////
/
10
5
0
SHOT- NOISE
I
LEVEL
SHOT-NOISE LEVEL
-0
:
(n
-5
-15
-10
-5
-I5
-15
-20
-20
0
5
10
15
20
25
30 32.5
0
DISTANCE(CM)
5
K)
15
20
DISTANCE(CM)
25
30 32.5
Fig. VIII-5
Noise power vs cavity position for convergent-flow pulsed electron beam.
-7
V
= 1500 volts; pressure~ 7 X 10
mm; beam diameter
Icoll = 16.6 ipa;
0. 035 inch.
-~-
H - 344 GAUSS
H . 258 GAUSS
/--_/
2C
5
I1
o~ IC
5-
10
5
SHOT-NOISE LEVEL
SHOT-NOISE LEVEL
U
cn
WC
U,
- r,
10
15
20
DISTANCE (CM)
5
6ANODE
30 325
5
O
10
15
20
25
30 32.5
DISTANCE (CM)
Y
C
62
25
5
0.62
CL68
H 581 GAUSS
H 430GAUSS
20
15
10
5
SHOT- NOISE LEVEL
SHOT-NOISE LEVEL
-5
0
5
10
15
20
25
30 32.5
0
DISTANCE(CM)
5
10
15
20
25
DISTANCE (CM)
Fig. VIII-6
Noise power vs cavity position for temperature-limited pulsed electron beam.
IColl = 1. 1 La; V ° = 1500 volts; pressure -
pressure7
volts;
=1500
7 x 10 -coil
mm.
30 32.5
(VIII.
MICROWAVE
TUBE RESEARCH)
present in the growing waves from approximately 10 db above shot noise.
The noise
growing wave was considerably more prominent in the unneutralized pulsed beams than
in the completely neutralized dc operated ones.
Therefore,
the growing noise was caused by the ions in the beam.
it seems improbable that
Assuming that the noise curves
consisted of a periodic standing wave and a superimposed growing wave,
observe a delay of the growing part as the magnetic field was increased.
For high mag-
netic fields the growing part was absent in the drift space under observation.
well be that in a longer drift space the growing part would still occur.
we could
It might
The level of the
successive minima also changed with the magnetic fields, as shown in Figs. VIII-2,
VIII-3, and VIII-4.
Growing noise waves were present even in temperature-limited beams as Fig. VIII-6
shows.
A rise up to approximately 15 db above shot noise level could be observed. Again
a delay of the growing portion was noticeable with the increase of the magnetic field.
Increasing magnetic field also caused a decrease of the standing-wave ratio in the periodic noise waves.
This decrease was due to the raising of the noise-wave minima and
the simultaneous lowering of the maxima as well.
Further experiments will be made with confined-flow electron beams.
C. Fried
B.
INTERNALLY
COATED CATHODES
Two electron guns with internally coated cathodes have been designed for low-voltage
as well as high-voltage operation.
Both guns are under construction at present.
C.
C.
Fried
PROPAGATION OF SIGNALS ON ELECTRON BEAMS
Data have been taken, both on a direct current electron beam and a pulsed electron beam, following the outline given in the last Quarterly Progress Report (1).
A 3000-Mc/sec 1-mw signal, well above the noise level but still small enough for small
signal theory to apply, was fed into one (fixed) cavity.
picked up by the other (movable) cavity.
The alternating beam current was
In continuous-wave operation,
standing-wave
ratios of current as high as 41 db were observed at magnetic fields higher than 1. 5 times
the Brillouin field.
A typical curve is shown in Fig. VIII-7.
The value of the Brillouin
field for this case was 300 gauss.
A plot of the current standing wave under pulsed operation is
figure.
The pulse repetition rate was 4000 cps,
the length of the pulse 0. 75 4.sec.
can be seen that the plasma wavelength is considerably longer.
-41-
given in the same
It
This can be attributed
(VIII.
TUBE RESEARCH)
MICROWAVE
32
30
.0
-
0
28
26
-\
BEAM VOLTAGE1.5KV
CURRENT 3.2MA (CONTINUOUSWAVE)
MAGNETIC FIELD 290 GAUSS
SBEAM
22
P ULSED
0-
/
-
27
920
z 18
I
-O
S16
o
I
I!
0 12
I
CAVITY GAP0.070 INCHES
-
(D
CAVITY HOLE DIAMETER 0.120 INCHES
MEASURED BEAM DIAMETER0.020 INCHES
I I
-
42
-I
4
2 -2
1
2
4
6
4
6
8
10 12 14
16
IO
16
12
14
IB
20 22 24 26 28 30
32 34 36
20
32
22
24
26
28
30
34
36
POSITION OF MOVABLECAVITY (CM)
Fig. VIII-7
Standing-wave pattern of current.
to the fact that ions are essentially absent under pulsed operation, whereas
in con-
tinuous-wave operation the focusing action of the ions confines the beam to a smaller
diameter.
It may be that the smaller standing-wave ratio observed under pulsed opera-
tion can be blamed on the shape of the pulse, which was not ideally square.
At high beam current and low magnetic field, growing noise of the nature observed
elsewhere (for example,
see ref. 2) was known to exist.
No anomalies were observed
in the standing-wave pattern of the signal current.
H.
A.
Haus
References
1.
Quarterly Progress Report, Research Laboratory of Electronics, M.I.T.
1953, p. 26
April 15,
2.
Quarterly Progress Report,
1953, p. 43
April 15,
Research Laboratory of Electronics,
-42-
M.I. T.