A modified Aarhus negative-ion source

NUCLEAR

INSTRUMENTS

AND

METHODS

164 (1979) -1-10;

(~)

NORTH-HOLLAND

PUBLISHING

CO.

A MODIFIED AARHUS NEGATIVE-ION SOURCE H. VERNON SMITH, Jr.*

Department of Nuclear Engineering and Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, U.S.A. Received 22 December 1978 We report intensity, emittance, and brightness measurements both for the C u - beam as a function of source parameters and for several atomic and molecular negative-ion beams when our modified Aarhus negative-ion source (ANIS) operates on a cold PIG discharge (CPD). A hot PIG discharge (HPD) produces a significantly larger (factor of ~ 6) C u - beam intensity. However, the larger C u - beam from a HPD also has significantly increased emittance, resulting in only a small improvement (~4096) in C u - beam brightness. In the HPD mode the bias of the filaments is important. The relative yields of the atomic negative-ion beams are only in fair agreement with the predictions of the Saha-Langmuir equation. Finally, we compare ANIS with other negative-ion sputter sources.

1. Introduction Negative-ion sputter s o u r c e s M7) are widely used on tandem accelerators. The early development of these sources is reviewed in refs. 18 and 19. One of the more successful sources is the Aarhus negativeion source (ANIS) developed by Andersen and Tykesson at the University of AarhusS,g). Their ANIS source operates on a hot (low voltage, high current) PIG discharge (HPD) fed by filaments whose power consumption (400W) necessitates water cooling of the source. If these filaments could be eliminated and the source run on a cold (high voltage, low current) PIG discharge (CPD), perhaps the source power consumption and complexity could be reduced. We have constructed an ANIS source t which can operate on either a HPD or a CPD. In this paper we evaluate the CPD mode of operation, compare the CPD and HPD modes, and compare ANIS with other negative-ion sputter

coated spherical surface 2°-22) and accelerate across the plasma boundary layer. Since the energy gained in traversing this layer ( - 1 keV) is much greater than the energy of the negative ions after sputtering ( ~ 10 eV), the sputtered negative ions focus to the center of the sphere, which is at the center of the 0 i

,

, , ¢:PalA

,

5cm i

ALUMINUM ARMCO E~

COPPER I~~

Present address: AT-2, University of California, Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, U.S.A. t An earlier description of our ANIS source is contained in ref. 11. *

STAINLESS STEEL

D,t.

sources.

2. Source design and performance Our version of the ANIS source is shown in figs. 1 and 2. As described in ref. 5, for HPD operation the spacing S between the exit aperture and the sputter cathode is set equal to the radius of the concave spherical surface RSPH machined in the end of the sputter cathode (see fig. 3 insert). The plasma boundary layer assumes the spherical shape of the cathode. Negative ions sputter from the cesium-

BORON N I T R I D E

CATHODE POSITION._=. ADJUSTMENT LEGEND

A-

ANODE

B C -

P I G CATHODE S P U T T E R CATHODE

DE -

POLE PIECE GAP LENS

G H J -

BASEPLATE SOURCE HOUSING ALUMINUM GASKET

K -

CATHODE FLANGE

L -

F I L A M E N T (HPD ONLY)

M -

To HEATER COIL (CPD ONLY)

N -

S P U T T E R CATI'K)OE CLAMP

GUIDE

Fig. 1. ANIS source (top view). The electrical feedthrus and a portion of the PIG cathode support structure have been left out for clarity.

2

H.V.

SMITH

exit aperture. A gap lens forms these ions into a beam and discriminates against those negative ions Aluminum and electrons which are at plasma potential. An=--Baseplote dersen and Tykesson showed that the ions are Stainless Steel [~ Source formed on or near the sputter-cathode surface since I ~ Boron Nitride Housing the negative ions extracted from ANIS have sputCopper ter-cathode energyS). Gap Sputter When operated with a HPD, two filament loops Cathode are placed between each PIG cathode and the anode Beam ~ _ C o t h o d e Position.... (see fig. 1). The filament power dissipation is 100 W Adjustment Out and the total power dissipation is 120 W (table 1). _~:'~ Csln © Since the filaments operate at -2500 K, they can feed either electrons or surface-ionized Cs ÷ into IX ,4-.---Flange the discharge, depending upon the bias voltage applied between the filament and the anode. When ~ C s Flow operated with a CPD the filaments are removed I x)~0 Adjustment and a Ta heater coil is placed at the rear of the discharge chamber to dissipate enough power "" eoled) ( - 2 0 W) to prevent Cs condensation on the insulaL~ Valve Heater Block tors, During CPD operation the total power dissipation is only 20W (table 1). The discharge is Cs Pool--, Cs Oven sustained with argon support gas for both the HPD Cs Heater Block and CPD modes of operation. The exit aperture is Cartridge 1.5 m m in diameter. An Alnico horseshoe magnet Heater (not shown) with soft iron pole tips provides a Fig. 2. ANIS source (side view). The PiG cathodes and their support structure, the electrical feedthrus, the filaments, and the magnetic field of 0.086 T at the PIG cathodes and discharge magnet have been left out for clarity. 0.061 T at the center of the anode. Table 1 shows o

