Sns-Note-ap-127, 2004-06-29

EXperimental SETUP for the SNS H^-^ laser stripping

> A.Aleksandrov, S. Assadi, J. Barhen, Y. Braiman, V. Danilov, W. Grice,
>
> A. Shishlo, S. Henderson, B.Lang, Y. Liu, G. Murdoch, K. Potter
>
> **Oak Ridge National Laboratory**
>
> **Oak Ridge, TN 37830**
>
> D. Berkaev**,** A. Evstigneev, I. Koop, A. Otboev, Yu. Shatunov, D.
> Shvartz
>
> **Budker Institute of Nuclear Physics**
>
> **Novosibirsk, 630090, Russia**
>
> H. Nakayama, I. Yamane
>
> **KEK, High Energy Research Organization,**
>
> **OHO 1-1, Tsukuba-shi, Ibaraki-ken, 301-0801 Japan**

This memo presents a scheme for 3-step stripping of an H^-^ beam (see
\[1\]). First, H^-^ atoms are converted to H^0^ by a magnetic field,
then H^0^ atoms are excited from the ground state to the upper levels by
a laser, and the excited states can be converted to protons by a
magnetic field. In addition, we can measure the beam energy by observing
the maximum of one electron detachment with the laser beam angle (see
\[2\]) Also, it is convertible to 2-step method of stripping, suggested
by Y. Iamane, by using the Stark effect (see \[3\]). The scheme is
presented below.

![](media/image1.wmf){width="4.679861111111111in"
height="3.1347222222222224in"}

Figure 1 Setup of the experiment

# I. Insertion Specification

The full length of the insertion is **125 cm** length. But only 122.224
cm are available for magnets and a bellow -- the rest will be occupied
by a current transformer and its flanges. The height from the floor to
the center of the beam axis is **50"=127cm**. Figure 2 shows the
transfer line, the place for experiment (between last two quads) and
type of mechanical supports for magnets.

![](media/image2.jpeg){width="5.897222222222222in"
height="3.9194444444444443in"}

Figure 2 Part of the HEBT line for the laser stripping experiment

# 

# II. Magnets Specifications

The first magnet has to have maximal horizontal field (2 Tesla at the
boundary of iron saturation). The maximal field is needed to avoid big
spread of angles of the Hydrogen atoms after conversion from the
negative Hydrogen atoms. The resulting angular spread (about 10^-3^) is
still very large to perform good laser stripping; therefore the magnetic
field should bend the ions in vertical direction since the laser beam is
in horizontal plane. **Horizontal angular spread should be avoided as
much as possible to achieve the laser stripping.** The magnet field and
the edge configuration preferably have to be designed to minimize this
spread. It is given by the following formula:

$\sigma_{\phi} = \sqrt{\int_{0}^{\infty}{\frac{\text{ds}}{\beta c\text{γτ}(s)}\text{exp}( - \int_{0}^{s}\frac{ds^{'}}{\beta c\text{γτ}(s^{'})}) \cdot (\int_{0}^{s}{\frac{ds^{'}}{\rho(s^{'})} - \phi)^{2}}}}$,

where
$\phi = \int_{0}^{\infty}{\frac{\text{ds}}{\beta c\text{γτ}(s)}\text{exp}( - \int_{0}^{s}\frac{ds^{'}}{\beta c\text{γτ}(s^{'})}) \cdot \int_{0}^{s}\frac{ds^{'}}{\rho(s^{'})}}$
,
$\tau(s) = \frac{A_{1}}{\text{βγ}c|B(s)|}\text{exp}(\frac{A_{2}}{\text{βγ}c|B(s)|})$
is the lifetime of H^-^ in the rest frame of the ion, with
A~1~=2.47±0.09 10^-14^ MVs/cm, A~2~=44.94±0.10 MV/cm, and
$\rho(s) = \frac{\gamma\text{mv}}{\text{eB}(s)}$ is the radius of
curvature for the H^-^ ion trajectory in the given magnetic field
*B(s)*.

The second magnet is needed for a precise control of the magnetic fields
(both vertical and horizontal) at the interaction point. **The fields
have to be controlled at the level of 10 gauss to prevent Stark shifts
of the excited Hydrogen level in the volume about 1 cm^3^ in the center
of the magnet.** The magnetic field in both directions has to vary from
zero to about 150 Gauss to compensate the stray magnetic fields from the
first and the third magnets.

