Linear accelerator frequency control syste

Gilkison, C. A.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
REPLY TO 
ATTN OF. 
TO : 
WASIIINGTON. n (1 10546 
March 298 1971 
c & Technical Information D i v i s i  
iss Winnie M. Morgan 
III . . 
c h n i  ion  
At ten t ion :  M i s s  
FROM: GP/Office of A s s i s t a n t  General 
Counsel f o r  Pa ten t  Matters  
SUBJECT: Announcement of NASA-Owned 
U.S. Pa ten t s  i n  STAR 
I n  accordance w i t h  t h e  procedures contained i n  t he  Code GP 
t o  Code U S 1  memorandum on t h i s  s u b j e c t ,  dated June 8, 1970,  
the  attached NASA-owned U.S.  p a t e n t  i s  being forwarded f o r  
a b s t r a c t i n g  and announcement i n  NASA STAR. 
The fol lowing information i s  provided: 
U.S.  Pa t en t  No. : 3,366,886 
Corporate Source : Space Technology Laboratory 
Supplementary 
Corporate Source 
NASA Pa ten t  C a s e  No. :  XGS-05441 
Please note  t h a t  t h i s  p a t e n t  covers an invent ion  made by an 
employee of a NASA con t r ac to r .  Pursuant t o  Sec t ion  305(a)  of 
t h e  Nat ional  Aeronautics and Space A c t ,  t h e  name of t he  
Adminis t ra tor  of NASA appears on t h e  f i rs t  page of t h e  pa t en t ;  
however, t h e  name of t h e  a c t u a l  inventor  (au thor )  appears a t  
t h e  heading of Column N o .  1 of t h e  S p e c i f i c a t i o n ,  following t h e  
w o r d s  ' I .  . . w i t h  r e s p e c t  t o  an  invent ion  of .  . . . It  
G a + l e  Parker 
Enclosure: 
Copy of Pa ten t  
https://ntrs.nasa.gov/search.jsp?R=19710013486 2020-03-17T02:50:58+00:00Z
Jan. 30, 1968 HUGH L. D R Y D E N .  D E P U T Y  3,366,886 
ADMINISTRATOR OF THE N A T I O N A L  A E R O N A U T I C S  
A N D  S P A C E  ADMINISTRATION 
LINEAR ACCELERATOR FREQirENCY CONTROL SYSTEM 
Filed Oct. 24, 1965 2 Sheets-Sheet 1 
L 
i? 
. - 
(3 
LL 
BY 
INVENTOR 
Charles A. Gilkison 
UGH L. D R Y D E N .  D E P U T Y  3,366,886 
O R  OF T H E  N A T I O N A L  A E R O N I > J T l C S  
D S P A C E  ADMINISTRATION 
L I N E A R  ACCELE?&ATOR F R E Q U E N C Y  C O N T R O L  SYSTE 
Fi led 'Oct .  24, 1965 2 Sf 3ets-Sheet 2 
r' 
I 
B 
r- 
2 
- 
BY 
9 88 Unit ateneed Jan. 30, 1968 
2 
digital counter starts counting the timing pulses from 
the master fixed frequency oscillator when the particle 
passes through a first measurement grid and stops when 
the particle passes through a second measurement grid, 
~~~~r~~~~~~~~~~~~~~ ~ ~ ~ ~ ~ ~ ~ ~ * , o ~ i ~ ~ ~ ~ ~ ~ ~  5 and this count is maintained until the particle has passe,d 
an A. G1lBcison, Mills through the linear accelerator. The grids are spaced at a 
Estates, Calif. distance equal to the length of the first drift tube and the 
Filed Oet. 24, 1965, Ser. NO. 505,321 count accumulated by the digital counter represents a 
6 Claims. (GI. 325-233) time equal to one half of the period of the accelerating 
voltage applied to the drift tubes. 
At the end of the measurement interval the count 
A ~ S T ~ A C T  OF THE DISCLOSURE accumulated in the measurement counter is transferred, 
A system for generating a signal wave having a fie- in Parallel, to a freClUeilCY counter which repetitively 
quency proportional to a measured physical phenomenon, counts timing Pulses from the fixed frequency oscillator, 
particularly useful in controlling the frequency of an al- 15 beginning with the count established by the measurement 
ternating voltage applied to a linear accelerator for counter and ending at zero. Thus, the time required for 
charged particles, and including counting and control ap- the frequency counter to reach zero is equal to the inter- 
paratus for applying a correction signal for varying the val initially measured (i e., the desired half period of the 
period of the generated signal Wave in response to vari- accelerating voltage of the linear accelerator). Upon 
ations in a control signal representing the measured physi- 20 reaching Z2r0, the frequency counter is automatically re- 
cal phenomenon. set to the count still held in the measurement counter, and the countdown process of the frequency counter repeats. 
