J. Plasma Fusion Res. SERIES, Vol. 8 (2009)

Multi-anode photomultiplier tube as a tool for spatio-temporalmeasurements in a plasma
Ole Waldmann and Werner Bohmeyer
Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Wendelsteinstr. 1, D-17491 Greifswald, Germany(Received: 30 August 2008 / Accepted: 1 December 2008)

Measurements of fast phenomena in the vicinity of a limiter inserted into a linear plasma column are shownin this work. The results were conducted at the plasma generator PSI-2 (Humboldt Universitaet zu Berlin, Germany) which provides a stationary plasma produced in a DC arc discharge. The plasma near a tungsten limiter
was imaged on a photomultiplier tube equipped with 16 anodes. The photomultiplier array was being attached
to oscilloscopes. In comparison with fast cameras this arrangement is inferior with respect to spatial resolution
but superior with regard to temporal resolution which is more important in the present context. Two diﬀerent
structures with enhanced photon emission regions were investigated. On the one hand the rotation of the plasma
column could be seen, with and without limiter. On the other hand, when immersing limiters inside the column
structures with a high photon emission travelling radially into the shadow downwards the limiter were investigated. These investigations deal with basic questions, they could help to understand the anomalous transport inlinear devices in general and the transport in plasma shadows behind limiters in particular.
Keywords: convective plasma transport, plasma blobs, optical diagnostics, plasma rotation

1. Introduction

with short exposure times (t ≈ 1 μs) but have long readout times between frames (t = 10 μs). It is shown in thefollowing that such long intermissions in between frames
would not be tolerable.
The measurements presented in this work were performed
using a multi-anode photomultiplier, which will be described in section 2.2. This diagnostic tool has alreadybeen used in other ﬁeld of physics [10–12], but not inplasma diagnostics. For the observation of the scrape-oﬀ
layer in NSTX two rectangularly crossed lines of photomultipliers were used [13], which cannot provide a full
two-dimensional spatial resolution. In the course of thiswork, we will ﬁrst introduce the plasma generator PSI-2,
the multi-anode photomultiplier and the limiters brieﬂy. In
the main part of this work we will concentrate on the measurements with and without limiter and afterwards conclude the article.

Observing the plasma in the vicinity of a material limiter on a short time scale can provide information about
intermittent processes possibly being responsible for an
anomalous transport. In an earlier paper anomalous transport was proposed for the linear plasma generator PSI-2 [1]but a convincing explanation how this transport mechanism works could not be given.
The enhanced transport perpendicular to the magnetic ﬁeld
lines in magnetically conﬁned plasma was explained by
Antar et al. [2] with a convective transport process. Thisanomalous transport cannot only be found in toroidal but
also in linear conﬁgurations [3]. In the just mentioned
works Langmuir probes and fast imaging cameras wereused as diagnostics.
Groups at other linear devices investigated intermittent
transport processes in the past [4–7], other references canbe found in the review article about turbulence in toroidal
devices by Zweben et al. [8]. In the frame of this work
Carter’s article [6] is especially interesting, because limiters where also used in that work. But the plasma wasa non-stationary and non-rotating one (pulses of 10 ms),compare section 2.1 for the plasma used in this work. He
measured a fairly high amount of density in the shadowof the limiter as it was also shown by us in the past[9]. He used arrays of negatively biased Langmuir probes
which measured enhanced signals moving outwards anddecreased signals moving inwards stochastically with a velocity of at least 1000 ms . Our probe measurements in thepast were not in an array but in a single probe conﬁguration and too slow to measure such intermittent events.
One advantage of an optical measurement is its noninvasive nature. The mentioned fast cameras can be used

2. Experimental Setup
2.1 Plasma Generator PSI-2

PSI-2 is a linear plasma device with a stationary arc
discharge. A steady discharge current in the range of 20 toabout 500 A can be chosen. The plasma is produced in the
discharge region which consists of a heated, hollow, cylindrical LaB6 -cathode and a cylindrically-shaped Mo-anode.
This leads to a hollow proﬁle in electron density and temperature. Guided by an axial magnetic ﬁeld the plasma
streams with a typical cross-section of 60 mm through aso called diﬀerential pumping system and a target chamber until it is terminated at a neutralizer plate (cf. Fig. 1).
The limiter was placed in the ﬁrst target plane and the optical system was installed under a rectangular angle to theplasma (cf. Fig. 1). We used helium and argon as working
gases. Typical plasma parameters are: IAC = 50 . . . 200 A,

author’s e-mail: ole.waldmann@ipp.mpg.de1©2009 by The Japan Society of Plasma
Science and Nuclear Fusion Research

