I ndian Journal of Pure & Appl ied Physics Vol. 39. January-Febru<lry 200 1 . pp. 45-49

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High current microwave proton ion source

S K Jain*, Akhi lesh Jain & P R Hannurkar

RF & M icrowave Section, Accelerator Programme, Centre for Adv<lnced Technology, CAT, I ndore 452 0 1 3

Received 7 November 2000

A high current, h igh-density microwave proton ion source having 40 keY beam energy and 30mA proton beam current has been designed and under development at CAT, Indore. The source wi l l be used as an injector to Radio Frequency Quadrupole (RFQ). The major components of the M icrowave Ion Source (MIS) are. plasma chamber and vacuum system, microwave system, electromagnet and its power supply, high voltage i nsulator dome and platform, beam extraction system and supervisory control system. This paper descri bes the design in detail of h igh current microwave proton ion source and i ts various subassembl ies.

1 Introduction

Microwave based ion sources are classified into two types according to their operating conditions. The first type is Electron Cyclotron Resonance (ECR) is used to obtain highly multiply charged ions. An ECR plasma is produced by matching the cyclotron frequency of an electron, in a dc applied magnetic field, to microwave frequency. The second type is operated at off-resonance/higher magnetic field than that of ECR to obtain high current singly charged ions. This type of source is called as Microwave Ion Source , ·4 (MIS) which is used as injectors5 to RFQ and in ion implanter machines3.5.r. This non-fi lamentary ion source has distinct advantages over the conventional ion sources l ike higher beam current, longer l i fe time, low beam emittance, less energy spread/dispersion, higher power efficiency, higher rel iabi l ity, stable ion beam and high ionization efficiency. Invariably, all the microwave ion sources are built at ISM band (at 2450 MHz) using magnetron source. Hence the cost of the source is less and the size of the source is compact. For a high current ion source, a high temperature, high-density plasma is required . In ECR ion source, the plasma density is l imited by microwave frequency. For example, the cut-off plasma density (ncJ corresponding to the microwave frequency of 2450 MHz is 7 .46 x 1 0"1 cm··1• Also, it is found experimental ly that the plasma density saturates nearly at Il,,, and coupling of higher power

... emai l : skjai n @ cat.ernet. i n

into plasma does not help for density enhancement. However, h igh-density plasma can be obtained at off-resonance (COee > COrf, where COee and COrf are electron cyclotron and microwave frequencies respectively), with low gas pressure ( 1 0.1- 1 Wi mbar) using the off-resonance technique/higher magnetic field. The plasma density can be obtained nearly 1 00 times h igher than the cut-off plasma density l7. The microwave ion source is operated in of1'­resonance mode. From the microwave ion source current densityX obtained is of the order of - 200 mNsq. cmK, hence max i mu m beam current can be obtained. The design detai l s of the high current microwave proton ion source are presented in this paper.

2 Source geometry and Microwave System

The schematic diagram of the integrated assembly of the microwave proton ion source is shown in Fig. I .

The plasma chamber is made water-cooled . The plasma chamber and water-cool ing jacket are fabricated in seamless SS-304 L. The dimension of the plasma chamber is <jl I 1 4.3 mm, length 500 mm. The plasma chamber requires a base pressure of I (l'r. mbar and the operating pressure is 1 0.4 mbar. Turbo molecu lar pump of 400 l itre/sec . is used. High purity hydrogen gas is fed to the plasma chamber through an SS tube of 5 mm and tlow is monitored through a micro precision valve.

The microwave system consists of a microwave source, wave-guide system for transferring

46 INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

MICROWAVE SYSTEM (2.45GHz.2kW)

MAGNETRO'-J

C O NTROL ELECTRONICS

5kV , 1 AMP DC SOURC�

ISOLATOR

DIRECTIONAL COUPLER

CHAMBER

ELECTRODE

TO VACUUM PUI.1� (TU"" O<l rjuc.)

W.lT[H .)uT

Fig, I - Schematic diagram of the integrated assembly of the microwave proton ion source

microwave power from the source to p lasma chamber and microwave coupl ing device. The mIcrowave system is sourced by a 1 .9 kW (CW) magnetron (variable power source) , at a frequency �450 MHz. A h igh power i solator ( isolation 25 dB) IS used to protect the magnetron from l oad i mperfections by directing the reflected power to the load . Forward and reflected power i s monitored using a 50 dB loop d irectional coupler and a power meter. The power supply for the magnetron was fabricated at CAT, Indore. The microwave transfer l ine has been designed and developed using WR-284 waveguide section . The output of the magnetron is co-axial type; hence we require a wavegui de to co-axial adapter. The magnetron mount was procured from Mis Richardson Electronics Ltd ., USA. The co-axia l port i s located at the d istance of (11./4) from the end plate of the wave-guide. A WR-340 to WR-284 waveguide transit ion has been designed and developed. The length of the transition is approximate equal to one wavelength . The measured VSWR is 1 .06 . A loop type directional coupler') has been designed and developed at 2450 MHz for 50 dB coupl ing factor us ing WR-284 wave-guide. The coupl ing was calcu lated using the relat ion:

C :: 20 In [h " A� A ,l2 n A ] dB

where,

b :: Narrow d i mensions of waveguide, cm; A, =

Guide wavelength, cm; A., :: Free space wavelength , cm and A :: l oop area, cm2•

In our case, for a coupl ing factor of 50 dB the hole d iameter comes to be 4.0 mm. The i mpedance of the plasma is not fixed but has a variable i mpedance nature due to its chaotic behaviour. A water-cooled triple stub tuner (TST) has been designed and developed . So that maximum power can transfer between the generator and load i .e . plasma. The TST consists of a variable depth screw which presents a shunt capaci t ive susceptance and separated by a d istance of 3 ')"..18. A Teflon sheet of thickness 3 mm is used as a

< waveguide window.

The window provides the vacuu m isolation to the p lasma chamber from the microwave source side and also prevents the flow of p lasma into the microwave l ine. The teflon sheet (as a window) is sandwiched between a choke type f1anCTe and an ordinary flange. A die lectric window t::> made of a lumin ium n i tride or a lumina w i l l be used when the system wi l l be energized for h igh power. The solenoid coils are electrical ly isolated from the p lasma chamber us ing h igh voltage glass epoxy insu lator dome.

JAIN et al. :MICROWA VE PROTON ION SOURCE 47

2.1 Microwave launcher (as a microwave­

coupling device)

To match the i mpedance of the l ine to the equivalent i mpedance of the p lasma a homogeneous quarter wave transformer I I I has been designed and fabricated . A rectangular four section homogeneous transformations to achieve e lectric field breakdown eas i ly i .e . microwave launcher was des igned with a field enhancement factor of 1 .9 1 3 . The d i mensions of the larger side, and hence, the guide wavelength is the same for all four sections:

a = 72. 1 4 mm

A� = 23.09 cm

The length of each section i s (A/4) i .e . 57.45 mm and the total length of the transit ion i s equal to one guide wavelength .

For plasma i mpedance of 1 3 1 .70. and waveguide i mpedance of 5270. the computed values are:

Section i mpedance d i mensions

N Z o. b mm

482.700 3 1 .20

2 34 1 .322 22.00

3 203 .460 1 3 .00

4 1 43 .870 09 .30

The microwave launcher is used to i nject the microwave power to the plasma chamber.

2.2 Electromagnet and power supply

The magnetic fie ld required for ECR action I S gi ven by :

let. = 2.8 B ( from CD,o = e Blm)

where,

f = microwave frequency in GHz; B = critical magnetic field in kG and m = mass of an electron, kG.

The axial magnetic fie ld profi le of the electromagnet coil as a function of the coi l current is shown in Fig. 2.

The resonant magnetic field corresponding to the microwave frequency 2 .45 GHz i s 875 Gauss. In our case, solenoid coi l s are required to produce �reater than 875 Gauss magnetic field of 1 400

Gauss i s chosen, because microwave ion source I S operated at off-resonance principle . To obtain uniform magnetic fie ld axia l ly in the center of the p lasma chamber large numher of i teration was done us ing the POISSON software. Three water-cooled solenoid coil s were desi gned and developed. The fie ld patterns for the coi l I and 3 are s imi lar. The magnetic fie ld in the plasma chamber was produced us ing the three e lectromagnet coil s was placed i n core. Thi s core was fabricated us ing soft M S . The core of the electromagnet was fabricated in five parts consisting of two s ide p lates and three cy l indrical shapes bore equal to e lectromagnet. The solenoid coil s was p laced in it and was covered by two s ide p lates fixed with A l len bolts. The core and electromagnet was having bore of 1 50 mm, so that plasma chamber/water-cool i ng jacket can be fitted i n this . One side flange of the plas ma chamber was fabricated in spl i tt ing type, so that electromagnet with core can be fitted to p lasma chamber. A l l three e lectromagnet coi I s were energized by three constant current power suppl ies of 0-32V, 1 00 rnA ratings . The e lectromagnet coi I s, produces the un iform field of 1 00 mm axial d irection in the center region of the plasma chamber. The coi l s were water-cooled and they were fabricated using OFHC rectangular copper tubes of 5mm x 5mm with bore d iameter of 3 mm. The sal ient features of the system and solenoid coi l design parameters are presented i n Table I .