Each time the frequency counter reaches zero and re- 
The invention described herein was made in the per- sets it triggers a bistable device such as a flip-flop and a 
formance of work under a NASA contract and is subject 25 square wave is generated having a frequency related to 
to the provisions of Section 305 of the National Aero- the initial measurement interval. 
nautics and Space Act of 1958, Public Law 85-568 (72 The accuracy of a system of the described type is de- 
Stat. 435, 42 U.S.C. 2457). termined by the ratio of the frequency of the master 
This invention relates generally to a frequency control fixed frequency oscillator to the desired frequency of the 
system, and more particularly to a system for generating 30 OlItPUt square wave. For example, in the specific instance 
a signal wave having a irequency proportional to a of a linear accelerator, in order to generate a 100 kc. ac- 
measured physical phenomenon. celerating voltage to an accuracy of 1 part in 100 (or 1% ), 
There are a number of instances where it is desirable a 10 mc. master timing signal is required. Thus the fre- 
that the frequency and phase of a periodic wave, such as quency response of the digital components used in the 
a square wave, be precisely related to a control signal. 35 measuremmt and frequency counters provides limitations 
The control signal itself may be a series of spaced pulses, as to the accuracy which may be achieved, and result in 
a sqiiare wave or a periodic signal whose spacing, pulse- practical restrictions on the velocity to which a particle 
width or repetition rate is indicative of the detected or may be successfully acceierated. Although a larger spac- 
measured physical phenomenon. ing between the measurement grids will allow a longer 
As a more specific example, the control signal may be 40 time to accumulate the initial count (count into the 
spaced pulses derived from the drift tubes of a linear measurement counter) to allow greater accuracy with a 
accelerator such that the spacing or time interval be- given master timing frequency, the frequency counter 
tween pulses provides a msasure of the initial velocity which is fed by the measurement counter must ordinarily 
of a charged particle that is to be accelerated by an operate at the higher rate to provide the desired fre- 
alternating electric field applied to the drift tubes of the 43 quency for the accelerating voltage. 
accelerator. In this application, to insure successful ac- In accordance with present invention, the accuracy and 
celeration of the particle there must be a proper phase response Of the frequency control system of the above- 
reversal of the accelerating voltage producing the electric described type may be increased by providing a greater 
field as the particle travels, at extremely high speeds, measurement interval such that the count accumulated by 
through successive drift tubes, and this depends on the 50 the measurement counter is greater by one or more digits 
accuracy with which the frequency of the accelerating than that required to achieve the desired frequency of the 
voltage can be controlled in relation to the initial velocity output signal. The extra digits, the least significant digits, 
of the particle injected into the drift tubes. If the proper are transferred to a correction register. The most sig- 
phase reversal fails to occur, the particle is subjected to nificant digits of the measurement counter, representing 
a retarding rather than an accalerating voltage and is lost. 55 an interval equal to or slightly less than a half-period of 
Details of such a linear accelerator and the manner in desired frequency of the output signal, are transferred to 
whkh frequency control of the accelerating voltage in the frequency counter. The frequency counter repetitively 
response to the panicle initial velocity is utilized to SUC- counts timing pulses, beginning with the count received in 
cessfully accelerate the particle is fully described in co- the measurement counter, and ending at zero. During each 
pendiTg application, Ser. No. 505,076, filed Oct. 25, 1965, 60 counting cycle the least significant digits of the measure- 
now Patent No. 3,317,846 and commonly assigned. ment counter are transferred out, summed, and stored in 
Digital frequency control systems are preferred for ap- the correction register. When this sum exceeds a prede- 
plications of the type mentioned above in that they offer termined amount an overflow pulse occurs which is utilized 
higher accuracy, ease of implementation, and greater re- to delay the triggering of the bistable circuit generating 
liability than analog systems such as those utilizing more 65 the square wave for the period of a selected number of 
conventional voltage controlled oscillators. One such digi- additional master timing pulse. Thus the half-period of the 
tal system utilizes a master fixed frequency oscillator to output signal is delayed whenever the accumulation of the 
generate timing pulses, and a digital counter counts the least significant digits into the measurement counter ex- 
timing pulses during the interval indicative of the mea- ceeds a predetermined amount, with the result that cor- 
sured quantity. In the specific application where the 70 rection is provided by periodically allowing the error to 
measured quantity is the initial velocity of a particle in- build up and then re-referencing the phase and frequency 
jected into the drift tubes of a linear accelerator, the of the output signal before the error exceeds a tolerable 
3,366,886 
LINEAR TOW FREQUENCY 
SYSTEM 
15 and a stop pulse on line 17. The time interval between 30 
the start and stop pulses provides a control signal repre- 
sentative of a desired measurement interval, and as will 
be subsequently described may be related to the initial 
velocity of a particle injected into the drift tubes of a lin- 
ear accelerator. In addition, both counters 12 and 14 re- 35 
ceive master timing pulses from a k e d  frequency oscil- 
lator 18. Measurement counter 12 counts the number of 
master timing pulses received from fixed frequency oscil- 
lator 18 during the measurement interval, and this count 
is retained until subsequent start and stop pulses estab- 
lish a new measurement interval, at which time a new 
40 
count is registered. 