O. Waldmann and W. Bohmeyer, Multi-Anode Photomultiplier Tube as a Tool for Spatio-Temporal Measurements in a Plasma

discharge
regiondifferential
pumping
stage

oscilloscope (Tektronix 2440) was used to trigger the otherthree oscilloscopes simultaneously (cf. Fig. 2). One channel of the photomultiplier was used as a trigger signal.
All oscilloscopes measure continuously. Therefore, pretrigger events were also measured. The three oscilloscopes
were connected to a computer via GPIB and controlled bythis computer.
The background signal was quite high therefore all channels were attached to the oscilloscopes in AC. This improved the signal-to-noise ratio for the interesting ﬂuctuations. When measuring in AC conﬁguration (cf. Fig. 3)
one has to consider the critical frequency fcrit = 2πC1c Rt
with Cc as the coupling capacitor and Rt as the terminating resistor [15]. The frequency of the signals in our casewas much higher than the critical frequency fcrit ≈ 10 Hz(Cc = 0.016 μF, Rt = 1 MΩ).

dump
regiontarget chamber

Limitercathode (LaB6 )neutralizer
plate

x
z

gas inlet
Mo anode

0.5 m

Photomultiplierplasma flow

B

Fig. 1 Plasma Generator PSI-2.

B = 0.1 T, ne = 1 . . . 5 · 1018 m−3 , T e = 2 . . . 15 eV,
T i = 0.5 . . . 0.7 T e , pneutral ≈ 0.05 Pa.

2.2

Multi-Anode Photomultiplier�����������

U in

�

� �� ��

�� �� ��

�� �� ��

�

�

� ��

�

�

�

�

�

�

�

�

�

�

�

�

Cc

R t U out

Fig. 3 Capacitor coupling with the coupling capacitor Cc , theterminating resistor Rt , input Uin , and output voltage Uout ,respectively.

���������������

2.3

Limiters
120 mm�

half mask�������

100 mm

80 mm
35 mm

���������������

plasmaplasmaFig. 2 Multi-anode photomultiplier with four oscilloscopes.
Three for data acquisition and one for their external triggering.

aperture mask

Fig. 4 Limiters used in the investigations.

To achieve a high temporal resolution, a photomultiplier (PMT) was used in this work. To obtain also spatial
resolution we used a multi-anode photomultiplier (Hamamatsu H6568-01) [14]. It consists of a photo cathode, anarrangement of dynodes and sixteen anodes. The setup
strives for compactness to minimize the inﬂuence of an external magnetic ﬁeld.
A 4 × 4-grid is installed in front of the photo cathode andwith the arrangement of the dynodes each cell of this gridis mapped to one anode. The cross-talk between neighbored channels is very small (< 1% [11]). Each channel
is calibrated and normalized on its own to handle with theunequal sensitivity of the channels.
A lens maps the plasma onto the photomultiplier. A greyﬁlter and a narrow band interference ﬁlter (589 nm in thecase of helium and 435 nm in the case of argon) were set in
between the lens and the photomultiplier. For the data acquisition three fast oscilloscopes with four channels each
(Tektronix TDS 540A, 744A, 754C) were used. A fourth

Two kinds of limiters were used. Both are made oftungsten and brought into the plasma under ﬂoating conditions. First is a rectangular mask which cuts away half
the plasma (half mask (HM)), the second one has an oriﬁcewith an opening of 35 mm (aperture mask (AM)).
The half mask breaks the rotational symmetry of theplasma while the aperture mask cuts away the hotter anddenser part of the plasma column.

