Installation of the
Laser Driven Target (LDT)
at BLAST
Introduction:

LDT is similar to ABS as both targets are a
source of nuclear spin polarized H or D atoms.
Atomic Beam Source
• Well established technology
• Can create pure spin states
• 8 x 1016 atoms/s
• 84% degree of dissociation
• 80% polarization
Laser Driven Target
• Compact design
• Active pumping—higher flux
• 2 x 1018 atoms/s
• 60% degree of dissociation
• 50% polarization

Active pumping: The nuclei of H or D atoms
can be polarized by initially polarizing alkali
vapor through optical pumping in a high
magnetic field.
The vapor and atomic
hydrogen or deuterium flow through a spinexchange cell where the vapor is pumped and
polarization is transferred to the nuclei
through collisions.
1
2

Important dates:
 Now
until September 2002
→ optimization
 September
2002 to September 2003
→ prepare for installation
 September

2003 → ready for installation
Outline of this talk:
 Current
 Result
design of LDT
update
 Installation
plan
 Conclusions
3
Current design of LDT:





Gas flow and vacuum system
Dissociator and spin-exchange cell
Magnetic field coils
Pump laser and probe laser
Polarimeter
4
5
6
Gas flow and vacuum system:

H2 or D2 pass through an MKS mass flow
control into the dissociator
 The target chamber is a six-way cross with
10” flanges on the to and bottom and 8”
flanges on the sides
 There are two holes in the target cell allowing
the gas to be sampled along its length
 The background pressure is ~10-7 Torr
7
Dissociator and spin-exchange cell:
 The
RF can is typically driven at 30W and
100-240 MHz
 The spincell is heated to 220oC and has a
drifilm coating to prevent depolarization
 The spincell is recoated after about 150-300
hours of operation
 The
RF can is 3” in diameter
Magnetic field coils:
 Two
144 turn coils in a Helmholtz-like
configuration centered on the spincell
 Current typically 285A (300 max) and
voltage 50V (100 max) and the maximum
field is 1 kGauss
8
Pump laser and probe laser:

The pump laser will be 1.4” in diameter,
power less than 3 W and circularly
polarized at the spincell
9

The probe laser parts have not yet arrived

It will be 2mm in diameter, power less
than 3 mW (fractions of a mW may be
typical) and linearly polarized

The probe laser will propagate through the
spincell in the opposite direction to the
pump
10
Polarimeter:
 Blank
copper gaskets with 3 mm holes in the
center are placed between each stage for
beam collimation and differential pumping
 The first stage houses a 1 Tesla permanent
sextupole magnet for spin-state separation
 This stage is pumped with a Varian Starcell
300 l/s ion pump to 10-7 Torr
 The second stage is pumped to 5×10-9 Torr
with a 300 l/s ion pump and NEG sorption
pump
 The beam is detected with a Balzers Prizma
QMA and to enhance the signal, the beam is
chopped at 20 Hz
11
Result update:
Molecular signal
35
RF Off
RF on
30
Qma x (mV)
25
20
15
10
5
0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Distance at target cell (mm)
Degree of dissociation
100
bgrd subtracted
uncorrected
90
deg of diss (%)
80
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Distance at target cell (mm)


Large background
Uncorrected degree of dissociation =
1 M2
RF on
M 2 RF
off
12

90% degree of dissociation at 2sccm flow
rate has been achieved with a complete
spincell
13
Heating of the oven and potassium at 1 sccm flow rate
95
Degree of dissociation (%)
90
85
80
75
70
65
Spincell heating up
60
Potassium heating up, spincell at 180C
55
20
40
60
80
100
120
140
160
Temp (C)