Frequency Storage OverAow, 
Counter Register and Correction 
Cycle Contents 
No l-. - - -. . . . . . -. - 3 
2 _._._ ~ _ _ _ _  _ _ _ _ _  0 No 
3-. -. . . . . - - - - -. 9 No 
4-. -. -. -- - - - - - - - 2 Yes 
5 _ _ _ _ _  ~ ...-.- ~ - .  5 No 
6..  . . . . . - - - - - - - - a No 
Yes  7-... ___.__._._. 1 
8 --...- _ - _ _  _ _ _ _  ~ 4 No 
9 _ _ _ _ _  ~ __._.._.. 7 No 
Yes 10 __._._. . __._ 0 
11 - _ _ _ _  ~ __._ _ _ _  3 NO 
12 ._.__ _ _. - Etc. _ _ _ _ _ _ _ _ _ _ _ _ _ *  
3,366,886 
5 
age of 3 cycles out of 10 (in this example), with the applications, periodic frequency and phase correction is 
result of permitting the error to build up and then re- achieved lengthening appropriate half cycles of the sig- 
referencing the output frequency and phase of the square nal to be controlled, thus preventing the error from ex- 
wave out of flip-flop 20 on a periodic basis. ceeding a tolerable level, High speed circuits are not re- 
The counters, sum and carry circuit, connection reg- 5 quired for accurate control, and the periodic corrections 
ister, delay logic, and other associated circuiting and are readily generated by simple arithmetic techniques. 
gating may be implemented by standard digital tech- Although a specific embodiment of the invention has 
niques, such as those desciibed in Logic Design of Digital been described with particularity, it is not limited to the 
Computers by Montgomery Phister, Jr., New York, Wiley, specific system and particular circuit arrangements herein 
1958. disclosed, and modifications and variations thereof should 
The manner in which the above described frequency be obvious to those skilled in the art. It is therefore to be 
control system may be used in conjunction with a licear understood that within the scope of the appended claims 
accelerator of the type wherein a particle is injected with the invention may be practiced otherwise than specifically 
an initial velocity into a series of drift tubes and sub- set forth. 
sequently accelerated by an alternating potential app'ied 15 What is claimed as new and desired to be secured by 
between adjacent drift tubes is illustrated in FIGURE 2. Letter Patent of the United States is: 
Details of one such linear accelerator are set forth 1. Apparatus for generating an alternating signal wave 
in the afore-mentioned Patent No. 3,137,846. having a frequency proportional to a measured physical 
Briefly, particle generator 30, which may be a Van phenomenon, including in combination: 
de Graaff accelerator, injects charged microsized par- 20 means for providing a control signal representing a 
ticles into the drift tubes designated by 3Z1-32,. An measurement interval substantially greater than the 
accelerating potential is applied to the drift tubes such period of a half cycle of the alternating signal wave 
that there is polarity reversal for alternate periods of to be generated; 
acceleration by connecting opposite terminals of an al- a fixed frequency oscillator for generating timing pulses 
ternating high voltage source 36 to alternate adjacent 25 at a rate substantially greater than the frequency of 
drift tubes. The lengths of the drift tubes are successive- the signal to be generated; 
ly greater such that with increasing velocity the time first counter means coupled to said k e d  frequency OS- 
required for a particle to pass through each tube re- cillator and responsive to said control signal for count- 
mains constant, thus allowing a constant frequency ac- ing and cyclically registering the number of timing 
celerating voltage to be used. pulses occurring during the measurement interval rep- 
The required constant frequency is directly propor- resented by said control signal; 
tional to the initial velocity with which the particle is second counter means coupled to said fixed frequency 
injected into the drift tubes. This velocity is detected by oscillator and responsive to the most significant digits 
a pair of measurement grids (or similar detectors) 37 of the count registered by said first counter means to 
and 39 in front of the first drift tube. As a particle 35 cyclically count said timing pulses during an interval 
passes through measurement grids 37 and 39 voltage established by said most significant digits; 
pulses are induced and fed, via amplifiers 41 and 43, to bistable circuit means coupled to said second counter 
frequency control system 44. Frequency control system means and responsive to its cyclic counting opera- 
44 is the system described in conjunction with FIGURE tion to produce an alternating signal wave; 