3. Experimental Results
3.1 Movement Without Limiter

For the collective movement we set up a trigger
threshold to measure enhanced or reduced intensity. The
trigger was set to signiﬁcant events. We averaged the signals over 50 events.
In Fig. 5 twelve channels are shown at diﬀerent times. The
successive response of the diﬀerent lines is very obvious.
First, the line at x ≈ −3 mm gains intensity, afterwards

2

1.0

where mi is the ion mass, into account and supposing thesame ﬁve-fold structure, this is roughly comparable to therotation in krypton. We found ( frot · mi )|Ar /( frot · mi )|Kr =1.15 as the ratio of the products of rotational frequency
and ion mass. The sequence of the successive response oflines corresponds with the sense of rotation of the plasma
column measured by spectroscopy. This supports the assumption of an observation of a rotation of the plasma column while looking at the collective movement. It shouldbe emphasized that the temporal resolution is very important for the observation of this movement. A fast camera
with a read out time of about 10 μs would not be able toreveal the whole process.
Looking closer at the reduced intensity (Fig. 6) the line byline behaviour can be recovered but in the inverted direction. However, due to the much broader signal an estimation of the velocity would be less accurate.

0.8
0.4
0.2
0.0

t= 4e-6 s

1.0

t= 5e-6 s

0.8
0.6
0.4
0.2
0.0

t= 7e-6 s

1.0

t= 8e-6 s

0.8
0.6
0.4
0.2

0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24
Axial position z [mm]Axial position z [mm]0.0

Axial position z [mm]Fig. 5 Argon discharge without limiter. Rotational movementof enhanced intensity. Averaged measurement.

3.2

the line at x ≈ 3 mm and at last the line at x ≈ 9 mm.
The reduction of the signal happens in the same manner.
This looks like a rotation of the plasma column with a nonhomogeneous intensity distribution in the column. Klose[16] showed in his PhD thesis a rotating ﬁve-fold structure
in a krypton plasma, with enhanced intensity in the corners at a radius rm = 27.6 mm. The rotational frequency
was determined to frot = 12 kHz.
In our case we have an average distance of 6 mm between
the channels and the structure moves with 1.2 μs fromchannel to channel. Assuming the same radius as in kyrpton, this results in a velocity and frequency ofm
6 mm= 5000 ,1.2 μssv
frot == 29 kHz.
2πr(1)Radial pos. x [mm]Radial pos. x [mm]Radial pos. x [mm]

v≈

12
9
6
3
0
-3
-6

t= 0e-6 s

t= 3e-6 s(2)

t= 7e-6 s

-0.2
-0.6
-0.8
-1.0

t= 10e-6 s

t= 13e-6 s

9
6
3
0
-3
-6

12
9
6
3
0
-3
-6

0.0
-0.4

12 t= 8e-6 s

0.0
-0.2
-0.4
-0.6
-0.8
-1.0

t= 14e-6 s

t= 15e-6 s

t= 16e-6 s

Movement with Limiters

The measurement shown in Fig. 7 is again an average
like in the case without limiter. But this time the half mask
is inserted at z = 0 mm and blocks the positive x-half space(the mask is marked in green in the ﬁgure). By comparison
with Fig. 5 the same rotational movement with a similar velocity can be found. The only diﬀerence is the behavior ofplasma in the column directly behind the mask. The movement in this column at z ≈ 3 mm is delayed in comparison
with the columns at bigger z-values.

0.0
-0.2

t= 0e-6 sRadial pos. x [mm]t= 3e-6 s

12
9
6
3
0
-3
-6

Radial pos. x [mm]t= 2e-6 s

0.6

12 t= 6e-6 s
9
6
3
0
-3
-6

t= 1e-6 s

12 t= 6e-6 s

Radial pos. x [mm]t= 0e-6 s

12
9
6
3
0
-3
-6

Radial pos. x [mm]Radial pos. x [mm]

12
9
6
3
0
-3
-6

Radial pos. x [mm]

12
9
6
3
0
-3
-6

Radial pos. x [mm]O. Waldmann and W. Bohmeyer, Multi-Anode Photomultiplier Tube as a Tool for Spatio-Temporal Measurements in a Plasma

12
9
6
3
0
-3
-6

t= 2e-6 s

t= 4e-6 s

1.0
0.8
0.6
0.4
0.2
0.0

t= 8e-6 s

t= 10e-6 s

9
6
3
0
-3
-6

1.0
0.8
0.6
0.4
0.2
0.0

t= 12e-6 s

t= 14e-6 s

t= 18e-6 s

1.0
0.8
0.6
0.4
0.2
0.0

t= 20e-6 s

t= 22e-6 s

t= 25e-6 s

1.0
0.8
0.6
0.4
0.2

0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24
Axial position z [mm]Axial position z [mm]Axial position z [mm]

0.0

-0.4
-0.6

Fig. 7 Argon discharge with half mask (positive x-half space at
z = 0 mm, marked in green). Averaged measurement.

