A single cavity accelerator for 1 MeV protons

NUCLEAR

INSTRUMENTS

A N D M E T H O D S 31

(I964) 2 8 5 - 2 9 2 ;

© NORTH-HOLLAND

PUBLISHING

CO.

A SINGLE CAVITY ACCELERATOR F O R 1 MEV P R O T O N S A. A. GLAZOV, V. A. K O C H K I N , D. L. N O V I K O V and L. M. O N I S C H E N K O

Joint Institute for Nuclear Research, Laboratory of Nuclear Problems, Dubna, USSR Received 27 February 1964

A 1 MeV proton injector of the ring-shaped phasotron model, designed and constructed at the Laboratory of Nuclear Problems at Dubna in 1960-62 is described. Protons are accelerated in the gap of a toroidal cavity excited on the basic frequency of about

60 MHz by a self-excited oscillator. A cold cathode of the Penning discharge type is used as an ion source. Proton current of 10 m A per 20#see pulse is supplied by the injector at the repetition rate of 50 Hz.

1. Introduction A proton accelerator in the form of a single cavity was developped to investigate the possibility of its use as the injector of a ring-shaped phasotron model with spiral magnetic field structure1). The choice of a cavityaccelerator as the injector is due to the fact that such an accelerator is a compact unit which can be installed inside the phasotron ring. It can be conveniently arranged together with an ion source and can supply considerable pulse currents. There are two disadvantages of the cavity accelerator: a) the accelerated beam is not mono-energetic; b) the accelerating voltage is not completely used. This is due to the principle of acceleration by means of the r.f. fields when particles are involved into acceleration at various phases of the accelerating fields, while the time of their flight in the accelerating gap tf makes a noticeable part of the period of the accelerating voltage T. The ratio t:/T can be decreased by reducing the

product fd, w h e r e f i s the frequency of the accelerating voltage, 2d is the gap length in which particles are accelerated. Therefore, to use the accelerating voltage more efficiently one should choose cavities with a narrow accelerating gap and low natural frequency. From this point of view as well as to satisfy the requirement of the unit compactness, the choice of a toroidal cavity as an accelerator seems to be most reasonable. In this case, if the diameter of the inner cylinder of the toroidal cavity is sufficiently large, the ion source and the buncher can be placed inside it, which makes beam transportation unnescessary and reduces the sizes of the device. Since the minimum value of the gap is restricted by its dielectric strength, fd can be decreased at the expence of f, i.e., by increasing the device dimensions. A compromise solution of the desire to have small fd, larger Q values and reasonable size of the unit is a cavity schematically shown in fig. 1.

l-

2. Eleetrodynamies of the cavity A detailed calculation of the cavity has been made in ref. 2). Here the calculation operations, their results are reproduced and compared with the experimental data. For fields varying in time according to the law exp(io~t ),the wave equation in the cylindrical coordinate system is of the form

2k-/,,, 2 & . /,~

girt : Q d m

£d-" O,0//m

( Fig. 1. Cavity diagram.

-

02~ F 02~

10~

1 02~

az

r ar

r z 0rpz

(1)

where k = ~o~//~8 = 2n/2 is the propagation constant. The azimuthal-symmetric solution with the electric field component Ez, i.e. the T M mode, is of particular interest. In this case three field components Ez,E, and H , are different from zero and in a definite way depend upon the solution of eq. (1). The latter is solved separately for regions I and II (fig. 1). The solutions represented as the Fourrier series of the unknown function are fitted at the boundary r = r 1. The boundary conditions are: 285

286

A.A. GLAZOV 0,

Iz > d

EzI(rl)

=

Ezii(rl),

Iz

< d

(2)

H~i(rl)

= H~li(rl),

iz

< d.

