Indian Journal of Radio & Space Physics Vol. 33, August 2004, pp. 267-277

Development of 20 kW amplifier at very high frequency (VHF)

Raghuraj Singh, Y S S Srinivas, Pankaj Khilar, B Kadia, Sunil Dani & Dhiraj Bora

Institute for Plasma Research (IPR), Bhat, Gandhi nagar 382 428, Gujarat (India)

E-mail: raghuraj @ipr.res.in ; dbora@ipr.res.in

and

D S Bhattacharya

Variable Energy Cyclotron Center (VECC), IIAF, Bidhan Nagar, Kolkata 700 064 (India)

Received 26 Jlllle 2003; revised 13 Oclober 2003; accepted 14 April 2004

A triode based 20 kW cw (continuous wave) amplifier has been developed and tested indigenollsly at 91.2 MHz and 70 MHz. The equipment will drive the pre-driver stage of 200 kW, which will, in turn drive the final stage of 1.5 MW of rf power to heat plasma in SST-I Tokamak for 1000 s. Various aspects of design and fabrication of the amplifier are di scussed in this paper.

Keywords: Amplifier, Very hi gh frequency (VHF) PACS No: 84.30 Le IPC Code: H03F 3/191

1 Introduction A 1.5 MW Ion Cyclotron Resonance Heating

(lCRH) system is being developed to heat the SST-I (Steady State Superconducting Tokamak-I) plasma during 1000 s operation. Different frequencies in the range 20-91.2 M Hz have been chosen to allow second harmonic and minority heating at 1.5 T and 3.0 T toroidal magnetic fields. Fast wave current drive is also planned at 70 MHz. Several stages of amplifiers will be used to achieve the high power output at the above-mentioned frequencies. Two rf generator chains would be used to cover the desired frequencies. One is tunable from 20 MHz to 45.6 MHz and the other one is fixed at 91.2 MHz and 70 MHz each, with 4 MHz bandwidth. A 20 kW rf amplifier is designed and tested at 91.2 M Hz. It is also tested at 70 MHz with minor modifications in the circuit. This would serve as input to the pre-driver stage of amplifier, delivering output of 200 kW. This output will be used at the input of the final stage to obtain an output of 1.5 MW of power at the desired frequency. We describe here the architecture of the 20 kW stage of the amplifier, which serves also as a prototype for the higher power stages. Test results are also reported in this paper.

A variety of power amplifiers exist at VHF range. The two main categories are based on solid-state devices and vacuum tubes. The former can yield only quite low power and large numbers must be operated

in parallel to reach even a few kilowatts of power level s. Individual bipolar transistor and field effect transistor can be made to give a few hundred watts of rf power at VHF. To get more rf power many transistors are operated in parallel. Appreciable power is dissipated in the splitter and combiner circuits. An important limitation of transistorized rf amplifier is high cost of power transistors. The other points on which they compare unfavourably with tube amplifier are low output of a single transistor and its sensitivity to overloads. Transistors are widely used in low power stages of amplifiers. In some higher power amplifiers (10 kW in the HF band), transistors are used in the output stages as well. In high power amplifiers the use of transistors does not results in a substantial reduction in size and weight. The high power stages of radio amplifiers (except those operating at lowest frequencies) mainly use vacuum tubes and special purpose microwave devices. Tubes will be an inevitable choice when an amplifier is to operate at elevated temperature.

1.1 Objective

The objective is to develop a rf amplifier with the following specifications:

Operating frequency: 91.2 ± 2 MHz and 70 ± 2 MHz Output Power: 20 kW (adjustable) Input and output Impedance: 50 Ohm

268 INDIAN J RADIO & SPACE PHYS, AUGUST 2004

2 Circuit configuration A triode based amplifier is selected due to easy

availability in wide range of power ratings and frequencies . It has low nonlinear distortion and low harmonic contents. It can be operated in ambient

temperature range from - 60°C to 70°C. Maximum operating frequency = (0.75-0.8)1.11ax . where 1.nax is the upper limit of frequency of the tube. Reduced plate voltage is necessary near 1.n"x' A chain of cascaded amplifiers is used to achieve the required output power as shown in Fig. I. A signal generator is used to get the low power signal of required frequency. The output of signal generator is fed to a low power amplifier, which gives 50 W output power with 5 dBm input. Simi larly the output of the low power amplifier is fed to the driver stage, which gives a maximum 2 kW output power to drive the first stage of the high power amp li fier. As much as 20 kW output power is expected from the first stage of the hi gh power amplifier with 2 kW input drive. Output power of each stage is adjustable according to