1, and accordingly, amplifiers 41 and 43 supp!y the start 40 first circuit means responsive to said first and second 
and stop pulses, respectively, for measurement counter counter means for producing a correction signal when 
12. The output of frequency control system 44, which the sum of the least significant digits of the count 
is the square wave derived from flip-flop 20 (FIGURE registered by said first counter means during the cy- 
1) is applied to high voltage source 36, synchronously clic counting operation of said second counter means 
triggering high voltage source 36 such that an accelerat- 45 exceeds a predetermined amount; and 
ing voltage of the desired frequency is applied to drift second circuit means for applying said correction sig- 
tubes 321-32,. nal to said second counter circuit means for varying 
AS has been mentioned, the spacing of measurement the period of a half cycle of said alternating signal 
grids 37 and 39 in this specific embodiment is 10 times wave a predetermined number of said timing pulses. 
the length of the first drift tube. Since only the most sig- 5~ 2. The apparatus of claim 1 wherein said means for 
nificant digits of the count accumulated by measurement providing a control signal includes particle velocity de- 
counter 12 is applied to frequency counter 14 to control tection means of a linear accelerator, said linear accelera- 
the triggering of flip-flop 20, there is effectively a divide- tor being of the type wherein a charged particle is injected 
W e n  action such that the square wave triggering high with an initial velocity into a series of drift tubes and 
voltage supply 36 has a hali-period approximately equal 55 subsequently accelerated by an alternating potential ap- 
to the transit time through the first drift tube. This pro- plied to said drift tubes, with said control signal represent- 
vides an accelerating voltage having the required phase ing the initial velocity of said charged particle, and with 
reversal as the particle passes through successive drift said alternating signal wave operable to control the fre- 
tubes. The least significant digit of the count accumulated quency of said alternating potential. 
by measurement counter 12, then, is supplied to sum and 60 3. Apparatus of claim 2 wherein said detection means 
carry circuit 24 and storage register 26 such that there includes first and second measurement grids spaced in front 
is a periodic re-referencing of the frequency of the square of the first of said series of drift tubes, with the spacing 
wave triggering high voltage supply 36 in the manner between said first and second measurement grids being 
previously discussed. As a result, the accuracy of the fre- substantially greater than the length of the first of said 
quency of the accelerating voltage is enhanced with a (35 series of drift tubes. 
corresponding increase in the number of times a particle 4. Apparatus of claim 3 wherein the half cycle of said 
receives an accelerating potential of the correct polarity alternating signal wave is approximately equal to the 
during transit through the drift tubes. transit time of said particle through said first drift tube. 
The invention provides, therefore, a frequency control 5. Apparatus of claim 1 and wherein said first circuit 
system capable of generating a signal having a frequency 70 means includes a sum and carry circuit coupled to receive 
proportional to a measured physical phenomenon. It is the least significant digit of the count registered by said 
particularly adaptable to be utilized in conjunction with first circuit means, gating means for applying said least 
a linear accelerator wherein an accelerating potential hav- significant digit to said sum and carry circuit in response 
ing a freqtlency accurately related to the velocity of the to the cyclic counting operation of said second counter 
particle being accelerated is required. In this, and in other 7.i means, and storage means for accumulating the sum of the 
30 
3,366,886 
7 8 
least significant digits applied to said sum and carry References Cited 
circuit. UNITED STATES PATENTS 
cuit means includes delay circuit moans for delaying the 3~296152s 1'1967 Sakuma ---------- 307-885 6. Apparatus of claim 5 and wherein said second cir- 
triggering of said bistabie circuit means a predetermined 5 
number of said timing pulses when the accumulated sum 
of said least significant digits exceeds a predetermined 
amount. 
JAMES w* 
S. A. SCHNEEBERGER, Assistant Examiner. 
Prinzary 


Linear accelerator frequency control system Patent

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