(3)

It follows from the calculation 2) that for the cavity under consideration owing to the smallness of the relation ~ = d/L ~ 1 one can neglect the field variations in region II. The accelerating electrical field in region II and the magnetic field in region I are described then by the expressions: Ez. =

Up Jo(kr) V/m, ~-ff

(4)

.~Up [

HqjI = 1-~-~

Mo(l" )-}-

+ 2k ~, M.(r) sin(nn~)cos-~-z] A/m, (5) n=l Tn nlto~ while the frequency is determined by:

1 Jl(krl) 0~ Jo(krl)

~ M.(rl) [sin(nna)] 2 L~-n-~n~--J' ' (6) Tn

= Mo(rl) + 2k L n=l

where t/= 120n ohm, Up is the amplitude of the voltage applied to the accelerating gap, Tn =

-- k 2

and J, N,/, K are the cylindrical functions in the notations taken in ref. 3).

Mo(r) = Jl(kr)No(kr2)- Nl(kr)Jo(kr2) Jo(krl)No(kr2) - No(kr~)Jo(kr2)" M,(r) =

IdT~r)K°(T~r2) + KdT"r)I°(T~r2)

(7) .

(8)

Io(T~rl)Ko(T~r2) - Ko(T~r~)Io(T.r2) Equation (6) determining oscillation frequency can be essentially simplified, if instead of T. and M, their expansion in series are used, the terms whose order of smallness higher than the first power L/2 being neglected. In this case eq. (6) is reduced to 1 Jl(krl) = M o ( r l ) XJo(krl) 2kL [lnln

- 0.338 + 2n--~1( 1 . 6 3 3 - }n2e)] , (9)

which is not very difficult for calculation. The function Mo(rl) in this exPression is known as "a small radial cotangent" and is tabulated4). The frequency calculated from eq. (9) differs not more

et al.

than by 0.5 ~o from the exact value obtained by solving eq. (6) on the electronic computer. The knowledge of the fields Htp I o n the conducting surface of the cavity is important for the determination of the Q-value of the cavity as well as for the calculation of the measuring loops and the coupling system. The expression for H~l(r2) can be essentially simplified by substituting the cylindrical functions of a large argument by their asymptotic approximation. The simplified expression agrees with the exact solution of eq. (5) within 0.5 per cent. The distribution H~, in the cavity volume obtained by solving eq. (5) on the electronic computer is given in ref. 2). From this distribution the Q-value of the cavity can be calculated. 2

j

I-I, dV ,o~. of . . . .

(10)

H~ dS

J cond. surf.

where ~ is the depth of the skin effect. The Q-value for the cavity under consideration found in this way is 29000.

3. Cavity design and actual parameters Basing on the above considerations and calculations the cavity in question was designed. It is a steel cylinder 1000 mm long and 1000 mm in diameter, closed at both ends with steel plates on which the inner cylinders are fixed. The cavity is copper-cladded, the inner cylinders are made of a copper tube and are chromium-plated at the edges. The cavity cross section is shown in fig. 2, a picture is seen in fig. 3. One of the inner cylinders is made movable to retune natural frequency. It is driven by means of a worm gear, from outside by a sliding gasket. The ion source is located in the stationary cavity cylinder. In order to observe sparkings there is a plexiglass viewing port in the side wall of the cavity. The cavity is evacuated by the vacuum pump BA-5-4 through a system of holes made in cladding. All the gaskets in the cavity and the pump are made of vacuum rubber. The pump with a nitrogen baffle provides the pressure of 1 x 10 -6 mm Hg. The oscillator is coupled to the cavity by means of a coaxial feeder, the inner conductor of which is terminated on a stationary inner cylinder near its basement. The construction of current contacts from the movable inner cylinder to the plate is seen in fig. 3. The contacts are soldered with a soft solder due to technical difficulties of soldering with silver. This and also insufficiently perfect contacts between the plate and the cylinder was the main reason of the fact that the Q-value found experimentally turned

A SINGLE

CAVITY

ACCELERATOR

FOR

1

MeV

PROTONS

287

i:ii,llil.ll;.iil;.iil;.i!