Signal -+

RF -+

LPA ~

Driver Generator Switch (SOW) A/r4>.(2 KW) t--

t I

Control L 1"HPA ~HPA 3"' HPA Signal (20kW) f-+ (200kW) -- (1 .5MW)

Fi g. I- Scheme of various amplifie rs and control (rf = radio frequency; LPA = Low power amplifier and HPA = Hi gh power amplifier)

requirement. Power amplification at VHF band has always been a technical problem. A grounded grid configuration (cathode input) of the amplifier is chosen because of the fact that a common amplifier may oscillate by itself at some undesired frequency . Therefore, the grounded grid acts as an electrostatic screen between the input and the output. It thus improves the stabi lity of operation and reduces the tendency of oscillation .

2.1 Layout of circuit Output power level and the frequency of operation

govern the layout of the amplifier. A coaxial cavity and strip line is preferred due to its low impedance, mmlmum radiation 10"ses and least undesired coupling to other circuit elements. Schematic of the ampli fier is shown in Fig. 2. A commercially available EIMAC make 3CW30,OOOH7 triode is used as shown in Fig. 3(a) and its technical specifications are given in Table 1.

Various components of the amplifier are di scussed below.

(a) Plate blocking capacitors

Low impedance should be offered by plate blocking capacitors at the operating frequency . Larger value capacitor tends to exhibit more internal inductance than smaller value capacitor. The voltage rating for the capacitors should be at least twice the operating voltage. It should also ha ve a high current handling capability . NPO ceramic capacitors are suitable at these frequencies. Impedance as a function

20kW RF AMPLIFIER (9 1.2MHz & 70MH z)

vi 0..

R

C, \oC,= 200pf VVC = S - 2S0p f

Cc = SOOp f

CL = S-l OOp f C. = l OOp! (8 NOS. IN PAR ALLEL)

L = SOn H

WC= Vacuum Variable Capacitor

. f ig. 2- Circuit diagram of the amplifier

SINGH et al.: 20 kW VHF AMPLIFIER 269

A

G

K F

(a) (b)

Fig. 3-(a) View of vacuum triode and (b) inter-electrode parameters of the triode

Table I-Technical specifications of triode 3CW30,OOOH7

Type Filament Voltage

Current, at 6.3 V

Direct inter electrode Cin

capacitance (grounded C oul

grid) Cpk

Amplification factor (average) Frequency of maximum rating cw DC Plate Voltage (max.) DC Plate Current (max.) Plate Dissipation (max.) Grid Dissipation (max.) Cooling

Thoriated tungsten 6.3 ±0.3 V 160A

61 pF

36 pF

0.2 pF 200 110 MHz 8000 V 6.0A 30,OOOW 500W Water and forced air

of frequency is shown in Fig. 4(a) for the selected capacitors with value of 100 pF. Lead inductance of the measuring probe should be compensated properly with the measuring instrument before the measurement.

(b) RF choke (RFC)

Plate and filament rf chokes should have impedance at least five times the value of plate impedance at operating frequency to protect the rf interference into the power supply. The self-resonance frequency (SRF) should be above the operating frequency and should have sufficient current handling capability. RFC response is shown in Fig. 4(b). The impedance of RFC is above 3000 ohm at 91.2 MHz and it must be modified for 70 MHz operation. Special care is taken during operation in VHF range. In this range the lead inductance to the output and

1000~----------------------~

E 100 .r. 0 ui 0

~ 10 a w Q.

~

0 20 40 60 80 100 120 FREQUENCY, MHz

a 10000

N 1000 :z: ::E ui u 100 ~ 0 w 10 Q.

;!