-0.8
0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24
Axial position z [mm]Axial position z [mm]Axial position z [mm]-1.0Besides this collective movement which can be found withand without a limiter, quite a diﬀerent structural behaviour
also shows up in the case with limiter. Because of thestochastical appearance of these events we refer to nonaveraged measurements in the following.

Fig. 6 Argon discharge without limiter. Rotational movementof reduced intensity. Averaged measurement.

Taking the 1/mi -dependency of rotational frequency,

3

Radial pos. x [mm]50 t= 0e-6 s

Radial pos. x [mm]50 t= 12e-6 s

Radial pos. x [mm]O. Waldmann and W. Bohmeyer, Multi-Anode Photomultiplier Tube as a Tool for Spatio-Temporal Measurements in a Plasma

50 t= 26e-6 s

t= 4e-6 s

t= 8e-6 s

40

1.0

the limiter and decreases after 15 μs. Afterwards it shows
up again directly behind the limiter moving towards theundisturbed plasma. This can be interpreted as a convective transport in a spiral movement.
These single measurements showed small spatial structures which moves independently from the rest of theplasma into the shadow of the limiter. In both cases withmasks we can measure radial velocities of about 3000 ms .
As a comparison the streaming velocity of undisturbed
plasma in PSI-2 should be mentioned which is in the rangeof 500 ms [17].

0.8

30
0.6

20
10

0.4

t= 16e-6 s

t= 20e-6 s

40

1.0
0.8

30
0.6

20
10

0.4

t= 30e-6 s

t= 34e-6 s

40

1.0
0.8

30
0.6

20
10
0

5 10 15 20 25 30 35 0
Axial position z [mm]5 10 15 20 25 30 35 0
Axial position z [mm]5 10 15 20 25 30 35
Axial position z [mm]

0.4

3.3
Intensity[a.u.]

Fig. 8 Helium discharge with aperture mask (x ≥ 17.5 mm at
z = 0 mm, marked in green). Single measurement.

Radial pos. x [mm]

18
15
12
9
6
3
0
-3
-6

Radial pos. x [mm]Radial pos. x [mm]

18
15
12
9
6
3
0
-3
-6

Radial pos. x [mm]t= 0e-6 s

t= 2e-6 s

Intensity[a.u.]

Intensity[a.u.]

1.0
0.8

1
0.75
0.5
0.25
0
1
0.75
0.5
0.25
0
1
0.75
0.5
0.25
0
1
0.75
0.5
0.25
0
-40 -30 -20 -10

0.4
0.0

t= 5e-6 s

t= 7e-6 s

Ar with AM

He with AM

Ar with HM

0

10

20

30

40

50

Fig. 10 Time traces of intensity in one cell. First measurement isaveraged over 50 times. The other three measurement are
single measurements.

0.2

t= 4e-6 s

Ar without

Time [e-6 s]

0.6

1.0
0.8
0.6

Looking at the temporal behaviour of the events a classiﬁcation can be made by the ratio of the rise time to the
decay time, cf. Fig. 10. In this ﬁgure the intensity of a particular cell is depicted. Especially the event with the halfmask shows a very fast rise and a slow decay. The diﬀerent events are summarized in Table 1 and compared to avalue of a blobby event in PISCES measured with Langmuir probes [3].

0.4
0.2
0.0

t= 9e-6 s

18
15
12
9
6
3
0
-3
-6
18
15
12
9
6
3
0
-3
-6

t= 1e-6 s

Intensity[a.u.]First a measurement in helium with the aperture mask isshown in Fig. 8. The oriﬁce has a radius of 17.5 mm andits center is positioned at plasma axis x = 0 mm, z = 0 mm.
The signal in this single measurement is quite noisy but one
can see that there is an enhanced intensity pattern moving
radially outwards directly behind the aperture mask. This
movement diﬀers from the collective one in a way that not
a whole line gains intensity at the same time. The pattern
is mostly in the ﬁrst column after the limiter with a slightoverlap to the second column. The radial movement will
be more noticeable in the next example.