P'7/~- S t e e l

- Bronze

I~

-Copper

~

- Plexiglass

Fig. 2. Cavity cross section. out to be two times smaller than the calculating one. The Q-value was evaluated either by the method of substituting by the RC-circuit (Q = nfoRC) 5) or from the build up time of oscillations in the cavity (Q = nfotb.Jln (1--E/Ema~)), where tbulld u p is the pulse rising time up to E. Before polishing the sheets of the copper cladding the Q-value was 9000, after polishing it was 14000. The cavity had three measuring loops. Two of them were at the bottom: St = 8 cm 2 and S 2 = 2 5 cm 2. And the third one (S 3 = 2 cm z) was in the side wall. The external parts of the seals are designed as r.f. plugs. The magnetic field distribution along the radius with Z = 0 was found with the measuring loops S = 0.36 cm 2 placed at the end of a copper tube 3 m m ~, the latter

being an external conductor of the coaxial line. The basic frequency of the cavity was measured with an electronic wave-meter when the cavity was excited by the standard signal generator. Owing to good agreement of theoretical and experimental results in determining " k " and the distribution of the magnetic field strength, it proved possible to use a side wall loop S = 2 cm 2 for measuring the absolute values of the voltage across accelerating gap. In this case, at the place where the loop is located H , = i3.27 x 10-3Up A / m and the voltage induced on the loop U I = - icopH, S. or Up[kV] = 3.11UI[V ].

(11)

4. Particle dynamics The motion of protons with v/c ~ 1 in the accelerating gap is described by the equation

eU o . .

m~ = - ~ - sin(cot + ~p)

(12)

while the first and the second integrals of this equation m 2

cos ¢p -- cos (cot + tp)] + vo

(13)

1 ~e Up,-. z = ~ [ ~ 2 - - ~ t s l n ~ --

Fig. 3. Cavity with a cover opened.

- sin (cot + ~p) + cot cos ~b] + cOtVo } Here e/m is the charge-to-mass ratio for protons; co is the cyclic frequency of the fields;

(14)

288

A.A. GLAZOV e t al.

q~

is the phase of the r.f. field at the moment when the ion enters the gap; /)o is the initial velocity of the ion. ~ . 2z

