a) structures with a fixed acceleration

(1)

RF SYSTEM DEVELOPMENT FOR THE HEAVY ION LINAC B.Stadler

UNILAC Group, Institut fiir Angewandte Physik Universitat Heidelberg, Germany Summary

The layout of the radio frequency system for a variable-energy variable-mass heavy ion linear accelerator will be described.

Results from specially designed vhf amplifier for variable power output as well as the phase and amplitude control system will be reported.

Introduction

The proposed UNILAC (1) combines two types of acceleration structures:

a) structures with a fixed acceleration profile(the Wideroe prestripper sections, the Alvarez poststripper sections).

b) the Single-Gap-Cavity structure for energy variation.

In the first structures the acceleration voltage is related strongly to the charge-

to-mass ratio of the ions, whereas the phase relation among the different sections remains unchanged. Since the charge- to-mass ratio, due to the ion sources provided, ranges from 0.5 to 0.046, the rf power must cover a range from about 1 to

100 for the proper excitation of the prestripper sections.For the poststripper sections the charge-to-mass ratio ranges from 0.5 to 0.1 and the rf power must oo- ver a 1 to 25 range only. For the lowest charge-to-mass ratio pulsed operation of the accelerator is provided with a duty of 2546 . The duty will be increased up to

lOO$ if the charge-to-mass ratio is increased ( for light particles) by a factor of 2 or more.

The acceleration voltage in the Single - Gap-Cavity structure and the phases in its

21 cells must be choosen not only accord- ing to the specific charge of the ions, but also with respect to the particle energy wanted at the end of the accelerator.

In cases where an energy for light particles far below the maximum energy speci- fied is wanted, it may advantageously hap- pen, that some of the cells at the end of

this structure are merely used to maintain the rf bunches of the particles. These cells are excited then with low power and the stable phase will be -90 degrees.

A one-line diagram of the UNILAC rf system is shown in Fig.1 . The prestripper sections are excited with a frequency of

27.12 MHz and all the poststripper sections and rebunchers use its 4th har- monic (‘108.48 MHz ).

Modular Structure of the RF Power Sources In designing the rf system with its control system it proved convenient and de- sirable to make use of a modular system as far as possible, since at least the Single-Gap-Cavity structure with its 21

cells has an inherent modularity. Start- ing from the peak power of 170 kW needed for one of the cells, a television-tube RS 1082 CW ( Siemens > was found to be economical,allowing at this power-level a duty factor of 25$ at 108 MHz. A boundary condition for the preceding stages was given by the use of a fast varactor- phaseshifter with an upper limit for the transmitted power of about 2W. A total gain of about 50 dB is needed for the amplifier chain. A complete modul for 108 MHz was designed and built with four stages,using the following tetrodes in grounded-grid circuit: 8122 ( 15dB ) RS 1062 / 7650 ( 12 dB ), RS 2022 C ('12 dB),RS1082CW (lldB).Thestages

driving the final amplifier ( RS 1082 CW) are conservatively designed,having a gain- reserve of 3 dB at least. These four stages with its control system and supplies except for the high voltage supply of the RS 1082 CW are mounted on a common frame.

A 108 MHz modul is shown in Fig.2 . This modul serves to excite one cell of the Single-Gap-Cavity structure as well as the Alvarez sections,where two or four moduls are working together on a common load. A similar arrangement modified for a frequency of 27 MHz will be used to excite the three Widerije sectiok. For the auxilliary cavities parts from this modul program will be used.

The Circuits

RS 1082 CW - Final Amplifier. The tube RS 1082 CW is able to Droduce at 108 MHz a peak power of more -&an 200 kW with a duty factor of 25 &if an anode voltage of 19 kV is used. 170 kW can be obtained with 12 kV, but with IO kV a mean-power of about 48 kW for duties of 50 $ and

100 '$ can be produced at this frequency.

The 108 MHz anode-tank, shown in Fig. 3 ,

© 1969 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers

or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.

(2)

consists of a capacitively loaded and folded full-wave resonator. Power is coupled out of the anode-tank by a motor-

driven coupling - loop located symmetrical- ly to a voltage-node . The length of the loop is x/4 and terminated at one end by a shorted x/4 -stub-line.Between its ex- treme positions the loading of the anode- tank can be varied by a factor of about 10 without markedly affecting the tuning of the tank. A change in rf power,.therefore, must not lead to a change of the rf voltage in the tank, maintaining the efficiency of of the tube. A capacitive probe in the anode-tank is used to gain a fast AVC-signal. The correspondin

to the preamplifiers f error-signal is fed

8122 and RS 1062 >.