I � 1600 r- - ----.... ---.- -- -... .... --- . -.. --- - --. .. - .- .. , � 1400 ., ..... +.+-+-+-+++ .... -+-+ .. "1'� ............. + .. � --- 45A ; , ,� 1200 !., ./ ... .JJ.:I :I:.X·XXZX X-J: z-:ra:-ltoz-x '+, ! � 50A I ! \,.I / ,:. J('X.)(<<)(·)(-X X )(-X )( -X-X_)(� �+ I . �1000 I �/�/� :::::::::::�+ =�� " I 1i: 800 . 4.. �� -x-65A � 600 /I. + -+- 70A .. � 400 +17" �S�i � 2� . +cf!" +� � ��'---::��:�* .. :

Distance(CM)

Fig. 2 - Graph between magnetic tield versus distance

2.3 High voltage insulator dome and platform

The h igh voltage insulator dome for 60 kV i nsulat ion designed and fabricated at Mis OS Electrical s, Indore. The dome was fabricated using high voltage glass epoxy with proper composition . The overal l thickness of the insulation dome was kept around 1 5 mm.The dome was tested at h igh

4X INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

voltage p latform of 1 00 kY at Power Supply Group, CAT, Indore. The dome was fitted over the p lasma chamber and electromagnet coi l s were fitted on it, to keep it at ground potential . The dome was fabricated in two parts having one side-corrugated flange, which also provides the sides of the magnet at ground potential . After assembly the complete set up was tested upto 40 kY.

Tanle I - The salient features of the system and solenoid design parameters

Beam energy (ke V ) Beam current (mA) Beam diameter (mm) Emittance (n mm- mrad) M icrowave frequency (MHz) Microwave power (kW) Magnetic field ( gauss) Coil NI (for ench coil I and 3) Coil NI ( for coi l 2) Power supply ( for each coi l )

2.4 Beam extraction system

40 30 08

(J .8 2450

2.0 1 350

1 6.380 9200

0-32V, JOOA

The plasma current density (Jp) i s gi ven by :

.II' = (jl1j + ,jCkTJml) where,

k = Boltzmann constant; Tc = electron temperature, eY; tnj = mass of the ion, k G and nt = ion density, cm- '

The space charge current density (J,pJ is computed using the expression:

./'Il<. = 4/9 En r ,j2e/l11.] VII'''� Ie!" where,

E n = permittivity ; d = extraction gap width and VII = extraction potential, k Y.

For practical cases, Kel ler l l proposes a handy formu la, where gap width = radius of extraction aperture, Now,

r ( rnA) = 0.7 I q/A J"� V"2 (kY)

where,

A = atomic weight of the ion, amu and

q = charge state of the ion.

Therefore, for a proton beam current of 1 00 rnA an extraction voltage of 27 kY is required . For a 50 kY extraction potential the current that can be extracted is greater than 250 rnA but in this case as

the plasma current density is less than the space charge current density the extracted current is control led by the value of .lr. High extraction voltage is required for high ion currents. A three­electrode system has been designed and fabricated to extract the proton ion beam current of 30 rnA and 40 keY beam energy. The schematic d iagram of the three-electrode system is shown in Fig. 3 .

Fig. 3 - Schematic d iagram o f the three electrode system

The plasma e lectrode (PE) wi l l be kept at a potential of 30 kY and accelerating e lectrode (AE) wi l l be negati ve with respect to ground and varies to few kY. The third e lectrode is at ground potential . The PE made water-cooled to prevent the emission of secondary e lectrons. The electrodes are separated by corrugated type Teflon flange that provides the dc isolation of 65 kY . The spacing between the plasma electrode (PE) and the acceleration electrode · (AE) is very critical and governs the beam current. To obtain maximum beam current the spacing has to be optimized . The beam current wi l l be measured by using Faraday cup.

The emittance of the ion beam E ll is calcu lated being the expressions given by Taylorl2 :

E n = 1 .6 X 1 0 -5 (q/A) B r 1t mm-mrad

where,

B = magnetic field in Gauss; r = radius of the extraction aperture in mm; A = atomic weight of the ion, amu and q = c harge state of the ion .

The computed emittance comes out to be between 0.22 < E n < 0.36 1t mm-mrad for 875 gauss < B < 1 400 Gauss.

JAIN et al.:MICROW AVE PROTON ION SOURCE 49

Supervisory Control system - Supervisory control system for MIS is designed with Computer eXtended Instrument (CXI) uni t. Raw d igital and analog data received frum various sub-systems of MIS are conditioned using Signal-condition i ng units. These conditioned data is fed to data acquisition cards i nside a Pc. GUI based software has been prepared to monitor and control various parameters of the system. This software presents whole of the system in block diagram format. Each of the b locks can be monitored in detai l by activat ing that b lock. This helps moni toring and supervis ing of the system.

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