Initial Experience in Achieving High Brightness From a DC H- CUSP Source
without Cs Injection
T.Y.T. Kuo, previously with TRIUMF, E-mail: kuo2003@aol.com
R. Baartman, TRIUMF, E-mail: Baartman@TRIUMF.ca
K. Jayamanna, TRIUMF, E-mail: keerthi@TRIUMF.ca
D. Yuan, previously with TRIUMF, E-Mail: dickyuan@Gmail.com
Prior to 1994, a DC H- Cusp source had been developed at TRIUMF having a current capability of 7 +/- 2 mA [1, 2, 3]. The best
brightness occurred at 5 mA with an associated normalized 4rms emittance of 0.375 pi-mm-mr giving a brightness of 5.8 mA/mm2mmr2. Since 1995, a new version was developed achieving 16 +/- 1 mA by using a double ring shaped Tantalum filament [4]. Quite a
few basic H- Cusp source parameters were studied since then and continued efforts were spent on further improvement on the H output and the associated emittance [5]. During 1999-2001, Cs injection was introduced and H- beam current up to 25 mA was
obtained. The corresponding emittance was 0.52 pi-mm-mr. In parallel, a non-Cs version using a Cusp field length of 25 cm achieved
20 mA with a normalized 4rms emittance of 0.39 pi-mm-mr. The best brightness also occurred at 20 mA point giving a value of 21
mA/mm2-mr2. Compared to the performance of the older source prior to 1994, the new source system seems to have a factor of 3 in
beam output improvement while the brightness improvement is also in the factor of 3. These results indicate that the TRIUMF source
has been able to keep its emittance quality while its beam output increases a factor of 3. In a way, the performance of our new nonCesiated source can be considered as having a complete match with that of a Cesiated source. In this paper, we shall describe the
source-extraction system and the emittance-brightness data in more detail. Future effort to expand the operation including pulsed
mode so that our result will be able to be compared with the performance from the mainstream ion sources for Linac injection [6, 7].
I. INTRODUCTION
Since 1988, TRIUMF’s H- DC cusp sources have been under constant effort of improvement, in particular, on the quest of better
brightness. Quite a few parameters have been studied and optimized whenever possible. The parameters affecting the brightness
strongly can be tentatively listed as the following: Beam energy, Arc power, Cs operation, Length of Cusp field filtering and cooling,
Electron filtering and cooling, Filament physics, Extraction physics [8], Magnetic field generated by any current flow, Metallic
properties of the plasma chamber, etc. The less critical parameters would be: Cusp field strength, Nature of Arc power, Extraction
optics, Plasma volume, e/H- ratio, Gas flow/vacuum pumping, Stripping and neutralization, Source tuning guides (diagnostic tools and
equipment, e.g., a fixed emittance-acceptance cyclotron), Source tuning philosophy/strategy/guidelines, Use of matching optics
devices (Einzel lens, solenoids, triplets), Cleanliness of the system (oil free, oil free pumping, leak free, contamination free, etc.),
Control compatibility of equipment (e.g. LaB6 offers lower temperature electrons but must be handled by fast electronic circuit with
precision feedback), etc.
Aside from the equipment aspect, one must work on the source plasma physics, making plasma parameters measurement, simulation
and theoretical studies. Most of the above listed parameters were reviewed [10], but some of them, such as changing the source bias
from 30 to 60 keV, could not be optimized because of budget limitation. To overcome the low beam energy, an accel-decel column
and a resister network were built. The 6 kW Arc power supply should have been a 100 V, 100 A unit, rather than the 150 V, 40 A
version which the first author of this paper had to borrow from another department. Certain equipment was supported by private
funding. From the parameter listing above we chose “Changing the Cusp Field Length from 15 cm to 25 cm” as the major theme of
the study for this paper.
II. MOTIVATION
From our optimization process we found that moving the filament ring closer to the filament end plate but farther away from the
extraction increases the H- production for the same arc power. We wondered if it is because the hot electrons from the filament
cooled down more by their longer path through the Cusp magnetic lines. The portion of hot electrons in the center magnet field-free
region will encounter more collisions with H2 molecules, thus losing energy as well. The number of excited H2 (υ”) molecules might
reduce somewhat when they are arriving near the extraction region because of the longer path they have to travel after they are
created near the hot electron source. The increase of the number of cool electrons and their cooler temperature thus higher Hproduction cross-section [10, 11], might more than compensate the effect of decreasing H2(υ”) molecules. The end result is larger
number of H- available. Then we asked ourselves, what would be the effect if the Cusp field length be increased in a larger step? At
TRIUMF, we had two sets of 10cm sources and 15cm sources which can be combined into 20cm, 25 cm and 30cm long sources. The
25cm source was chosen first because the drift length for both electrons and H2, H2(υ”) molecules increases by a factor of 2, from
10cm to 20cm. We should see some observable effects since both the confinement time and the electron cooling path are increased
by a factor of 2 as well.
To our surprise, the H- current did not decrease too much, only about 10%, while the emittance improves dramatically as we shall see
in the later part of this paper.
III. SYSTEM DESCRIPTION
Fig. 1. Schematic of experimental system setup.
The general layout of the source system for higher brightness development is shown in Fig. 1. There are a few special points worth
mentioning:

There are gas mixing and Cs injection penetrations from the end of the back plate of the Cusp source.

The distance L, between the filament-ring to the plasma plate can be changed in steps.

For this experiment, the total Cusp field length is 25cm, Cusp field strength on the plasma chamber wall is 5.4 kG. The L
value is 20cm; twice as long as before.

The usual simple 3-electrode extraction system was replaced by a new system which consists of an extraction Einzel lens,
an accel-decel column and a matching Einzel lens. This system is called Ext. #1 if voltage on these three devices are off,
Ext. #4 if on.

The water-cooled extraction Einzel lens is quite small, 10cm aperture at the front end and 12mm in length, embedded inside
the extraction plate assembly.

The first electrode of the accel-decel column can be biased up to 20 kV, dropped down to ground through a 4-step resistor
network.

The distance between the end plate of the extractor and the 1st electrode of the accel-decel column can be changed in
steps also.
Typical voltage used for the operation were:
Source body bias
−30 kV
Extraction to body bias
~+6 kV
Ext. Einzel lens to body bias
~+18 kV
First electrode of the a-d column
+20 kV
Matching lens
−20 kV
The beam energy variation along the beam path is shown in Fig. 2. An IGUN calculation was carried out to check that no beam will
hit any electrode. The H- beam was tuned to a central region model cyclotron first which has an acceptance of 0.5 pi-mm-mr.
Transmission was optimized first, then drop in the Faraday cup for H- current measurement. Magnetic and electric devices for
electron suppression and filtering along the beam path were provided. Finally, an Allison style emittance scanner was moved in for
emittance measurement.
IV. EMITTANCE and BRIGHTNESS
Fig. 3. Emittance scan with 15cm source/Ext.1 at 16.4 mA.
Fig. 4. Emittance scan with 25cm source/Ext.4 at 17.1 mA.
The emittance scan for the 15cm (1995) source and that for the 25cm (1999) source are shown in Figs. 3 and 4. At 16 -17 mA, the
En ratio
[En]25cm / [En]15cm = 0.37/0.74 = 1 / 2
shows that a factor of 2 in improvement on emittance was obtained which should give a factor of 4 in improvement on brightness.
Fig. 5. Comparison of beam profiles between two source lengths.
Fig. 6. Comparison of emittance ellipses from two source lengths.
In Fig. 5, we put the beam profile from these two scans together, normalized the peak point at the same height. We see that although
the profile for the 15cm source indicates a good symmetry, it shows a long shoulder on both sides. The image of this beam will show a
large halo. This characteristic appears in all scans from all of our shorter Cusp length sources.
The profile from the 25cm, longer source is different. There is no shoulder at the bottom. The image is very clean and sharp, no halo!
The comparison of emittance ellipses is equally dramatic as shown in Fig. 6. The ellipse from the 25cm source fits completely inside
the one from the 15cm source. The envelope for the 25cm source on both x, x’ directions are only ~ 43% of those from the 15cm
source.
From the emittance vs. beam percentage plots the transmission to the cyclotron should be 100% (at 17.1 mA) for the beam from the
25cm source, but only (80%)2 (10.5 mA out of 16.4 mA) for the beam from the 15cm source. The beam transmission figures derived
from the emittance scans are almost in complete agreement with the unbunched cyclotron beam ratio measured at 1 MeV, 1.27mA /
2.07 mA.
Fig. 7. Emittance scans vs. extracted H- beam current with various Source/Extraction combinations.
The emittance data as a function of extracted current for several source/extraction conditions are shown in Fig. 7. The
corresponding brightness plots are shown in Fig. 8. We see that the emittance/brightness were improved in stages.
First, the emittance at 20 mA from the 15cm source with 4.0 kG (SmCo bars) Cusp field strength was improved from 0.85 to 0.75 pimm-mr by increasing the Cusp field strength to 5.4 kG (NdFeB bars). The improvement was moderate, only 12% at 20 mA.
Next, we obtained the emittance from Cs operation with the 15cm, 5.4 kG source. Please note that it was necessary to use ext. #4 to
handle the optics tune due to large change of plasma condition. The emittance at 20 mA dropped from 0.75 to 0.52 pi-mm-mr. The
upturn slope to 25 mA is still steep, ~ 0.024 pi-mm-mr/ mA.
Then we achieved the best set of emittance data from the 25cm, 5.4 kG source with ext. #4, non-Cesiated operation. To our
surprise, the emittance at 20 mA was even lower than that from the 15cm source with Cs injection, being 0.39 pi-mm-mr. The
improvement at 5 mA was minimal, only from 0.32 to 0.29 pi-mm-mr. The most striking behavior is the linear and slow upturn from 5
mA to 20 mA. The slope is quite small, only 0.0066 pi-mm-mr/mA. A linear extrapolation to 30 mA would give 0.45 pi-mm-mr.
There are two brightness scales on Fig. 8. The left side scale used by Peters and the right side by Baartman. From left to right the
scaling factor is 2 x (90%)2 ~ 1.6. The best brightness for Cs injection case occurred at 12 mA, being 8.4 (Peters) and 13.4 (Baartman)
mA/mm2-mr2 respectively. After that it gradually drops down to 6.3 (10.1) at 25 mA. For the 25cm source and non-Cs operation the
brightness is still climbing upward at 20 mA, achieving 13.4 (21.4) mA/mm2mr2.
Fig. 8. Brightness corresponding to Fig.7. please note the difference between the curve from the 25cm source without Cs injection.
V. DISCUSSION
With repeated and consistent emittance measurement for several alterations of extraction geometry, as well as certain tight beam
properties experienced from the 25cm source, a high brightness merit without Cs injection seems to have been established. The
measurement with Cs injection has not yet been done at this time but should be carried out in the near future. This is necessary not
only for the completeness of scientific finding, it is also anticipated that further improvement in H - beam emittance at high current for
the 25cm source may still be possible.
We ask the question whether the linear relationship with beam current can be continued up to 30 mA DC. It is projected that if the
following conditions are met, there is a good chance to achieve such goal:

60 - 80 keV extraction

A 30-40 cm long Cusp field length with 6 kG field strength

An arc power supply supporting 100-120 Amp current at 80-100 Volts

Reliable and optimized Cs injection technique

Well-designed and equipped test station: such as oil-free vacuum, fast control, fully capable diagnostic system

Well-calibrated emittance scanner and Faraday cup which can take the high DC beam density heating

Well-designed optics system

Proper choice of matching system (e.g., a compact magnetic triplet rather than a negatively biased Einzel lens)
The dramatic reduction of emittance implies that doubling the confinement time, doubling the cooling path for electrons does also
reduce the temperature of the H- ions produced, resulting in smaller transverse momentum when extracted from the source. It is
desirable that independent measurement be done at another laboratory to confirm this effect of Cusp field length on emittance. It is
also desirable to expand the DC operation to include the pulsed mode. Will the low emittance merit be retained with high current
pulsed mode operation? If the result is positive, it may find its application in the linac source community.
REFERENCES
1.
R. Baartman et. al. “An Intense Ion Source for H- Cyclotron”, Proc. 11th Cyc. Conf. p717, Tokyo, 1987.
2.
D. Yuan et. al. “A Compact DC Cusp Source”, Fifth {IS-PN- NIB/B.NY}, p323, 1990.
3.
Jayamanna et. al. “The TRIUMF Compact DC H-/D- Ion Source”, Proc. 2nd EPAC, p647, Nice, 1990.
4.
T. Kuo et. al. Rev. Sci. Instrum. 67(3), 1314, 1996.
5.
T. Kuo et. al. Rev. Sci. Instrum. 69(2), 959, 1998.
6.
R. Welton et. al. Rev. Sci. Instrum. 77(3), 03A506, 03A508, 2006
7.
J. Peters, Rev. Sci. Instrum. 77(3), 03A528, 2006
8.
M. Bacal et. al. Rev. Sci. Intrum. 77(3), 03A502, 2006
9.
Y. S. Hwang et. al. Rev. Sci. Intrum. 77(3), 03A509, 2006
10. J. R. Hickes, J. Appl. Phys. 51, p4592, 1980
11. A. P. Hickman, Phys. Rev., A43, p3495, 1991