, 5cm

I SIco~e

L

. . . . . . .

TABLE 1 Typical parameters for operation o f ANIS with a cold PIG discharge (CPD) or with a hot PIG discharge (HPD). Parameter

Sputter cathode

voltage a Sputter cathode current Discharge voltage a Discharge current Sb Filament voltage c Filament current c Filament bias voltage a Filament bias current Magnetic field Ar pressure Cs partial pressure d C u - current e~' (85%) B~N (85%) a b c a

CPD - 1000 V 1.3 m A -625 V 0.5 m A 6.9 m m 2V 9A 0 0.061 T 3.2 Pa - 10 - 2 Pa 8.0/~A 3 . 5 n m m mrad M e V i 0.62 # A / m m mrad M e V t

With respect to anode potential. Sputter-cathode to exit-aperture spacing. Filaments in parallel. Estimated from k n o w n Cs c o n s u m p t i o n ( ~ 1 m g / h ) and conductances.

HPD - 1000 V 13 m A - 100 V 18 m A 9.5 m m 4.6 V 22 A - 10 V 360 m A 0.061 T 2.8 Pa ~ 10 - 2 Pa 38 ~tA l l n m m mrad MeV ! 0.91 # A / m m mrad MeV t

3

N E G A T I V E - I O N SOURCE

typical parameters for operation with CPD and HPD discharges. Fig. 2 shows the method for dispensing Cs' vapor into the discharge chamber. An all-metal, bellowssealed needle valve* contained in an aluminum heater block is used to regulate the flow of Cs vapor between the Cs oven and the discharge chamber. To prevent blockage with Cs the connecting tubing is short and both it and the valve operate at elevated temperature. Normally the valve is at 250°C, the Cs pool at 230°C, and the valve is set at 30% of its full-open position. A bellows-sealed mechanism (not shown) allows adjustment of the exit-aperture to sputter-cathode spacing S. The allmetal system eliminates organic contaminants.

3. Beam intensity, emittance, and brightness measurements Recently we upgraded our ion source test stand by (1) increasing the analyzing angle from 12° to 30° to improve the mass resolution and (2) incorporating Ames' improved version 23) of Billen's emittance device24). The design of the new test stand insured that the full beam extracted from the source is transported to the emittance device. At 20 keV the acceptance =t of the emittance device is 120 rc m m mrad MeV t in the plane of the emittance measurements and > 2 5 0 r r m m m r a d M e V t in the orthogonal plane23). The two-dimensional emittance t~(f) in the (y, ¢) plane is given by 23) ~ ( f ) = A ' ( f ) E ~,

(1)

plane emittance at a given fraction of the total beam. This difference arises because the analyzing magnet does not resolve the 63Cu- and 65Cu- ions for the emittance measurements. Indeed, at low discriminator settings on the emittance device (i.e., for high brightness contours) for a C u - beam two distinct ellipses appeared on the oscilloscope display of the (x,O) plane phase-space plot. The two observed ellipses are separated by magnetic field values corresponding to the mass difference between 63Cu- and 65Cu-. Only a single ellipse is observed at all discriminator settings for both the (Y, 0) plane and for mono-isotopic ions in the (x, 0) plane. The dependence of the C u - beam intensity I c , - , emittance e~', and brightness/~N on S for operation with a CPD is shown in fig. 3. The discharge and sputter cathode voltages were 400V and 1000V respectively. Optimal performance occurs for S approximately 25% less than Rsp a. The dependence of l c , - , t~, and /~N on the discharge voltage was also measured for the C u - beam from a CPD for ~ 4.o,r "--'-'~R

"u :~ e 3.~, .~ %'~ 3.o=

;

o

where AY(f) is the phase-space area in the (y, ¢) plane for f fraction of the total beam I_ and E is the beam energy. The resulting normalized two-dimensional brightness BsN(f) in the (y, ¢) plane is

RSPH

;

,~, I0

1~%,

.