~02

v

2,C0

tJ *

to*

°"ff

~~~.~o,~

....

Fig. 4. Calculation dependence of accelerated proton energy on the inlet phase.

2240 s

L ~ 2

2O

Solving the transcendental equation (14) for Z = 2d with respect to 0 = COot and substituting the solution into eq. (13) one obtains the dependence of the output proton energy upon the incoming phase. The proton energy as a function of initial phase for three values of the accelerating voltage is plotted in fig. 4. As follows from fig. 4 in order to obtain 1 MeV protons the amplitude of the accelerating voltage should be about 1.2 MV. In order to increase the amount of high-energy protons in the accelerated beam the phase bunching of particles before they enter the accelerating gap can be applied. Its role is to modulate the velocity of particles as shown on curve I (fig. 5). Thus, according to (13) and (14) the output proton energy is constant. The beam modulation velocity can be achieved by means of the r.f. field. The proper drift space allows to convert velocity variations into phase ones. Fig. 6 shows the function Vo(~0)at the buncher exit with the parameters: Uo = 2 0 k e V , Ub = 5kV, l = 5 m m , o9 = COp where I is the length of the accelerating gap of the buncher, Uo is the ion energy at the buncher input. After the beam passes the drift space 40 mm long the function of fig. 6 is transformed into that of fig. (5) (curve 2). The comparison of figs. 5 and 6 allows to make a conclusion that 35~o of ions entering the bunching gap from the ion source can be bunched into a 1 ~o energy spread. The real effect will be smaller owing to the loss of a fraction of protons in the buncher. A rather small drift space requires that the buncher be located inside the inner cylinder of the accelerating cavity. The performed calculations and experiments

2.2./eG 48

2.0 1.9"

d, de ~, d~ ds de ~7 d, dg ;o

Fig. 5. Required (curve I) and attanable (curve II) buncher characteristics.

-~.o

-0.5

o

i

I

i

oJ

t.0

t.s

Fig. 6. Function V0(9) on the buncher output (before the beam flies over the drift space).

A S I N G L E C A V I T Y A C C E L E R A T O R FOR 1

have shown the feasibility of constructing such a system in the form of a poly-cylindrical coaxial cavity with n = 2. 5. R.f. power supply The r.f. field in the cavity can be excited by various methods the advantages and disadvantages of which are discussed detail in refs. 6- lo). In the case in question use is made of a selfexcited oscillator for which the cavity is the only resonant circuit at the operating frequency 11). Since the shunt impedance of the cavity on the fundamental mode is 0.8-- 1.2 megohm (Q = 9 0 0 0 - 14000, respectively) the r.f. power required for the accelerator is approximately 1 MW. To obtain the r.f. power of the order of 1 MW a triode GI-4A is used in the commongrid circuit. The oscillator diagram is shown in fig. 7. The anode resonant circuit including 1) tube capacity, 2) inductance due to construction components 3) the feeder and 4) the cavity itself, is a complex oscillation system which is due to resonate at the frequency fa of the parallel resonance close to the cavity natural frequency fo. The impedance transformed from the cavity to the coupling system is just the loading of the coaxial feeder _

Z1

~

2

X

2

r2 + x2 x¢ - i - - r +2 x2 xc

(15)

where x = rQ(1 _fzc/f2) r is the resistance of the cavity losses related to the current antinode; x~ is the coupling impedance.

F-

MeV P R O T O N S

289

The input impedance of the feeder having the wave impedance Zo the length l and the loading Z1 is Z = Zo Zt + iZo tg 2~I/2 Zo + iZ1 tg 2~I/2"

(16)

If l = ½n2, Z = Z 1 and at frequencies in the vicinity of the resonance frequency fc represents an impedance of the parallel circuit and consequently, stable operation of the oscillator is possible at the resonance frequency. Because of the capacity and the inductance caused by the tube and the components of the oscillator unit, the minimum voltage in the feeder is near the anode GI-4A and the feeder considerable shorter than ½2 is to be tuned to the half-wave resonance (at the cavity frequency). The use of the conductive coupling allows to reduce the feeder length down to 30 cm (Z o = 88 ohm). The feedback is used in the oscillator at the expense of the capacity between the anode and the cathode, which allows to avoid the additional lead-in to the vacuum chamber. A fine regulation of the feedback is achieved by means of the cathode stub tuner. The application of the inner feedback in operating with the high quality cavity is one of the peculiarities of the oscillator. The proper operating conditions of the oscillator ( f = f p , Roc = Roc opt.) are provided by the choice of the wave impedance of the feeder and the place connecting the inner conductor to the inner cylinder of the cavity. The evaluation of the amount of losses in the coupling system shows that the efficiency of the system is about 100~o. The oscillator pulse operation is achieved by means of the anode modulator with an