The third magnet should have vertical field and the gap between its
poles has to accommodate the laser beam line with possibility to vary
angles form 20 to 60 degrees. For simplicity it is reasonable to make it
as C-magnet to avoid intersection laser beam with the magnet frame.
**1^st^ and 3^rd^ magnet lengths should be about 20 cm with the gap 5 cm
-- shorter magnets will have the field decrease at the axis and longer
magnets won't be economical.** In addition, in another scheme for the
laser stripping (see \[3\]) the magnetic field at the 2^nd^ interaction
point in the center of this magnet has to be large (of the order of 4
kG) in order to introduce Stark broadening of the excited state. This
should be achieved by making its coil out of two parts. If connected in
series they should give 2 Tesla field. If the smaller coil is powered,
the field from the same current should be 0.38 Tesla

**The second magnet length is 18 cm or less as allowed by geometrical
restrictions and shielding properties of the iron (it should shield the
region from strong fields of the 1^st^ and the 3^rd^ magnet). The larger
distance is not desirable to prevent beam size increase on its way from
1^st^ magnet front edge to the center of the second magnet (due to the
angle divergence) and decay of the excited state when it is moving from
the IP to the third magnet front edge.**

# III. Vacuum Chamber Specifications

The vacuum chamber should have 10^-8^ Torr vacuum. It has to have one
bellow and a current transformer after the third magnet. The magnet part
of the vacuum vessel will consist of quadrupole mating flange to match
the existing quadrupole flange, a reducer flange to make transition to
5cm OD vacuum chamber, vacuum chamber piece to the current transformer
mating flange, and the mating flange itself. It will be surrounded by 3
magnets. The 2^nd^ and 3^rd^ magnet will be manufactured such that they
allow laser beam to pass from the table laser beam manipulation assembly
(described in another Tech Memo) to two interaction points. The vacuum
vessel for the laser beam and the flange should be made accordingly.

The vacuum chamber doesn't need baking. The cleaning process has to
correspond to the SNS procedures for the HEBT vacuum chamber:

ALL PARTS TO BE FABRICATED IN ACCORDANCE WITH GOOD VACUUM

PRACTICE, WHICH AS A MINIMUM SHALL COMPLY WITH THE FOLLOWING

REQUIREMENTS:

1\. ALL CONTAMINATES e.g. SCALE, DIRT, GREASE & OIL SHALL BE

REMOVED PRIOR TO FABRICATION.

2\. ONLY WATER SOLUABLE NON-SULFUROUS BEARING CUTTING OILS

SHALL BE USED FOR MACHINING.

3\. ALL PARTS ARE TO BE DETERGENT WASHED TO REMOVE ALL CUTTING

OILS PRIOR TO FINAL WASHING & RINSE WITH DE-IONIZED WATER.

4\. ALL FABRICATED PARTS SHALL BE HELIUM LEAK TESTED & SHOW NO

DETECTABLE LEAK WHEN A MSLD PEAK TO A SENSITIVITY OF

\<2 x 10-10 STD. CC/SEC.

5\. FINISHED PARTS ARE TO BE INDIVIDUALLY WRAPPED USING UHV GRADE
ALUMINUM FOIL & STORED IN A POLYETHYLENE BAG WITH DESICCANT PRIOR TO
BEING PACKAGED FOR SHIPPING.

6\. SPECIFIC WAVERS TO ANY OF THE ABOVE REQUIREMENTS CAN ONLY BE GRANTED
BY AN SNS ASD VACUUM GROUP ENGINEER.

7\. WELD ACCORDANCE WITH A.W.S.D1.1-92.

Figure 3 shows the horizontal cut of the magnet, vacuum chamber and
diagnostics setup.

# IV. Laser Beam Absorbers

We have to have two beam stops for the experiments. The one should be
placed in the second magnet region, another- to the third magnet region.
They will be made out of white ceramic, since the tests show the
stainless steel gets too much damage even from 16% of nominal power
laser beam**. Their sizes -- transverse 15mm×15mm**, the thickness (the
dimension in the direction of the laser beam propagation) is **6 mm**.
The first beam stop ceramic piece surface should be perpendicular to the
incident laser beam, because any head-on reflections with the beam can
be harmful for the experiment. The second ceramic beam stop surface can
be parallel with the laser beam surface -- the Stark Broadening
experiment is less sensitive to the reflected light (see Figure 3). They
should be positioned such that the laser beam would hit the center of
the first if its angle with the axis is 39.51° , for the first, and
28.8° for the second laser stop, respectively. The deeps in the vacuum
chamber for the laser stops should have some elastic clips to hold the
ceramic pieces. Glues are not acceptable.

# V. Power Supply and Water Cooling Pressure

At time of experiment we are going to disconnect the main HEBT magnets
from their power supply and use it to power two main (the firs and the
third) magnets of the assembly.

It has the following parameters: **Current** -- 1405 A, **Voltage** --
390 V.

The available **Water Pressure** is about 100 psi.

