ng=10 cellspacing=0 width=100%>Yahoo! is not affiliated with the authors of this page or responsible for its content.
THE GRAY LABORATORY S 5 Me V LI NEAR ACCELERATOR
Copyright© Gray Cancer Institute 1995
1
THE
GRAY
LABORATORY
S
5
Me
V
LI
NEAR
ACCELERATOR
This document was prepared by B Vojnovic and provides the following information:
A brief historical overview of the accelerator.
A description of the operation of the different sections of the accelerator.
Waveforms present in various parts of the accelerator when operating under normal/typical conditions.
Circuit diagrams of the accelerator.
Staff involved in the commissioning of the linac:
Mr KD Allen
Mr BL Hall
Mr C Tootel
Mr B Bloomfield
Dr BD Michael
Dr B Vojnovic
Mr M Grundon
Mr D Nash
Dr P Wardman
Mr J Draper
Mr RG Newman
Mr G West
Introduction and history of the accelerator.
The linac had been in use in the Radiotherapy Department of Mount Vernon Hospital from 1979 to 1993
and was used to generate repetitive photon pulses for therapy purposes. In late 1993, it was dismantled and
most of the ancillary patient and accelerator movement and dosimetry equipment was discarded. At that
time, it was decided to reconstruct the linac in such a way that it could be used to generate both repetitive
and, more importantly, single-s
hot

pul
s
e
s

of

dur
a
t
i
on
s
hor
t
e
r

t
ha
n
t
he

4
s

pr
ovi
de
d
by
t
he

ma
c
hi
ne

i
n
i
t
s

original form. More specifically, the intention was to
r
e
pa
c
ka
ge
t
he

va
r
i
ous

powe
r

s
uppl
i
e
s

a
nd
pul
s
i
ng
circuits so that they were to be contained in a single assembly rather than in several separate equipment
racks and in a rotating gantry. This arrangement resulted in a system which may be conveniently screened to
prevent the intense radio-frequency interference, resulting from the generation of short radiation pulses,
from affecting relatively sensitive detection equipment used with the accelerator. The basic accelerator
structure was not altered and the only addition to the vacuum system was the installation of an output beam
line extension which incorporates an inductive pulse charge monitor used for dosimetry purposes.
The acquisition of the accelerator by the Gray Laboratory was initiated by BD Michael, in collaboration with
P Wardman and B Vojnovic. The dismantling of the original accelerator was performed by the Physics
Department of Mount Vernon Hospital in collaboration with the Mechanical Workshop of the Gray
Laboratory and the operation was supervised by Mr Godfrey Mott who had been formerly in charge of
accelerator maintenance at the hospital. G Mott also supervised the extensive mechanical modifications
needed to reshape the linac and the majority of this work was performed by B Bloomfield, D Nash, J Draper,
C Tootel and BL Hall. Some basic power distribution wiring was also completed at that time and operation
of the microwave generating circuits was confirmed. Vacuum problems were dealt with by BL Hall and G
Mott. In this initial phase of the commissioning, it was also established that the proposed methods of
interference screening would indeed be adequate for our purposes. At that time, it was also decided that all
control circuits would be housed within the accelerator so as to minimise the number of conductors which
are placed outside the shielded enclosure.
There were a number of ways by which short pulses could be generated, these included the installation of a
triode (Pierce) type of gun which can inject electrons into the waveguide by controlling the grid potential,
the installation of beam deflection circuits which sweep the beam across a slit and the installation of a
separately pulsed circuit which uses the existing (diode) gun to inject electrons for a short time during the
nominally 4
s RF envelope. All of these options were considered, but it was ultimately felt that the most
expedient way was to use the last option. This was somewhat facilitated by the availability of a pulse
transformer (just about adequate, salvaged from the Gray
La
bor
a
t
or
y
s

f
i
r
s
t

1.
8
Me
V
l
i
na
c
)

a
nd
by
t
he

f
a
c
t
Copyright© Gray Cancer Institute 1995
2
that it required no modifications to the vacuum system. It was also decided that the triode gun approach
would be ultimately installed and consequently the triggering/timing electronics were designed with this
eventual purpose in mind. A prototype thyratron pulsing arrangement was constructed by KD Allen under
the guidance of BD Michael. The limitations of the available pulse transformer became evident at that time
and the circuit was modified by B Vojnovic
t
o
i
nc
l
ude

a
doubl
e

Bl
uml
e
i
n
l
i
ne
whi
c
h
r
e
duc
e
d
t
he

s
t
r
e
s
s
e
s

on the coaxial delay line cable. This arrangement was eventually installed in the accelerator and is in use at
this time (October 1995).
The personnel safety interlock system used on the linac is essentially the same as that used on the
l
a
bor
a
t
or
y
s

Coba
l
t

60
s
our
c
e

whi
c
h
i
s

i
ns
t
a
l
l
e
d
i
n
t
he
DC
r
oom

of

t
he

Va
n
de

Gr
a
a
f
f

a
c
c
e
l
e
r
a
t
or
.