-IPq

Fig. 7. Principal diagram of the oscillator.

A . A . G L A Z O V et

290

artificial line (fig. 7). As a commutator the mercury thyratron TP-85/15 is used which is capable to commutare currents up to 1000 A in a pulse of 150 #s duration. The thyratron is switched by a triggering circuit producing 5 ~ts pulses of 250 V on the 150 ohm resistance. A pulse of an amplitude up to 5 kV shaped on the artificial line is transferred to the anode GI-4A through a pulse transformer with the transformation ratio 1 : 8. The multipactoring effect12' la)is the main obstacle in obtaining high voltage levels. Arising at the voltage level of about 1 kV across the gap, the discharge loads the cavity preventing further voltage increase. The discharge is suppressed only with the pulse operation of the oscillator. In this case it influences on the delay of the r.f. pulse with respect to the beginning of the anode modulating pulse and oscillation "misfires" when no high voltage arises in the oscillator.

al.

with the loop S = 8 cm 2. A spark gap is used in the coupling circuit for protecting the anode GU-34B from the overvoltage arising during the main oscillator operation. 6. Ion source

An ion source of the Penning type is used in the accelerator as a proton source. In order to increase current density in the discharge region a hollow cathode is employed in the source. A detailed description of this source is given in ref. 14). The design features of the ion source are due to its location inside the hollow inner cylinder of the toroidal cavity-accelerator. The magnetic field in the discharge region is produced by means of the armoured vacuumtight solenoid (800 Oe at the axis, the current is 200 A). The ion source components are fixed in a plexiglass

F--"

try ¢

Pulse

Fig. 8. Diagram of the pre-exciting oscillator. In order to suppress the multipactoring effect "preexcitation" is used, i.e., the initital voltage exceeding the discharge level is applied to the cavity with the aid of the oscillator loosely coupled with the cavity. The pre-excitation oscillator provides the output power of 500 W per 100 #s pulse. Its circuit diagram is shown in fig. 8. The instability of the frequency of the driving oscillator is Af/f= 2 x 10 -s per hour after an hour's heating. Pulse modulation is carried on the grid of the tube 6P9. The modulating pulse is chosen from the first channel GIS-2M which serves as a synchronizer of the entire system. The coupling with the cavity is provided

jacket which is installed into the solenoid. In order to improve the vacuum in the extraction region the volume between the anticathode and the extraction electrode is evacuated through the holes in the plexiglass jacket. The cathode and anticathode of the source are made of aluminum; the anode, the extraction electrode and lenses are made of brass and are chrome-plated. The source is fed to the set producing pulses for triggering and keeping the arc discharge and pulses for ion extraction. The arc pulse duration is 50#s (a 3 kV amplitude), the extraction pulse duration is 20/ts (20 kV amplitude). The set is synchronized from GIS-2M.

A SINGLE CAVITY ACCELERATOR FOR 1 MeV PROTONS The arc current whose supply circuits are under the extraction potential is controlled with a special circuit based on the Rogovsky belt principle15). A three-potential electrostatic lens is used in the ion source. The focusing efficiency is increased by means of tungsten grids made of thin wire 25/~m~. As has been shown by the investigation of various systems, such optics provides the best focusing with the extraction potential accepted. The developed ion source is a compact device and provides the output proton current of about 40 mA per pulse in the beam 10 mm ~, the extraction voltage being 20 kV. Gas consumption is 0.5 cm3/min. In this case the vacuum of 7 x 10 -6 mm Hg is maintained in the cavity volume. The use of the cold cathode allows to keep the fundamental characteristics of the ion source unchanged during the long period of operation. 7. Measurement of the accelerated beam characteristics