1 10 25 40 55 70 85 100

FREQUENCY, MHz

(b)

Fig. 4--(a) Self-resonance curve of plate blocking capacitor and (b) self-resonance curve of the plate rfc

input circuit are kept as low as possible. Short and wide straps are used for this purpose. Cathode bypass capacitor in this case is chosen appropriately. Efficiency is improved for enhanced tube life. Mechanical and electrical precautions are equally important during operation. To avoid cross-talk, separate compartments for input and output circuits are used. Parasitic oscillations are avoided by using low inductive (large surface and shorter length) wires. One must doubly ensure the tightness of contacts. Among electrical precautions, a good plate power supply regulation is used. At the same time crowbar protection for the tube, over voltage and over current protections must be ensured.

2.2 Input circuit A simple L-type matching network is used to match

the load impedance to the output impedance of the driver stage to couple the maximum power. The input capacitance (Cn) is measured around l.9 times higher than the data sheet value. Therefore the Cn value of 116 pF and calculated load (Rd value of 30 Q are considered for the calculation. The matching network will take care of required bandwidth and load.

270 INDIAN J RADIO & SPACE PHYS, AUGUST 2004

Parallel combination of -jXill and RL can be transformed into the series combination as:

- R Z= } LX;II =6.0-jI 2

RL - jX;1I

or Rs = 6.0 and Xs = - jl2

The series equivalent circuit is the combination of a 6.0 Ohm resistor and a - j12 .0 Ohm capacitor. Now, the equal and opposite reactance (+j] 2.0) is added in series for the cancellation of - j12.0 Ohm capacitive reactance in Fig. 5(c) leav ing only an apparent 6.0 Ohm resistor. The values of network components (inductance L and capacitance C) are estimated as follows:

For impedance matching the followin g condition should be satisfied:

f-Jf0g Q.1"= Qp= --I

R.I

where, Qs is the Q of the seri es leg, Qp the Q of the parallel leg, Xs the series reactance, Rs the series resistance, Xp the shunt reactance and Rg the shunt resistor.

The series inductive reactance is estimated as

[IJ j"--::-l

Rg. RB~ -jX. -- JX in R L

~ ~ R.

(0) (h)

l

Rg C RL

"v

EJ

Therefore the required total series reacwnce is

x = j16+j12 = j28 .0 Q

The parallel capacitive reactance is estimated as

Therefore the equivalent series inductor Land parallel capacitor C are around 50.0 nl-I and 94.3 pF at 91 .2 MHz. A vacuum variable capacitor (5-250 pF) is connected to get the active matching during operation of the amplifier. The bandwidth of the input-matching

network is estimated as BW =!..~ and it IS Qp

approximately ±16 MHz. A measured value of bandwidth is shown in Fig. 13(b), wh ich is measured at the input on VNA by simulating grounded grid active resi stance (dynamic resi stance) by physically putting 30 ohm carbon resistance.

A complete view of input circuit of the generator is shown in Fig. 5(f).

2.3 Output circuit A view of co-axial and strip line resonator

presently used in operating the ampli fier (20 kW) is shown in Fi g. 6(a). COlTesponding equivalent circuit is shown in Fig. 6(b).

(a) Resonator at 70 MHz

A co-axial resonator is used in the output circuit. A /.../4 coaxial cavity is Llsed for 70 MHz operation ,

(e) (d)

Fig. 5-(a) Equivalent of input circuit, (b) parallel to series equivalent, (c) equivalent of series resonance, (d) along matchi ng network, (e) final input circuit and (f) view of input circuit

SINGH el 0 1. : 20 kW VHF AMPLIFIER 27 1

having square type outer conductor and cylindrical inner conductor of copper. Charac teristic impedance

(20) for thi s structure is given by 20 == 60 In(b/a) , where b is the width of rectangular outer conductor and a the outer diameter of the inner conductor. 20 is

selected as 60 Q considering the high voltage breakdown strength. Schematic in Fig. 7 shows the relative positions of various components of the co­axial resonator.

Conductor dimensions are as follows:

Cross-section of outer conductor = 640 mm x 640 mm; and diameter of inner cylinder = 230 mm.