# VI. Request for Magnet and Vacuum Data

At the end of the assembly, the SNS group will need from BINP the
magnetic fields map along the beam axis.

![](media/image3.wmf){width="6.0in" height="4.5in"}

Figure 3 Horizontal cross section of the assembly.

# VII. Transfer Line Optics

The linac dump is one of the convenient places for the experiment. It
has total 17 quadrupoles and its length is equal to 77.873 meters. The
first 11 quadrupoles belong to the HEBT line also. Their values are left
intact and correspond to the standard optics. The linac dump has 6 more
quadrupoles. The whole experimental setup is placed between 16 and 17
quadrupoles with the distance between the interaction point and the
center of the first quad of the HEBT line equal to 60.75 meters. The
distance between the last magnets is equal to 2 meters. The last 4
magnets are varied to get minimal vertical size and minimal horizontal
spread. One possible set of their values shown in Table 1:

  ----------------------------------- -----------------------------------
  12 Quad gradient (T/m)              -3.106

  13 Quad gradient (T/m)              0\.

  14 Quad gradient (T/m)              2.3637

  15 Quad gradient (T/m)              2.3637

  16 Quad gradient (T/m)              -3.106

  17 Quad gradient (T/m)              2.3637
  ----------------------------------- -----------------------------------

Note: 14, 15, 17 magnets have one power supply and their values are the
same. 12 and 15 magnets also have same fields and one power supply.
Figure 1 shows the resulting beta functions of the linac dump. Their
values at the interaction point are 50 m (horizontal) and 20 cm
(vertical); their derivatives are equal to zero.

![](media/image4.png)

Figure 4 Beta functions of the linac dump line from the center of the
last SCL quadrupole to the interaction point.

The laser stripping idea is described in details in \[1\]. The
experiments on the experimental observation of the H^-^ Photodetachment
cross section is described in \[2\]. The laser stripping method via a
broad Stark state is described in \[3\].

# VIII. Laser Beam Parameters

The laser can be used at least for two purposes:

1)  to test the laser stripping for the SNS ring. It needs to excite the
    hydrogen from n=1 to n=3 state. For 355 nm wavelength and 1 GeV beam
    the angle of the laser beam with the hydrogen beam should be equal
    to 39.51 degrees.

2)  to measure energy of the beam. The negative hydrogen atom has
    Photodetachment cross section maxima at the photon energies of about
    10.93 and 10.98 eV. The angle in this case should be about 53.53
    degrees for the same wavelength and beam energy. The first magnet in
    this case has to be turned off. The third magnet field should be
    small to prevent stripping but large enough to direct the electrons
    to the electron detector.

3)  To do experiments with Stark broadening of the excited states ( I.
    Yamane's experiment brief description goes here). For n=4 state
    excitation one needs angle of 28.8 degrees. The laser angular spread
    is preferred to be equal to zero.

> The laser beam angle should vary from 20 degrees to 60 in order to
> have possibility to work below or above beam energy of 1 GeV.