Temporal Behaviour

t= 11e-6 s

t= 13e-6 s

1.0
0.8
0.6
0.4
0.2

t= 15e-6 s

t= 19e-6 s

t= 21e-6 s

0.0
1.0
0.8
0.6
0.4
0.2

0

3 6 9 12 15 18 0 3 6 9 12 15 18 0 3 6 9 12 15 18
Axial position z [mm]Axial position z [mm]Axial position z [mm]0.0

Fig. 9 Argon discharge with half mask (positive x-half space at
z = 0 mm, marked in green). Single measurement.

Experiment

Arwithout

ArAM

HeAM

ArHM

[3]

tincr /tdecr

0.9

0.4

0.3

0.2

0.2

Table 1 Ratio of rise time to decay time (cf. Fig. 10).

In the measurement shown in Fig. 9 the half mask is set
to the positive x-half space at z = 0 mm. At the time stept = 0 μs one cell inside the plasma column shows an enhanced intensity. This cell is moving into the shadow of

There are other groups (e.g. [18–20]) which observed intermittent structures that can be assigned to convective transport. In the case of an optical measurement one has to recall that electron temperature has a much stronger eﬀect on

4

O. Waldmann and W. Bohmeyer, Multi-Anode Photomultiplier Tube as a Tool for Spatio-Temporal Measurements in a Plasma

visibility than electron density. But the combined measurements of Langmuir probes and fast cameras undertaken by
the just mentioned groups suggest that these enhanced intensity events are due to density perturbations and therefore blobby plasma transport.

4. Conclusion

In this work we showed the use of a multi-anodephotomultiplier as a tool to measure fast phenomena withmoderate spatial resolution.
The advantage of this method in comparison to fast cameras is the possibility of taking data continuously without
intermission for read out. This gave us the opportunity to
see intermittent events on a very short time scale.
We could distinguish between two movements. A collective one which could be interpreted as a rotation of thewhole plasma column, and a convective one, indicatingtransport of small structures into the shadow of a limiter.
The movement of enhanced intensity cells into the shadowof a limiter across magnetic ﬁeld lines could be a possible
explanation for the unexpected high density level inthese shadows. To decide if these events contribute toparticle and/or heat transport it is intended to combine
these optical measurements with probe arrays in the future.
[1] O. Waldmann et al., Contributions to Plasma Physics 47, 692(2007).
[2] G.Y. Antar et al., Physical Review Letters 87, 6 (2001).
[3] G.Y. Antar et al., Physics of Plasmas 10, 419 (2003)[4] A.H. Nielsen et al., Physics of Plasmas 3, 1530 (1996)[5] G. Y. Antar et al., Physics of Plasmas 14, 022301 (2006).
[6] T.A. Carter, Physics of Plasmas 13, 010701 (2006)[7] T. Windisch et al., Physics of Plasmas 13, 122303 (2006)[8] S.J. Zweben et al., Plasma Physics and Controlled Fusion 49,
S1 (2007)[9] O. Waldmann et al., AIP Conference Proceedings 812, 443(2006)[10] A.J. Soares et al., Nuclear Instruments and Methods inPhysics Research Section A 477, 77 (2002)[11] P. Krizan et al., Nuclear Instruments and Methods inPhysics Research Section A 394, 27 (1997)[12] E. Albrecht et al., Nuclear Instruments and Methods inPhysics Research Section A 488, 110 (2002)[13] M. Agostini et al., Physics of Plasmas 14, 102305 (2007)[14] H. Kyushima et al., IEEE Transactions on Nuclear Science41, 725 (1994).
[15] G. Meyer, Oszilloskope (Huethig, Heidelberg, 1989)[16] S. Klose, PhD thesis, Humboldt Universitaet zu Berlin, Germany (2000)[17] T. Lunt et al., Physical Review Letters 100, 175004 (2008)[18] N. Ohno et al., Journal of Plasma and Fusion Research 80,
275 (2004)[19] S.H. Müller et al., Physics of Plasmas 13, 100701 (2006)[20] C. Theiler et al., Physics Plasmas 15, 042303 (2008)5