From the tube data C Olli = 36 pF and measured C Olli

= 36x l.8 = 64.8 pF and Xc = 35 Q at 70 MH z. The

required length of the resonato r e is g iven by

e = ~ tan - I ~ , because at resonance Xc = XL and 2n: Z 0

----I! :'-----. Ii Cr ~ -~ -::. §

C01l1 ~

LrCSOllator

(b)

,r"> . :> Load ~>, ;:.:

Fig. 6-- (a) Actual view of output c irc uit and (b) equivalent 01" ou tput ci rcuit

~rINNER CONDUCTOR

~ V-OUTER CONDUCTOR o OUTPUT

TOP VIEW

input impedance of a short-circuited co-axial

transmission line of length 'A/4 is given by 2s = XL =

20 tan ~e . Therefore, dimensions for co-axial

conductors are

Calculated length (I ) = 360 mm Spacing between grid and plate = 100 mm Total length of outer conductor - 500 mm Inner conductor length = 330 mm Thickness = 5 mm

During design, requirement for the resonator should also be kept in mind. Resonator must work with sufficiently high efficiency . It should have the capability to withstand suffi cient electri c fie ld strength and be ab le to di ss ipate heat generated. Plate

impedance in the range 600-800 Q is ideal for a power amplifier plate circuit. Therefore, one would like to des ign the system in such a way that the impedance li es in the above range. Resonator cavity

should have high Q and is g iven as Q = XL = _1_ and R (VCR

impedance of parallel resonator is Ra = ~ = (v o LQ. CR

As the formula shows, for hi gh Q circuit res istance (R) should be as less as possible. It can be obtained by using hi gh conductive contact surfaces and proper tightening for minimum contact res istance. To tune smoothly , fl ex ible finger contacts are put on the plunger. Contact area is sufficient for current density I A/mm. Equivalent circuit for the output is shown in

Fig. 6(b). Efficiency of the resonator (11 ) is determined by the power lost within it, as power is lost in cavity walls, contacts and in tube section of the resonator. Efficiency is defined as

17 = 1- Qlnodl'd = 1- Rloaded

Q /Ill loaded R /Ill loaded

SIDE VI EW

HORTING PLUNGER

LA TE BLOCf(ING CAPACITOR

TRIODE

Fig. 7-Schematic of output assembly

272 fNOIAN J RADIO & SPACE PHYS, AUGUST 2004

Here, Q'oaded depends on operating plate load impedance, which in turn . is dictated by coupling capacitor.

(b) ResonaJoraJ 91.2 MHz

At 91.2 MHz, a strip line resonator has been used. Strip line is used because of easy fabrication and fixing in the same resonator for use at 70 MHz. Three strip lines are connected in parallel. The characteristic

impedance (20) of each strip line is kept at 100 Q and it can also be calculated as

Zo(Q) = 120 n ~ JE,. a+b

for bla > 0.3.

With allowed current carrying capability of I A/mm a width of 125 mm for the strip would be enough for operation at - 255 A rf current, which will be divided into three parallel arms. Thickness of strip line is kept 3 mm for the mechanical support. From the plot in Fig. 8, bla is 2.5 for 100 Q, where b is the width of the strip and a the gap between strips. For b = 125 mm and 20 = 100 Q, a is 50 mm. ETP semi­hard copper sheet is used to fabricate the strip lines that are shown in Fig. 6(a) . For absolute maximum field strength, minimum separation between conductors is given by

6.25xKxV max Dmin

Emax,abs

600.---------------------------------~

400

200

100 q 80

N' 60

t- b-j

-~ o \\<\\\«\\,\\\~\«

40 einseitige teitung

-L 1ti:Bli2

o 1lI'£i!!I~ l-- b -l T

summetr leitlXl

20~--~~~~~~~~--~~~~~~~ 0.1 0.2 0.4 0.5 O.B 1.0 2 3 4 5 6 7 8 10

b/a

Fig. 8-Variation of strip line characteri stic impedance with the ratio of bla

where, DrrUn = minimum gap between the conductors, K = 0.435 for co-axial line and 0.2 17 for two-wire line, Emax,abs = 5-10 kV/cm. For Vnux = 6 kV and E'TJaX,abs = 5 kV/cm, Dmin = 3.26 cm and therefore, 5 cm gap is sufficient.

Strip line length is calculated using the formula mentioned before as follows:

From the tube data COUl = 36 pF. Considering COUl

as 72 pF, capacitive reactance (Xc) = 24,25 Q at 91.2 MHz and required XL = 24.25 x 3 = 72,75 Q (three lines are in parallel), calculated length is obtained equal to 328 mm. Taking care of stray inductance, length is kept at 325 mm.