DC and cooling water are supplied through ceramic feed-through-capacitors ( 1500pF/

20kV ) to the anode in a voltage node of the tank circuit.The two feed-through-capacitors will suppress on the supply-lines residual rf voltages between the connected sufaces. The anode-cylinder is fixed in po- sition by three ceramic supports. No de- coupling capacitor is used in the anode circuit,since the grid No.2 is kept at dc- ground potential.

The rf input power (pulsed or below 50 kW also CW ) to this stage drives the cathode.

The heater-supply is decou led from the rf input by a tunable loaded i/4 stub. This gives an isolation of the lines outside the confinement of -34 dB related to the rf input, With some additional ferrit rings put over these lines inside the confinement the isolation could be made better than

-70 dB.

The anode voltage is taken directly from a 260pF capacitor-bank and the protection against a dangerous flash-over is formed by a tapped 8R resistor, where a thyratron

( BBC-TQ 91 ) acts as a crowbar, handling fault-currenta up to 2000 A. Since the RS

1082 CW allows steadily the application of

22 kV (dc), a hard-tube modulator was in our ease not necessary, but in addition it was found more economical to use a large

capacitor-bank together with a fast acting crowbar ( total maximum delay time less than ~&AS). The time dependent voltage - drop leads to some additional loss in the anode and is taken into account.

Grid No.2 is directly grounded and con- sequently the filament is on a negative potential ( -1450 V ).Protection of the cathode and grid No.1 circuit is achieved by external spark-gaps and a thyristor crowbar circuit.

The RS 2022 C Driver Amplifier is e- quipped with an air-cooled 12 kW fele- vision-tetrode. The tube is operated in a grounded-grid circuit, providing driving pover in excess for the final amplifier.

The anode-tank is formed by a coaxial structure and the power is coupled to a 50SL-line by means of a tunable series in- ductance. The rf input signal passes a tunable w-section, built by concentric elements, that matches a 5OLL -line to the cathode. 9 kV are used for anode voltage when the duty is 25 $, but 5kV are sufficient for the lower power levels at duties of 50 $ and 100 $. For grid No.2

800V are used and the grid No.1 is operated with fixed bias of -16OV from a transistorized shunt regulator. With 9 kV the circuit is adjusted to deliver up to 25 kW in an ohmic load, Only about 12 to 16 kW are needed for maximum driving-power.

For protection of this tube against a dangerous flash-over thyristor-crowbar- circuits are used.

The Preamplifier Stages containing the tubes 8122 and RS 1062 / 7650 are me- chanically almost identical except for the tube bases. They are operated in grounded-grid circuits. Matching of the inputs and outputs to 5OR-lines is accom- plished by tunablev-circuits built with concentric elements. An anode voltage of 1000 V for the 8122 and of 2000 V for the RS 1062 / 7650 is used. For both tubes the voltage for grid No.2 amounts to about 25OV. Transistorized shunt regulators supply the grids No.1. These regulators re- ceive the error-signal from the AVC - circuit of the final amplifier. With the aid of these shunt regulators it is also possible to switch-off the rf drive in cases of a fault in the RS 2022 or the RS

1082 CW stages.

The Phase and Amplitude Control System Four different parameters are affected by

the control system used to maintain the phase and amplitude in the driven cavity within close toleranc'es.

a) The resonance frequency of the excited cavity must be brought close to the driving frequency by means of a tuning-servo.

Its driving signal is gained from a rf- bridge, where the phase inside the cavity with the phase of the running wave in the

driving line is compared. If the resonance frequency of the cavity is correct, the

(3)

b) Since the motor driven tuning element moves slowly with respect to the 5 ms rf-pulse, the shortest pulse-length, a fast phaseshifter is used to correct small-phase deviations, resulting either from a mistuning of the cavity or phase- shifts in the amplifiers during a pulse.

The fast phaseshifter consists of 5 cascadedr-sections on a printed circuit, where the ca acitors of theTT-sections are. partial f y substituted by varactor- diodes (BAY 96 / Valvo). The network as shown in Pig.4 has a characteristic

impedance centered at 50 ohms. The phaseshift of each section is limited to 2 9 degrees, giving a total phaseshift of + 45 degrees. The dc-bias ranges from 15 To 100 V for a power level of 2 W, transmitted to the 8122 stage. The insertion-loss never exceeds 0.7 dB.

The error-signal driving the phaseshifter is ained by comparing the (assumed correct 7 phase of the preceding cavity with the phase in the controlled cavity.

Special care is taken, that the phase- error detector, connected to the out- PA

uts of a 3 dB directional coupler /4-ring circuit), may not produce false signals for different rf levels.