Pulse current, energy distribution and beam sizes in the cross section of the exit flange are the basic characteristics of the accelerated beam. One may get an idea about the accelerated proton energy from the value of the accelerating voltage. The accelerating voltage amplitude was determined by measuring voltage on a measuring loop. To measure the accelerating voltage by the independent method the gamma-spectrum produced due to bombarding the inner cylinder with parasitic accelerated electrons was investigated. The accelerating voltage determined from the high energy spectrum cut coincided with the data obtained by the measuring loop within the error of the spectrum measurements. A quartz-target was used to determine the shape of the proton beam. The size of the beam in the cross section of the exit flange was about 30 m m ~ . The pulse beam current (summed) is 10 mA. An electrostatic analyzer was used to study the

t0

Lj/I ,,o,

~

\

Fig. 9. Energy spectra of accelerated protons.

291

Fig. 10. General view of the system. energy distribution of the beam. Ions deflected in the condensor of the analyzer hit the system of 12 isolated lamellae. The lamellae were of different thickness so that similar energy interval should correspond to each of them. Large time constants of the current measuring circuits allow to produce a stable picture of energy spectrum at small repetition rate. The use of the step-by-step-switch for reswitching the lamellae allowed to employ only one electrometer-amplifier. The energy spectrum was registered with a recording potentiometer. The analysing circuit was calibrated with the help of a proton beam from the ion source with constant (not pulsed) extraction voltage The spectra of accelerated protons obtained with various values of the accelerating voltage are presented in fig. 9. The general view of the accelerator is shown in fig. 10. 8. Conclusion

The accelerator described has been developed to be used as an injector. However, it can be widely used in the field of low energy nuclear physics as a basic equipment. As compared with d.c. generators it profitably differs by its small size, easy construction at low cost and what is most important, it can produce considerable pulse currents of accelerated particles. The single cavity accelerator can be of special importance as a powerful secondary neutron source. Thus, for instance, for the reaction t(d,n) He* it is possible to obtain up to 1012 of 17 MeV neutrons per pulse of 1 0 + 20#see duration. There are some other reactions providing large neutron yields. Since the most intense neutron flux arises in the stripping reaction (d,n), experiments on deuteron acceleration were carried out with the cavity.

292

A.A. GLAZOV et aL

As with the existing parameter f d = 252 M H z . c m the deuteron energy gain was extremely small (300 keV with the accelerating voltage amplitude o f 1000 kV) an attempt was made to reduce the accelerating gap length down to 3 c m . Despite the fact that the operating frequency was thus reduced by 6 M H z , the accelerating voltage o f 1.0 M V was obtained without additional tuning up o f the oscillator. The dielectric~l strength o f the gap 3 c m long turned out to be sufficient t o withstand this voltage. The m a x i m u m energy o f acceierated deuterons is 900 keV with a 1 M V amplitude on the gap 3 c m long. The ion source o f the accelerator produces ion pulse current o f 20 mA, deuterium instead o f hydrogen being introduced into it. By improving the ion source and introducing additional beam focusing in the drift space, one is able t o increase the secondary flux up to 1013 in the pulse. The authors are indebted to V. P. Dmitrievsky for valuable advice and the discussion o f the results, to J. Schwabe and M. K u z m i a k for assistance in designing separate components o f the accelerator and also to V. V. Kudryashov, V. A. Akkuratov, P. T. R y b a k o v

and M. G. A k i m o v for help in assembling the electronic equipment and the accelerator units.

References 1) Yu. N. Denisov, V. P. Dmitrievsky, V. P. Dzhelepov et al., Nucl. Instr. and Meth., 21 (1963) 85. 2) A. A. Glazov and L. M. Onischenko. Preprint 936, Dubna, 1962. 3) S. Ramo and J. R. Whinnery, Fields and waves in modern radio, (Wiley, 1944). 4) WaveguideHandbook, 5, 10 (MIT, Radiation Laboratory Ser., New York, 1951). 5) F. H. James, PIEE, part B, 106, no. 29 (1959) 489. 6) E. Regenstreif, CERN, 60-26, 1960. 7) B. J. Polyakov, B. T. Zarubin and V. V. Kushin. Proc. Int. Conf. on High-Energy Accel. and Instr., 670, CERN, 1959. 8) L. W. Alvarez et al., Rev. Sci. Instr., 26 (1955) 111. 9) Yu. D. Beznogikh, L. P. Zinoviev et al., Preprint 907, Dubna, 1962. 10) A. L. Mins, I. Kh. Neviazhsky and B. J. Polyakov. Radio and electronics, 1 (1956) 893. 11) A. A. Glazov, V. A. Kochkin, L. M. Onischenko and J. Schwabe. Preprint 1103 (1962); Nukleonika, 8 (1963) 89. 12) L. B. Mullet, R. E. Clay and R. J. B. Hadden, AERE, GP/R, 1076 (1957) 13) A. A. Glazov and D. L. Novikov, Zh. Tekhn. Fiz., 28 (1958) 2295. 14) A. A. Glazov, M. Kuzmiak, D. L. Novikov and L. M. Onischenko. PTE, 1 (1964) 36. 15) A. A. Glazov and L. M. Onischenko. PTE, 2 (1964) 100.