I
t

wa
s

largely designed by BL Hall and constructed by M Grundon. It makes minimal use of electronic equipment
but rather relies on electromechanical relays which have been shown in the past to be adequately rugged and
r
e
l
i
a
bl
e
.

I
t

i
s

t
i
e
d
t
o
t
he

Va
n
de

Gr
a
a
f
f

a
c
c
e
l
e
r
a
t
or s

i
nt
e
r
l
oc
k
s
ys
t
e
m
f
or

i
ndi
c
a
t
i
on
pur
pos
e
s
,

s
i
nc
e

t
he

l
i
na
c

is installed in one of the Van de Gr
a
a
f
f s
r
a
di
a
t
i
on r
ooms
,
t
he

pul
s
e r
oom.
The
s
ys
t
e
m r
e
qui
r
e
s
t
he

s
e
que
nc
e
l
i
na
c

AC
powe
r

on,

r
oom
s
ur
ve
y,

c
l
os
e

out
e
r

door
,

e
na
bl
e

r
a
di
a
t
i
on
on

t
o
be

c
ompl
e
t
e
d
be
f
or
e

c
l
os
i
ng
a

pa
i
r

of

c
ont
a
c
t
s

whi
c
h
e
ne
r
gi
s
e

t
he

ma
c
hi
ne s

ma
i
n
HT,

gun
f
i
l
a
me
nt

a
nd
f
ocus power supplies.
Ope
ni
ng
t
he

r
a
di
a
t
i
on
r
oom
s

door
s

ope
ns

t
he
s
e

c
ont
a
c
t
s

a
nd
de
-energises these power supplies. In addition
t
o
t
hi
s
,

a
c
t
i
va
t
i
on
of

a
ny
of

a

s
e
r
i
e
s

of
pa
ni
c

but
t
ons
a
r
ound
t
he
pul
s
e
r
oom
r
e
move
s

a
l
l

AC
powe
r

t
o
t
he

accelerator and to the associated pulse radiolysis detection equipment.
The final stages of commissioning of the accelerator were undertaken by G West, by RG Newman and by B
Vojnovic. This involved the wiring up and testing of all control circuits and the installation of a control
computer. First radiation beam output was obtained in mid October 1995 and the machine was almost
completely aligned in late October 1995. The linac was put into service in late November 1995.
The end result of this work was that a very stable and, more importantly, electrically quiet accelerator was
a
dde
d
t
o
t
he

e
xt
e
ns
i
ve

a
r
s
e
na
l

of

t
he

l
a
bor
a
t
or
y
s

r
a
di
a
t
i
on
s
our
c
e
s
.

I
t

s
houl
d
pr
ove

t
o
be

a

ma
j
or

a
s
s
e
t

f
or

pul
s
e

r
a
di
ol
ys
i
s

wor
k,

e
s
s
e
nt
i
a
l
l
y
pe
r
mi
t
t
i
ng
a
t
ur
nke
y

ope
r
a
t
i
on
of

pul
s
e

r
a
di
ol
ys
i
s

e
x</b>periments to be
achieved.
Description of accelerator.
It is assumed that the principles of operation of linear accelerators are understood by the reader. Useful
descriptions of the operating principles can be found in the documents in the appendix. The present notes are
specific to the accelerator installed in the Pulse Room of the Gray Laboratory and are intended to be updated
as required when changes to the installation are implemented. This document was started in October 1995 at
which time the accelerator was first commissioned to generate single-shot pulses required for pulse
radiolysis work. When a reference is made to a ########.SCH diagram, this refers to an EASYPC
schematic file. All diagrams are can be found in directory C:\PCB\GLLIN on the Electronics and Van de
Gr
a
a
f
f

PC
s

a
s

we
l
l

a
s

on
B
Voj
novi
c s

PC.

If any work is carried out on the linac for servicing or other purposes, it is absolutely essential that
adequate safety procedures are observed. Electrical and radiation safety procedures must be adhered to
and no unqualified personnel are permitted to remove the copper enclosure shields. It is stressed that the
accelerator is a potentially lethal piece of equipment, both from the radiation and electrical points of view.
Lethal voltages are present in the accelerator enclosure even when it is switched off. These include a
permanently energised ion pump (240 V AC and ca 2.5 kV DC)as well as potentially charged delay lines
and pulse-forming networks. These items may have voltages in excess of 10 kV present. It is thus essential
that all high voltage circuits are discharged using earthing rods provided prior to working in the
vicinity of the gun, the magnetron and either of the two thyratrons. Remember that the radio-frequency
sections of the accelerator are designed to provide power levels of up to 2 MW and that equipment Copyright© Gray Cancer Institute 1995
3
associated with this must be treated with respect. Do not work on this if you are tired and do not work on
this if you are alone. There can be no better advice than that which appears on every circuit diagram
provided with the original installation: IF IN DOUBT, ASK. In addition, remember that it is