Overall accuracy is better than 0.5 degrees for rf amplitudes between 2 V and 20 V fed to the phase comparator.

In the phase-detector selected hp-5082- 2800 diodes are used.

c> Por the Single-Gap-Cavity structure a power range of 1 to 16 is sufficient, since at low particle energy the cavities not used for acceleration could be switched in phase to -90 degrees. With the aid of the variable coupling loop in the final amplifier a range of 1 to

10 in output power is covered with a constant anode swing. The latter is maintained by the AVC - feed - back

system , mentioned before. Reduction of the anode dc-voltage from 12 kV to 10 kV leads to a reduction of the anode swing from 10 kV peak to 8 kV peak and enlarges the total range of output power from 1

to 16. The feedback system does not only stabilize the anode swing, but in

addition it prevents rising of the anode voltage swing to dangerous values in cases of a strong mismatch of the load.

d) Since normally the cavity is precise- ly matched to the power line, the running wave gives a good measure for the power - in the cavity. Therefore. the rectified signal picked up in the iine by-a directional-coupler is compared with its reference, the resulting error-signal is fed to the coupling-loop-servo.

General Layout

The one-line diagram shown in Pig.1 gives a survey on the essential parts of the rf system. A master-oscillator for

27.12 MHz applies the signal to the pulse- modulator. The amplified pulsed or CW

signal is distributed to the modules exciting the prestripper sections. All phases are related to the phase of the Wideroe section No.2. At the end of the

27.12 MHz driving line a frequency multi- plier generates the frequency of 108.48 MHz for the poststripper sections. The phase of the Alvarez tank No.1 is bound directly to the phase of the Wideroe tank No.2. The phase of the Alvarez tank No.1 will serve further as a reference for the other cavities of the poststripper sections. The rebuncher No.1 transforms the phase-space of the end of the Wideroe into the double-gap-

cavity, called interpolation cavity. The latter seems necessary to correct for the energy loss in the stripper. Either the (two parts of the) Alvarez No.2 or the rebuncher No.2 will be excited, depending on the input energy (3 MeV/AMU or 4.5 MeV/AMU) wanted for the Single- Gap-Cavity structure.

The Alvarez sections are powered by 2 or 4 modules fed together by 3 dB-couplers.

In the Single-Gap-Cavity structure the

21 cells are individually excited and phased. Operation set points for the amplitudes and Phases will be

7 iven from a small computer on-line which also will control the status of the various equipments of the accelerator).

Precision motor driven mechanical phaseshifters with a range of 360 degrees are used for setting the phases wanted. They are added in the reference lines as well as in front of,the varactor phaseshifters.

The phase of the last cell will be checked against the phase of the first cell, since here all cells are bound to the preceding neighbour cell. Random distributed phase-errors in the indi- vidual cells within + 5 degrees will not seriously affect the beamquality as long as the mean-phase-error is less than ;f: 1 degree.

References

1. UNILAC-Bericht 3-68

and Ch. Schmelzer, Study of a Variable Energy Heavy Ion Linear Accelerator, 1968 Prot.Lin.Acc,Conf., Brookhaven

(4)

Wtd.rl,lll

1 R.bunth*rt ml **i ,10**, MB E”WW

t Mdchlng cwty

I I I

I 1 I 1 r---SF

P

::-;1g

I-.--...---.I

JK

q * -cmpamtor ,*, ,ur*- EL.rnMI k;; Direttmt town I

[II 7 -cmgcrobor tw Ekctra* )csittr -

lg Pow., tntrc4 JF p:,” 7:; bnnlnwn

I3 NC +I Plotor d,im” milq E,rrrmt

(TJ vnmdw Pho*l*klt.r # Poww 3 de-c.¶up,.r

Cdl No 20

Cd NO 21 ,D.b”“ChW,

Fig.2 108 MHz Amplifier Modul

Fig.1 One-Line Diagram of the UNILAC KF System

(5)

Filament U,.lOV

I,.2ooA

\ 1200 pF

\wpliw camcitars

D,...D,: Varactor-Diode Type BAY 96 (Volvo)

C,,C,.15pF L,... L,. 75nH

C&.37pF Brq,Bu,.SOn

V m w I

? -wl

uq ;j B,n 3

x1 20 Q +. -. c --as -

_i _{l b} _‘b _A- _{_} _{,I# 0}_T--

AP -,

Fig.4 Varactor Phaseshifter 8) Electric Diagram

b) Phaseshift vers. Bias- Voltage

Air & FilamPn)

Fig.3 Section of the 108 MHz Final Amplifier