As the spincell and potassium are heated the
degree of dissociation decreases
14
180
Positive helicity polarization test
5.5
QMA mass 1 0.4
Laser Power
5
QMA m1
4.5
0.35
4
3.5
0.3
3
2.5
2
0.25
118
116
114
112
110
108
106
104
102
100
98
96
wavelength (770 +.x nm)
Negative helicity polarization test
5.5
QMA mass 1
0.4
Laser Power
5
4.5
QMA m1
0.35
4
3.5
0.3
3
2.5
2
0.25
118
116
114
112
110
108
106
104
102
100
98
wavelength (770 + .x nm)

Two resonances for the pump laser wavelength,
770.105 and 770.112 nm  33% and 22% pol
 Flow = 1.0 sccm
15
Target Performance
0.60
FOM (1017 atom/sec)
0.50
0.40
0.30
0.20
0.10
0.00
150
160
170
180
190
O
K temp ( C)
200
210
220

The target performance is determined by the
flow rate, degree of dissociation and
polarization
 The design goal is 1.8 x 1017 atom/s
16
Installation plan:

Gas flow and vacuum system

The current gas flow system will be used
and the gas will be piped from the gas
panel (where?) to the dissociator
 The vacuum system of the ABS target
chamber can be used
17

Dissociator and spin-exchange cell

Small modifications to the height and
angle of the dissociator to accommodate
the magnetic field coils are required
 The RF can is typically driven at 30W and
100-240 MHz and a new can needs to be
built and tested to resonate at 13.5, 27, 54
(?) or 108 (?) MHz.

The spincell will need to be recoated after
about 150-300 hours of operation
 The time required with the current setup to
swap spincells is about 4 hours and the
estimated time after installation is 8 hours

Several spincells will be constructed and a
separate RF station made from the current
setup and duplicate parts
 This will be used to bake-out and test a
recoated spincell before installation
18

Target chamber

This will be the same as the ABS
chamber with modifications
 The cone (magenta) has a 10” flange that
will be bolted onto the target chamber
 The target chamber may need a trough
(V-shape) to reduce the separation
between the magnetic field coils
19

The current cone needs to be modified to
prevent obstruction of the acceptance
 There is room above the cone to make
this modification

The target cell will be heated to about
220oC

Target thickness and spin temperature
equilibrium→ there must be a good seal
between the glassware and the target cell
 As the target cell is fixed, this seal will
require that the glassware be made to very
small tolerance and bolted down with
precision
 Thin bellows with adjustment screws
may be added to the cone
 0.3” is available on the inside of the cone
if the oven width is reduced
20

Pump laser and probe laser

Both lasers will be transported to the
spincell from a laser hut (where?) and the
probe laser needs to be returned

The direction of the pump and probe laser
at the spincell must be parallel to and be
able to change with the magnetic field

A laser hut is needed near the target

The lasers will be transported in
enclosures from the hut to the spincell
using mirrors 2” in diameter in a
compensating configuration
 The
spincell and periscopes near the oven
will be completely enclosed to prevent
reflections into the hall and have vents to
allow heat to escape from the oven
window
21

The probe laser will return to the hut along
the path of the pump laser or a separate tube

The transmitted pump laser relative power
will be monitored using a beamsplitter after
the spincell
22

Magnetic field coils

Two layers of coils and iron, similar to
ABS
 Direction of the field may be changed by
adjusting coil currents
 Jason will talk about the design of the
magnetic field coils

Polarimeter

The ABS polarimeter will be used with
the entrance below the target cell
23
Conclusions:
 The LDT has produced high electron polarized
H and D atoms at flow rates exceeding 1018
atom/s, which have been calculated to be in
spin temperature equilibrium
 The design goal for LDT is a flow rate of 2 x
1018 atoms/s with 60% degree of dissociation
and 50% polarization
 Further improvements are expected with the
use of an electro-optic modulator and
optimization of the operating parameters of
the new spincell

For installation into BLAST several problems
need to be addressed including the:





RF can (specific frequencies)
Magnetic coils (must not block acceptance)
LDT coupling to the chamber (acceptance)
Spincell coupling to the target cell (leakage)
Laser transport (safety and stability)
24
Spin-exchange collisions
25