6

'

g i v e n b y 23) B ~ N ( f ) ---- fI_/eY2(f),

Sputter [ ./Cothode

I

(2)

~

where e:~(f) is given by eq. (1). In general, we report emittance values for the y, 0 (vertical) plane. The emittance values for the x, 0 (horizontal) plane are assumed to be near those measured for the (y, 0) plane. This assumption was verified within 30% for the mono-isotopic ions Cand AI~-(fig. 4). However, for C u - the observed (x, 0) plane emittance is almost double the (y, ¢)

-

* Model SS-4BMG-TSW Bellows Metering Valve, manufactured by the NUPRO Company, Willoughby, Ohio 44094, U.S.A. t ,, = (Phase space area). (E)½.

::L

600

4

z

~, oO

i

4

~s 2°°~=

,

i

-

6

8 S,

•

0

I~ 6 "

tO

mm

Fig. 3. C u - beam intens!ty l c u - , emittance e~, and brightness B~N as a function of exit-aperture to sputter-cathode spacing S for the ANIS source operated with a cold PIG discharge. Representative error bars (-+5%) are indicated on the figure. The curves serve only to guide the eye.

4

H.V.

TABLE

SMITH

2

The intensity I _ , emittance 8~, and brightness B~'N of several negative atomic and molecular ion species obtained from the ANIS source when operated with a CPD and for the V - and C u - beams obtained with a HPD. The electrical current and emittance values are obtained from the measurements shown in figs. 4 and 5 and the brightness values are calculated from ect. (2).

Ion

CC~ AI2VVNi Ni2Cu Cu Nb -

Discharge mode CPD CPD CPD CPD HPD CPD CPD CPD HPD CPD

/_ (/tA) 6.7 3.8 0.43 0.18 1.1 4.3 0.17 6.4 38 0.46

e~' (9096) - (mm mrad MeV ~)

B~'N (9096) t ~ A / m m mrad MeV )

14re 9.0 rc 2.1 n 2.7 n 9.9 7r 2.1 lr 1.7 rr 3.6 n 13 n 2.7 n

0.14 0.12 0.060 0.019 0.032 0.60 0.028 0.51 0.84 0.049

S = 7 . 5 m m and a sputter cathode voltage of 1000 V. The optimum discharge voltage is 625 V, but source performance is not critically dependent on discharge voltage since/i'~r~ is within 10% of its maximum value between 350 V and 850 V. The optimum discharge voltage depends upon the sputter cathode voltage- the higher the sputter cathode voltage the higher the optimum discharge voltage. We did not study e~ and/7~N as a function of sputter cathode voltage. However, as is expected from the dependence of the sputter ratio S' (atoms ejected/incident ion) on the energy of the bombarding species (see fig. 17c of ref. 25), the Cu- output current rises monotonically with sputter cathode voltage up to the power supply maximum of 2000 V. We have measured the intensity I_, e~, and B~r~ for a wide range of negative atomic and molecular ion species from the ANIS source operated with a CPD and for the V- and Cu- beams obtained with a HPD. The results of these measurements are given in table 2 and displayed in figs. 4 and 5. Fig. 5 "compares operation of ANIS with a HPD and a CPD. Even though the maximum beam intensity is much higher (~ a factor of 6) when ANIS is operated on a HPD, the brightness for 90% of the total beam is approximately the same (within 40%) for the two modes of operation (see table 2). Table 3 shows the maximum currents obtained for the negative-ion beams that we studied. Also shown in table 3 are the maximum currents obtained by the Aarhus groupS). Andersen and Tykesson5) report the existence of two different plasma modes for operation on a H P D - a dense plasma mode for which the maximum beam is

IO7/"

w

>~

•

.......

C'(y)

, 6.7p.A

•

....

C-(x)

, 6.71u.A

o

------

C2

,3.8FA

•

-----

Al~,(y) , 0 . 4 3 F A

•

. . . .

Ai;(x),

o

.......