3 Calculated parameters Operating points for the tube are selected 25 %

below the maximum rating of the plate voltage, considering upper frequency limit of the tube. Accordingly other operating parameters are calculated with the help of load line drawn on constant characteristic curve of the tube as shown in Fig. 9 and these parameters are shown in Table 2. The amplifier is selected to operate in class B or AB2, therefore peak rf plate current may be around 2.5-3,0 times of the average dc plate current. The load line is drawn between maximum plate voltage to be applied on plate and crossing point of minimum plate voltage swing to the peak rf plate current. The operating parameters are calculated as

dc Plate current = Ii (O.SA + B+ C + E + F + G)

Peak fundamental rf current = Ii (A + 1.938 + 1.73C + l AW + E + 0.S2F)

Output power = Peak fundamental rf current x Peak rf Voltage

2 Resonant load impedance =

Peak rf Voltage --------Peak fundam ental rf current

Output Power x lOO Efficiency = --'-----dc Input power

Plate dissipation = dc Input power - rf Output power 2

Driving power = V g

Rin

V (Peak) Input impedance = ---'g----­

I P (Peak)

w o o I f­« U

SINGH el al .: 20 kW VHF AMPLIFI ER

I '~ ..

~~~~F· ·t~ .... ~ .... ~ .. k@' - - i - _ . ' !"

- - .-- - .. , .----.~~-. 10

. . :_'_' _i '_

. . ': : r:

~~~~~'5DD 1<:: .001

\

~-2 3 4 S

PEAK rf PLATE VOLTAGE PLATE TO GRID VOLTAGE, kV

6 7

-'---1 CURVE #4043

Fig. 9-Constant current characteri sti c curve and load line

Table 2-0perating parameters o r the tube calculated from load lines r·----i /4 "'"veiength----1

Parameters From 1st load line From 2nd load line

dc plate voltage 6.0 kV 6.0 kV .,d ! L-__________ ~

dc plate current 5.5 A 3.2 A Peak fundamental 8.2 A 4.6 A plate current Power output 20.5 kW 11 .5 kW Resonance 6 10Q 1086Q impedance

,,~c\U,.ellt ] t \r"oitClge 1 ~ IlllpOli,nce

Input impedance 29.3 Q 34.8 Q Dri ving power 2.0kW 736 W Efficiency 6 1.8 % 60.0 % Plate di ssipatioll 12.7 kW 7.7 kW Grid current 1.0 A 525 mA Grid di ssipation 170W 60.0W

3.1 Voltage and currcnt distribution in rcsonator

Voltage and current distribution in % wavelength

section of a transmission line are seen as in Fig. lO(a) and Fig. lOeb). These parameters can be estimated as

Vt = Verest sin ~e and 1= l erest cos ~e

where

Dis l<Ull'e .. long transmissIOn hne

a

f---1 /4 wavelength I I ~Shorted end

('9= = = = = = = = = = = = = ~ ~------------------~

Conent

Voltage Impedance

273

VI Vt V crcst = , I crcst = I sc = .

sin{ Pe) Z OS II1( Pe) Fig. IO--(a) Voltage and current di stribution in resonator and (b) voltage and current di stribution in short-circuited transmi ssion line of length 1J4

274 INDIAN J RADI O & SPACE PHYS, AUGUST 2004

The calculated values of vo ltage and current are shown in Fig. II .

4 Output power coupling Coupling circuits may be inductive, capaciti ve or

direct tapping type. In our circuit a capacItI ve coupling is used which so lves the problem of impedance transformation as we ll as power coupling

RF Voltage Distribution in Resonator r-------------------------------,0000

6000 > wi 0 -<

4000 E-...J 0 >

2OOO t;; a

0 35 30 25 20 15 10 5 0

ELECTRICAL LENGTl-I, deg

(a)

RF Ourent Distribution in Resonator 205

200

-< 195 ~

Z 190 ~ 185 ~

U

180 t;; a

175

170 35 30 25 20 15 10 5 0

ELECTRICAL LENGTl-I, deg

(b)