**Table 1: Parameters for the proof-of-principle experiment.**

+--------------------------------------+-------------------------------+
| **H^-^ Beam and H^0^ Parameters**    |                               |
+--------------------------------------+-------------------------------+
| H^-^ Beam Kinetic Energy             | 1000 MeV                      |
+--------------------------------------+-------------------------------+
| Relativistic β,γ                     | 0.875, 2.066                  |
+--------------------------------------+-------------------------------+
| Linac output energy spread           | 0.33 MeV                      |
+--------------------------------------+-------------------------------+
| Horizontal, vertical RMS beamsizes   | 3.53, 0.22 mm                 |
+--------------------------------------+-------------------------------+
| Horizontal, vertical RMS angular     | 0.07, 1.0 mrad                |
| divergence                           |                               |
+--------------------------------------+-------------------------------+
| H^0^ *n=1* to *n=3* transition       | 102.6 nm                      |
| wavelength, λ~0~                     |                               |
+--------------------------------------+-------------------------------+
| Doppler frequency spread, ∆ω~0~^D^   | 3.6x10^12^ sec^-1^            |
+--------------------------------------+-------------------------------+
| H^0^ *n=3* lifetime                  | 5.3 nsec                      |
+--------------------------------------+-------------------------------+
| H^0^ *n=3* mean decay length in      | 2.87 m                        |
| lab-frame                            |                               |
+--------------------------------------+-------------------------------+
| ####### Laser Properties             |                               |
+--------------------------------------+-------------------------------+
| Laser type                           | Nd:YAG                        |
+--------------------------------------+-------------------------------+
| Laser wavelength, λ                  | 355 nm (frequency tripled)    |
+--------------------------------------+-------------------------------+
| Laser pulse length                   | 8 nsec                        |
+--------------------------------------+-------------------------------+
| Laser repetition rate                | 30 Hz                         |
+--------------------------------------+-------------------------------+
| Laser pulse energy                   | 150 mJ (at 3^rd^ harmonic)    |
+--------------------------------------+-------------------------------+
| ####### Laser-                       |                               |
| Particle Beam Interaction Parameters |                               |
+--------------------------------------+-------------------------------+
| Laser beam angle w.r.t. H^0^ beam, α | 39.5°                         |
+--------------------------------------+-------------------------------+
| Interaction time, T~0~               | 15 psec                       |
+--------------------------------------+-------------------------------+
| Frequency sweep range, ∆ω~0~^S^      | 1.1x10^13^ sec^-1^            |
+--------------------------------------+-------------------------------+
| Angular divergence of laser beam, ∆α | 2.36 mrad (1 mrad rms)        |
+--------------------------------------+-------------------------------+
| N=3 excitation efficiency, *f*       | 0.98                          |
+--------------------------------------+-------------------------------+
| Vertical laser beamsize, min         | 0.48 mm (.2 mm rms)           |
+--------------------------------------+-------------------------------+
| Horizontal laser beam size           | 4.8 mm (2 mm rms)             |
+--------------------------------------+-------------------------------+
| Rabi Frequency, Ω                    | 1.3x10^12^ sec^-1^            |
+--------------------------------------+-------------------------------+
| Rest-frame laser power density,      | 410 MW/cm^2^                  |
| *Q~0~*                               |                               |
+--------------------------------------+-------------------------------+
| Lab-frame laser power density, *Q*   | 34 MW/cm^2^                   |
+--------------------------------------+-------------------------------+
| Laser beam cross-sectional area      | 0.041 cm^2^                   |
+--------------------------------------+-------------------------------+
| Required lab-frame peak laser power  | 1.4 MW                        |
+--------------------------------------+-------------------------------+
| Required laser pulse energy          | 11 mJ                         |
+--------------------------------------+-------------------------------+
| Laser spot rms size at IP,           | 5 mm(0.2 to 1 (variable)) mm  |
| horizontal (vertical)                |                               |
+--------------------------------------+-------------------------------+
| Laser beam divergence θ at IP,       | 1.(or 0)-4. mrad(variable),   |
| horizontal (vertical) (θ=W/S, W-is   | (3 mrad max)                  |
| the waist in Gaussian beam, S is the |                               |
| Raleigh range. See, e.g. 1)          |                               |
+--------------------------------------+-------------------------------+
| Laser beam mirror angle step, less   | 0.1 mrad                      |
| or equal to                          |                               |
+--------------------------------------+-------------------------------+
| ####### Stripping Magnet Parameters  |                               |
+--------------------------------------+-------------------------------+
| H- stripping magnet gap              | 5 cm                          |
+--------------------------------------+-------------------------------+
| H- stripping magnet length           | 10 cm                         |
+--------------------------------------+-------------------------------+
| H- stripping magnet pole-tip field   | 2 T                           |
+--------------------------------------+-------------------------------+
| H- mean decay length                 | 78 μm                         |
+--------------------------------------+-------------------------------+
| Angular spread from Lorentz          | 1 mrad                        |
| stripping                            |                               |
+--------------------------------------+-------------------------------+
| H^0\*^ stripping magnet gap          | 5 cm                          |
+--------------------------------------+-------------------------------+
| H^0\*^ stripping magnet length       | 10 cm                         |
+--------------------------------------+-------------------------------+
| H^0\*^ stripping magnet pole-tip     | 2 T                           |
| field                                |                               |
+--------------------------------------+-------------------------------+

**References:**

1.V. Danilov, *et al*, SNS-NOTE-AP-48, 2002
<http://it.sns.ornl.gov/asd/public/doc/sns0048/sns0048.doc>

V. Danilov, A. Aleksandrov, S. Assadi, S. Henderson, N. Holtkamp, T.

Shea, A. Shishlo, Y. Braiman, Y. Liu, J. Barhen, and T. Zacharia
"Three-step H\^{-} charge exchange injection with a narrow-band laser",
PRSTAB, 053501, May, 2003

<http://link.aps.org/abstract/PRSTAB/v6/i5/e053501>

2\. H.C. Bryant *et al*, " Observation of resonances near 11 eV in the
Photodetachment cross section of the H^-^ ion", Phys. Rev. Letters,
January 1977

3\. I. Yamane, " Feasibility of Laser Stripping via a Broad Stark State
for Charge-Exchange Injection into a High-Intensity Proton Ring", KEK
Report 2001-20, February 2002