V-

, O.I8/J.A

A

-----

Ni-

,4.3#iA

v

............... Ni:.:

0.43FA

, O.17/.zA

o - -

Cu-

,6.4/zA

i, . . . . .

Nb-

,0.461J.A

"1o

E E

571"

E

/~/O

.

0

:

°

0

20

0

i

I

40 Percentoge

I

I

I

60 of B e o m ,

I

80

I

I

I00

%

Fig. 4. Two-dimensional emittance e,Y2 vs percentage of total beam included in the measurement for several atomic and molecular negative-ion beams from the ANIS source operated with a cold PIG discharge. The two-dimensional emittance e~ in the (x, 0) plane is plotted for the AI 2- and C 2- beams. Representative error bars ( + 5 % ) are indicated on the figure. Note that the AI2-0') and the N i - curves coincide and that the ordinate scale for the C 2- and both C - curves has been multiplied by a factor of 2. The currents given for AI2-, C2-, and Ni 2- are electrical currents. The curves serve only to guide the eye.

NEGATIVE-ION

157r

-'--~--

>~

HPD CPD HPD CPO

!

f

5 0 p . A Cu-, CPD (F~b Vapor) I I

I I

E

E E

107r

/7 iI

//

,?Y

/~

57r

O

I O

I

20

I

I

40

I

I

I

60

I

I

80

5

modes. However, we note that our HPD measurements are probably for the dense plasma mode since they were obtained for S = Rsps and for an Ar + Cs discharge. Fig. 5 also shows emittance measurements for the C u - beam from a CPD operated with Rb vapor in place of Cs. The small difference observed for operation with Cs and Rb vapor is probably not significant since there is 5% error in the emittance and the percentage of the total beam values23). Although we found little difference in C u - output when K or Rb were used in place of Cs vapor22), Cs vapor gave the maximum V - beam while K was twice as efficient as Cs in producing V - (efficiency = I_/sputter cathode current). We also studied the effect of additions of Hz, 02, and NH 3 gas to the A r + C s mixture. Sputter cathodes constructed of AI, V, Ni, Cu, Nb, and Ta were used but not every combination of gas and cathode material was investigated. A residual gas analyzer monitored the impurity gas addition. Generally H2 and NH 3 restored the negative atomic beams to their maximum values whereas 02 reduced the negative atomic beams but increased the negative metal-oxide beams. The resulting metal-oxide beams are not as intense as the metal atomic beams. The addition of Oz to the A r + K mixture increased the V - and C u - beams by a factor of 3-5. In fact, we obtained as much C u beam (8/zA)with an A r + K + O 2 mixture as with an A r + C s mixture (CPD).

207r

- - - - e - - - - I.I /u.A V-, ~ a ~ O.18p.A V-, --•-38/J.A Cu-, --o~ 6.4FLA Cu-,

SOURCE

1OO

Percentage of Beam, %

Fig. 5. Two-dimensional emitttance eY2 vs the percentage of the total b e a m included in the m e a s u r e m e n t for the V - and C u b e a m s from the ANIS source operated with a cold PIG discharge (CPD) or a hot PIG discharge (HPD). M e a s u r e m e n t s obtained using Rb in place o f Cs vapor for the C u - b e a m from a C P D are also shown. Representative error bars (--_5%) are indicated on t h e figure. T h e curves serve only to guide the eye.

obtained near S =RspH and a thin plasma mode (discharge operating on Cs only) for which the maximum beam occurs near S = ]RsPa. We did not study the difference between these two plasma

4. Discussion The most significant findings of this study of the AN!S source are (1) - 9 0 % of the total beams

TABLE 3 M a x i m u m (electrical) b e a m currents obtained from t h e ANIS source w h e n operated with either a cold PIG discharge (CPD) or a h o t P I G discharge (HPD). Typical b e a m currents are ~½ o f t h e reported m a x i m u m values. Also s h o w n (for c o m p a r i s o n purposes) are t h e m a x i m u m b e a m c u r r e n t s reported by t h e A a r h u s groupS). Ion

Wisconsin CPD

/max (pA) LiCC2AIAI 2VNiCuNbMoTa-

0.020 7 4 0.14 1 0.8 16 8.4 1 O. 14 1.2

Attempts many one one few few many few many few one one

Wisconsin HPD

/max (pA)

Aarhus HPD Attempts

Attempts

0.015 -

few

1.6

one

/max (/zA) 1 20 15 1 2

-

-

-

90

6 30

few . . -

. .