Fig. l l-(a) Voltage distribution used in resonator and (b) current distribution used in resonator

(a)

to the load . An equi valent tank circuit of a resonato r including its coupling circuit is shown in Fig. 12(a). The coupling circuit consists of the reactance of coupling capac itor X collple connected in series wi th the load impedance ZL = RL +jXL . Transfo rm the seri es connection into a parallel one. T he transformed coup ling circuit is show n in Fig. 12(b), where the para ll el res istance Rp and reactance Xp are given by

2 2 R' X ' _ RI. + X d X -_=~_+ __ -R - an 1' -

P RL RL

where X = X colipJe + XL when, X is substantiall y greater than RL. then

X 2 R" := - and Xp = X. Therefore the equi va lent

RL

res isti ve and reacti ve componen ts as shown In Fig. 12(c) can be given by

R pRlI1 R := orR

eq R + R I' I' III

Req R 1I1

RIII - Req

The required coupling reactance is

X := X collple + X L := ~ R I' R L

In case of standard load, XL can be ignored because it is the lead inductance, wh ich appears in series with the load. Therefore the value of coupling capac itor can be g iven as

C cOllple := --J--;=== R l. Req

(j) --

11

where, R eg = RJoaded and Ru J = RunJoaded

5 Testing Pass ive testing has been performed first to assure

the rf parameters. All measuremems are performed using Vector Network Analyzer (V NA), Spectrum

.-----,--.--~L_J--J

X cruple

c;r pi L-.... ~[~ _o

(b) (c)

Fig. 12-(a) Output equivalent c ircuit, (b) series to parallel equivalent of Xcouplcand RL and (c) equi valent impedance

SINGH el al.: 20 kW VHF AMPLIFIER 275

BAND WIDTH OF INPUT SIDE

CH I MARKERS

A RE~...':. 1 2: 3.0334 Db - 3.60596 MHz ~ ~

3: 3.022 1 Db 3.74091 MHz

t---~ I---t---.. ~ ,..--

')-~ r

CENT[[~ SPAN 9L!00 000 MH.~ 50.000000 MHz

(b)

Fig. 13- (a) A complete view of amplifier (b) Measured bandwidth of input circuit

Analyzer (SA), oscilloscope and multimeters. A non­standard open-ended BNC cable is calibrated for measurements, therefore 1-3% en'or is poss ible. A complete view of the amp lifier is shown in Fig. 13(a). Measured bandwidth of the input circuit is presented in Fig. 13(b).

From the Smith Chart in Fig. 14, it is seen that resonator efficiency is 95 %. Harmonic content ha also been measured. It is performed with the help of VNA and spectrum analyzer. Harmonic contents in output are shown in Fig. 15(a). It is around 30 dBc down at our working frequency .

6 Cooling A minimum of 10 GPM flow rate is required for an

output of -20 kW. The water conductivity required is

11 NOV 2002 !J!lI S l1 1 U fS 1:581.34 n - 1.6875 n 1.0099 nF

15:19:53 93.391 .200 MHz

....

(0)

tHII 511

Po.

(b)

I

I I

, /'

/" /

\ , -('

/' \ / /' , /

I,

I

I

I

/ , I

/ ,-----START STOP 88.000000 MHz 100.000000 MHz

11 NOV 2002 14:49:02 1 U '1:12.359 kn - 188.00 n 9.0806 pF 93.227.936 MHz

I

I

I I

/

/

START "", 87 .989198 MHz

I

\

} - ----, Hz.. "'" \

/ /

)0., / ,

-' /'

\ )'

/ ' , I '

/" , , / , - - 1:_, ;::-' ;;

STOP 100.000000 MHz

Fig. 14- (a) Loaded plate impedance and (b) unloaded plate impedance

276 INDIAN J RADIO & SPACE PHYS, AUGUST 2004

I )lS to avoid the short-circuit of high voltage, because anode and cooling chamber are same. Forced ai r-cooling is used for the base of the tube. The required water-cooling for different plate dissipat ions is as fo llows:

(a)

(b)

Fig. 15-(a) Harmonics level in output and (b) filter response

Plate dissipation

15 kW 20kW 30 kW

7 Filter

Flow required

12GPM 13 GPM 14GPM

Pressure drop

13.5 PSI 15.0 PSI 17.0 PSI

A filter is used to protect the dc power supply from the rf interference. It is shown in Fig. IS(b), that atten uation is around 60 dB at the high voltage terminal.