. . -

few many many few few one many

. . 3

one

6

H.V.

produced-during operation with a CPD and a HPD have comparable brightnesses even though (2) operation with a HPD allows production of substantially more total beam, (3) in the HPD mode the bias of the filaments is important (see below), and (4) a focussing mechanism still operates during a CPD even though S ~ 0.7 RspH and substantial asymmetric erosion occurs on the side of the sputter cathode. The power consumption and, to a lesser degree, the complexity of the ANIS source are reduced when it is operated with a CPD. Also, we find that for the refractory metals (i.e. groups Va and Via of the periodic table) the predominant beam is the atomic metal (or metal-hydride). The predominant refractory metal beams reported for the UNIS source are one of the metal oxides (see table 2 of ref. 15). We find that the maximum C u - and V - beams produced by the ANIS source are obtained when it is operated on a HPD (table 3). However, the brightness of 90% of the total beam is nearly the same (within 40%) for operation with either a HPD or a CPD (table 2). The larger output current for a HPD is offset by increased emittance. Tykesson et al. suggest that the increase in emittance with output current may be due to space-charge effects in the vicinity of the exit apertureS). However, estimation of the space-charge effects in the exit-aperture region indicates that this may not be the

SMITH

introduce the space-charge parameter By:

6y = 2 AI_ b3/e~ (a + b), (4) which is the ratio of the space-charge term to the emittance term in the Kapchinskij-Vladimirskij equation [terms 4 and 3 respectively in eq. (3)]. If 6 e < l , emittance dominates the beam growth; if 6y>l, space charge dominates29). For a 1 keV, 4 0 a A Cu- beam in the 1.5 mm diameter exitaperture with our source geometry, ey= 310 mm mrad so6y = 0.05. Since 6y.~ 1, space-charge effects are not expected to cause a significant increase in emittance due to increased lens aberrations. The magnitude of the direct, space-chargeinduced emittance growth is estimated with a crude model which assumes that in the exit-aperture region the ANIS beam is an infinitely-long pencil beam with uniform charge density. Assuming that the exit-aperture radius R is /> the beam radius RD, the maximum transverse energy E t gained by a beam particle due to space-charge forces is A E t = (el_/47reo v) {1 + 2 [In (R/Rb) ] }.

(5)

If R
NEGATIVE-ION SOURCE

7

geometry, i+ is the positive ion current incident on the sputter cathode, S' is the sputter ratio (atoms ejected/incident ion), o)n and 090 are the surface partition functions for negative ions and neutral atoms respectively, A is the negative ion electron affinity, ~ is the work function of the sputter-cathode surface, and T is the temperature parameter 3t) of the sputter-cathode material. The production of high-intensity, negative-ion beams from sputter sources is accomplished by reduction of 0 to values near 1.5 eV with sub-monolayer Cs films (column 8 of table 4). This reduction in allows achievement of (,4- ¢) values comparable to k T [ k T ~ 0 . 2 - 0 . 7 e V 31)] and results in a dramatic increase in the exponential term in eq. (6) and in the negative-ion yield I_. Fig. 6 shows a plot of I _ v s i+S'(co,/O)o) exp[(A-~0)/kT] for our CPD data reported in table 4. The values of S', o)0, A, ~, and T given in table 4 are taken from refs. 31-40. There is only fair agreement between the measured values and the dependence predicted by the Saha-Langmuir equation. The straight line plotted in fig. 6 corresponds to a source geometry factor G = 0.1. Recent data 4~) for the Mo- yield from a cesiated molybdenum surface exhibits the functional dependence of I_ on 0 (at low Cs coverage) given in eq. (6). In addition, refs. 42-44 show that a Cs coating increases the negative secondary-ion emission coefficient o~_{a~_ is the sputtered negative-ion fraction = (con/wo)exp[(A-())/kT]} to values >0.1 for many metals. The negative ions from the ANIS source are known to originate from the sputter cathodeS). Therefore, adding an impurity gas (e.g., H 2) to the A r + C s discharge may alter the work function of the sputter-cathode surface, cause the formation of compounds which subsequently sputter, or alter the transmission of negative ions through the discharge. Coadsorption of Cs and H2 and also Cs and O2 has been reported to result in lower optimum work functions for Ni 36'45, Mo 46'47, W 48'49, and Ta 4o) than for Cs alone. We found little change or some improvement in negative-ion yield from adding H2 or NH 3 to the discharge mixture. However, the result of even the smallest addition of 02 was a dramatic reduction of the atomic metal ion output of ANIS (CPD), except for the case of Ni where little change was observed. We did find that, unlike the case of an A r + C s discharge, when O2 gas is added to an A r + K mixture the V - and Cu- beams where G is a constant determined by the source increase. The difference in behavior for K and Cs

by 40 keV Ar + compared to that of C - sputtered from the same target3°). An alternative explanation is that the emittances of the diatomic ions are smaller because their intensity is less than that of the atomic ions. Andersen and Tykesson operate their HPD filament at PIG cathode potentialS.S). We find that independent bias of the filaments is important since we can increase the C u - output from 12~tA to 70/~A by adjustment of the filament bias from - 1 0 0 V (PIG cathode potential) to - 3 0 V . The above factor of 5.8 increase in C u - current was accompanied by a factor of 6.0 increase in the sputter-cathode current so perhaps the role of the reduced filament bias voltage is to allow more efficient injection of electrons into the plasma, thereby increasing the plasma density. The uniform erosion pattern that we observe on the sputter cathode confirms the Aarhus group's observation that when ANIS is operated on a HPD (dense plasma mode), the plasma sheath assumes the spherical shape of the sputter cathode. When operated in this manner, very little wear occurs on the sides of the sputter cathode. However, operation of ANIS with a CPD requires S ~ 0.7Rsp H for optimization of lcu-, el, and ~ N (fig. 3). When S ~ 0.7Rsp a the plasma sheath no longer fills the spherical cathode uniformly; accelerated erosion occurs at the edges and sides of the sputter cathode facing the PIG cathodes. Although the wear of the sputter cathode is not spherically symmetric and heavy erosion of the sputter cathode sides occurs, some focussing still occurs during a CPD since the beam intensity and brightness depend sharply on S (fig. 3). The optimum value of S~0.7Rspr~ probably represents an average or effective radius of the concave surface since the radius varies from RspH where the erosion is slight to a value ,~RsvH where the wear is substantial. The erosion of the sputter cathode with time has very little effect on the brightness of the extracted beam. During one test with Ni we compared the Ni- beam after 5 h and 50 h of operation with a CPD and found only a 996 decrease in brightness during this time. In ref. 22 we use a modified Saha-Langmuir equation to describe the negative secondary-ion emission 2~) from the sputter cathode. The output current I of the ANIS source is given by [eq. (2) of ref. 22] I_ = (Gi+ S' con/COo){exp [(A-dp)/kr]}, (6)

8

H.v.

SMITH

TABLE 4 Calculation of i+ S'(ogn/Ogo)exp[(A-@)/kT]. This quantity (plotted versus I_ in fig. 6) is related to I_ by the geometry factor G of the source [eq. (6)]. Ion

/_ (/zA)

i+ (mA)

S 'a (atoms/ion)

(o0b

(,On

Ac (eV)

@

kT e

(eV)

(eV)

i+ S'(~On/Og0) × exp [(A - @)/kT]

(A) CAIVNiCuNbMoTaa b c d e f s

7 0.14 0.8 16 8 1 0.14 1.2

2.5 6.1 8 7.3 1.3 6 5 8

0.12 1 0.65 1.45 2.4 0.6 0.8 0.6

2 1 1 1 1 1 1 1

9.3 5.9 48.8 30.6 2.4 25.4 8.7 16.4

Table 7.1 of ref. 32. Table 3 of ref. 33. Ref. 34. W. C. Lineberger, private communication. Column 16 from table 1 of ref. 31. Ref. 35. Assumed to be equal to that of Ni.

1.268 0.46 0.5 1.15 1.226 1.0 0.7520 0.6

h i J 'k

1.37 f 1.60g 1.53 h 1.60 i 1.64J 1.53 k 1.541 1.55 m

0.685 0.371 0.196 n 0.214 0.184 0.196 n 0.196 0.261

5.6×10 -5 4.8× 10 -5 5.6x 10 -7 4.2× 10 -5 6.5× 10 - 4 9.5x 10 -6 8.3× 10 - 6 7 . 7 x 1 0 -6

Assumed to be equal to that of Nb. Ref. 36. Ref. 37. Ref. 38.

i Ref. 39. mRef. 40. n Assumed to be equal to that of Mo.