8 Safety and interlocks The power supply (Fig. 16) can be operated locall y

or remotely from the central control system. A crowbar protect ion is provided to protect the tube in the event of an arc fault. ]n the event of an arc fault in the tube, the HV should be switched off with in few microseconds. This cannot be reali zed with the conventional circuit breakers, which takes approx imately 100 ms. So an ignitron is used as crowbar switch , which short circuits the dc output of the power supply. The crowbar cire it is tested and the time of operation of the crowbar is measured to be approximately 2 )ls. This power suppl y is tested for its fu ll rated values of dc output volrage, current and power. Its ripple and regulation are found to be within I %. The power supply also has several interlocks, which ensures safety of the tube and operating personnel. Some of these are listed in the Table 3.

AX. POMR CONTROLLER

SHIES .--.--.,..-J/IIIIt._TO LOAD

CROWBAR TRIGGER

L-----L--L-__ TO LOAD

Fig. 16-20kW stage power supply schematic

Table 3-Various interlocks in the power supply

SI. No. Description. Adjustabi lity Sensing Action

I. dc Over current o to 100% Shunt Crowbar 2. ac Over current o to 100% CT's Pulses block 3. dc Over voltage o to 100% Potential divider Pulses block 4. Doors open Door limit switches MCCB trip 5. Filament warm-up Up-to 5 min Electronic timer MCCB di sable 6. Filament trip Contactor MCCB trip 7. Emergency off Emergency switch MCCB trip 8. Control voltage off Contactor MCCB trip 9. Coolant now Adjustable Pressure switch Pu lse block

SINGH et al. : 20 kW VHF AMPLIFIER 277

Table 4-Calcul ated and measured operating parameters for the tube

Various Ca lcu lated Measured parameters parametcrs parametcrs

dc plate vo ltage 6.0 kV 6.5 kV dc plate current 5.5 A 4 .9 A Power output 20.5 kW 20.3 kW Drivi ng power 2000 W l600W Effic iency 6 1.8% 63.7 % Plate di ss ipatio n 12 .7 kW 11.6 kW Grid current 1.0 A 850 mA

,.1 HPA (2 0 KW Stage ) Test Result at 91.2MHz

20

16

12

o 0 .2 0.. 0 .6 0 .8 1 1.2 1 .4 1 .6 1.8 INPUT POWER IkW)

(a)

WAVEFORM

(b)

Fig. 17-(a) Variation of output power with input power and (b) theoutput waveform

9 Results and discussion At 91.2 MHz, the target of 20 kW cw has been

achieved without any trouble. All the measured

S DHARAN RSP-S2 Aug formatted

parameters are found within the safe limits and close to the calculated values. No harmonic distortion is observed . Input driving power is varied from about 200 W to 1600 W. Table 4 shows the calculated and the measured operating parameters for the tube. The measured parameters are for operation at 20 kW.

Variation of output power as a function of input power is shown in Fig. 17(a) and output waveform is shown in Fi g. l7(b) .

10 Conclusion A 20 kW cw amplifier at 91.2 MHz has been

successfully fabricated and tested at 20 kW . No voltage breakdow n and arcing is observed during testing . Output power 20 kW is achievable wi th proper provi sion for cooling. Contact surfaces should be properly cleaned to get the operational loaded plate impedance. Intense care should be taken to avoid harmonics and parasitic oscillations.

Acknowledgement Contribution of Mr V George, IS acknowledged

during the initial phase of the work.

References I Radio Transmiller Design, edited by Vagan V Shakhg ildyan

(Mir Publi shers, Moscow), 1987.

2 Care and Feeding of Power Grid Tubes, Prepared by Laboratory Staff, VARIAN EIMAC, Fourth Printing, 1982.

3 "3CW30, OOOH?" tube data sheet, Printed in USA by Varian.

4 Power Vacullm Tubes Handbook, edited by Jerry C. Whitaker (CRC Press, LLC Head Quarters, Florida, USA), 1999.

5 Technical Note by Max Marki (TRIUMF, Vancouvers, Canada).