when exposed to O2 is puzzling. Perhaps at the entrance) and for a terminal voltage of 3.5 MV, we operating temperature of the arc chamber the O2 obtained 215 nA of Cu 3÷ (charge current) through a gas chemically ties up the Cs vapor but does not 3 mm diameter aperture in front of the specimen react with K so it therefore can further reduce the chamber located 9 m from the accelerator exit. At work function. this transmission value (2.4% particle transmission) The C~-, AI~, N i - , and C u - beams from the studies with the refractory metal beams from the ANIS source (CPD) have been accelerated with the ANIS source will give acceptable damage rates. For model EN tandem Van de Graaff accelerator for example, 1.6/zA of V- results in a damage rate of use in radiation damage studies of reactor mate- 3.9× 10 -4 dpa/s for 15 MeV V on vanadium when rialsS°). For 3/zA of C u - injected into the accelerator electron microscope specimens are prepared at a (measured on a Faraday cup at the accelerator depth of 1/zm below the specimen surface and 2 . 7 x l 0 - 3 d p a / s when specimens are prepared at io-4 the depth corresponding to the peak of the damage curve. c- /

1°-5

io.6

Cu-

To_/

v-

"

• ~~Mo-4b-Al10-7.

........

1o-r

,

io-6

......

m.,

io-S

.

. .m. . . . . ,

........

1o-4

,

io-3

i+S'((On/(,%)e (A''lb)/kT , A Fig. 6. A plot o f / vs i+S'((On/COo)exp[(A-@)/kT] for various negative-ion beams from the ANIS source operated with a CPD.

The values plotted in this figure are from table 4. The straight line corresponds to a source geometry factor G = 0.1.

5. Comparison of ANIS with other sputter sources Table 5 compares the performance of the modified ANIS source with o t h e r 4's'14'15'17'51'52) negativeion sputter sources. The Cu- performance of all the sources listed in table 5 is good, with comparable outputs and brightnesses. For Cu- the SNICS source ~7) has brightness advantages over the other sources. The ANIS source can produce f> 1/zA of atomic-metal beam for all of the investigated refractory metals (V-, Nb-, and Ta-) except molybdenum. 6. Conclusions We have shown that by operating the PIG discharge under cold-cathode conditions the power

N E G A T I V E - I O N SOURCE TABLE 5 Comparison of ANIS with other negative-ion sputter sources. Source

Exit aperture

Ion species

Current ~A)

e~'(85%) , (ram mrad MeV t)

~s(85%)

,

Reference

( n A / m m mrad MeV t)

(mm) ANIS

SNICS SPIGS UNIS

1.5 1.5

Cu- a Cu- b

1.5 1.5

Ta-

1.5 1.5 1.0 1.0 3.0 3.0 3.5 1.0 1.0

a b c d • f g

a,e

Cu- b C- b Ta- b CuTa -¢'e Cu Ta- ¢ Cu-

8 38 1.2

30 10 3 17 0.25 22 0.2 10

3.5 zt 11 rt

620 910

-

-

4.4 r~(79%) d 2.4 n 8.4 rc 3.1 n (90%) f

570(79%) d 1920 710 920 (90%) f -

Cu-

7

-

Ta- +Tall- ~

0.016

-

This work This work This work 8 8 8

17 51 52 4

14 15 15

CPD. HPD. Perhaps a sum of T a - , Tall ~, and Tall2-. Calculated from fig. 6 of ref. 8. 2.5 ttA of T a O - . Although not stated in ref. 14, we assume emittance measurement is for 9096 of total beam. 6 / i A of TaO2-.

consumption and, to a lesser degree, the complexity of the ANIS source are reduced. A factor of 2-10 lower output current but only -5096 lower brightness results for operation with a CPD in place of a HPD. The bias of the filaments is important during HPD operation. The ANIS source produces multi# A beams suitable for use on tandem accelerators for many negative-ion species. Although extensive lifetime measurements were not made, we obtained 150 h of continuous operation on Ni-. The output of different negative-ion species from the ANIS source is only in fair agreement with the predictions of the Saha-Langmuir equation. The author is grateful to D.R. Benson and R.A. Douglas for assistance in the attempts to obtain a Li- beam and also to G.L. Kulcinski and H. T. Richards for critical comments on the manuscript. This work was supported in part by the United States Department of Energy under Contract Nos. AT(11-1)-2206 and EY-76-C-02-0007.

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